src/engine/runtime.js

const EventEmitter = require('events');
const {OrderedMap} = require('immutable');
const ExtendedJSON = require('@turbowarp/json');

const ArgumentType = require('../extension-support/argument-type');
const Blocks = require('./blocks');
const BlocksRuntimeCache = require('./blocks-runtime-cache');
const BlockType = require('../extension-support/block-type');
const BlockShape = require('../extension-support/block-shape');
const Profiler = require('./profiler');
const Sequencer = require('./sequencer');
const execute = require('./execute.js');
const compilerExecute = require('../compiler/jsexecute');
const ScratchBlocksConstants = require('./scratch-blocks-constants');
const TargetType = require('../extension-support/target-type');
const Thread = require('./thread');
const log = require('../util/log');
const maybeFormatMessage = require('../util/maybe-format-message');
const StageLayering = require('./stage-layering');
const Variable = require('./variable');
const xmlEscape = require('../util/xml-escape');
const ScratchLinkWebSocket = require('../util/scratch-link-websocket');
const FontManager = require('./tw-font-manager');
const { validateJSON } = require('../util/json-block-utilities');
const Color = require('../util/color');
const TabManager = require('../extension-support/pm-tab-manager');
const ModalManager = require('../extension-support/pm-modal-manager');
const MathUtil = require('../util/math-util');
const Cast = require('../util/cast');
const ExtensionStorage = require('../util/deprecated-extension-storage.js');

// Virtual I/O devices.
const Clock = require('../io/clock');
const Cloud = require('../io/cloud');
const Keyboard = require('../io/keyboard');
const Mouse = require('../io/mouse');
const MouseWheel = require('../io/mouseWheel');
const UserData = require('../io/userData');
const Video = require('../io/video');
const Touch = require('../io/touch');

const StringUtil = require('../util/string-util');
const uid = require('../util/uid');

const coreVariableTypes = [
    Variable.SCALAR_TYPE,
    Variable.LIST_TYPE,
    Variable.BROADCAST_MESSAGE_TYPE
];
const defaultBlockPackages = {
    scratch3_control: require('../blocks/scratch3_control'),
    scratch3_event: require('../blocks/scratch3_event'),
    scratch3_looks: require('../blocks/scratch3_looks'),
    scratch3_motion: require('../blocks/scratch3_motion'),
    scratch3_operators: require('../blocks/scratch3_operators'),
    scratch3_sound: require('../blocks/scratch3_sound'),
    scratch3_sensing: require('../blocks/scratch3_sensing'),
    scratch3_data: require('../blocks/scratch3_data'),
    scratch3_procedures: require('../blocks/scratch3_procedures'),
    pm_liveTests: require('../blocks/pm_live tests')
};

const interpolate = require('./tw-interpolate');
const FrameLoop = require('./tw-frame-loop');

const defaultExtensionColors = ['#0FBD8C', '#0DA57A', '#0B8E69'];

const COMMENT_CONFIG_MAGIC = ' // _twconfig_';

/**
 * Information used for converting Scratch argument types into scratch-blocks data.
 * @type {object.<ArgumentType, {shadowType: string, fieldType: string}>}
 */
const ArgumentTypeMap = (() => {
    const map = {};
    map[ArgumentType.ANGLE] = {
        shadow: {
            type: 'math_angle',
            // We specify fieldNames here so that we can pick
            // create and populate a field with the defaultValue
            // specified in the extension.
            // When the `fieldName` property is not specified,
            // the <field></field> will be left out of the XML and
            // the scratch-blocks defaults for that field will be
            // used instead (e.g. default of 0 for number fields)
            fieldName: 'NUM'
        }
    };
    map[ArgumentType.COLOR] = {
        shadow: {
            type: 'colour_picker',
            fieldName: 'COLOUR'
        }
    };
    map[ArgumentType.NUMBER] = {
        shadow: {
            type: 'math_number',
            fieldName: 'NUM'
        }
    };
    map[ArgumentType.STRING] = {
        shadow: {
            type: 'text',
            fieldName: 'TEXT'
        }
    };
    map[ArgumentType.BOOLEAN] = {
        check: 'Boolean',
        shadow: {
            type: 'checkbox',
            fieldName: 'CHECKBOX'
        }
    };
    map[ArgumentType.MATRIX] = {
        shadow: {
            type: 'matrix',
            fieldName: 'MATRIX'
        }
    };
    map[ArgumentType.NOTE] = {
        shadow: {
            type: 'note',
            fieldName: 'NOTE'
        }
    };
    map[ArgumentType.IMAGE] = {
        // Inline images are weird because they're not actually "arguments".
        // They are more analagous to the label on a block.
        fieldType: 'field_image'
    };
    map[ArgumentType.POLYGON] = {
        check: 'math_polygon',
        shadow: {
            type: 'polygon'
        }
    };
    map[ArgumentType.CUSTOM] = {
        fieldType: 'field_customInput'
    };
    map[ArgumentType.COSTUME] = {
        shadow: {
            type: 'looks_costume',
            fieldName: 'COSTUME'
        }
    };
    map[ArgumentType.SOUND] = {
        shadow: {
            type: 'sound_sounds_menu',
            fieldName: 'SOUND_MENU'
        }
    };
    // VARIABLE, LIST and BROADCAST are actually fields
    // they'll be handled similarly to IMAGE
    map[ArgumentType.VARIABLE] = {
        fieldType: "field_variable",
        fieldName: "VARIABLE"
    };
    map[ArgumentType.LIST] = {
        fieldType: "field_variable",
        fieldName: "LIST",
        variableType: 'list'
    };
    map[ArgumentType.BROADCAST] = {
        fieldType: "field_variable",
        fieldName: "BROADCAST",
        variableType: 'broadcast_msg'
    };
    map[ArgumentType.SEPERATOR] = {
        fieldType: 'field_vertical_separator'
    };

    return map;
})();

/**
 * A pair of functions used to manage the cloud variable limit,
 * to be used when adding (or attempting to add) or removing a cloud variable.
 * @typedef {object} CloudDataManager
 * @property {function} canAddCloudVariable A function to call to check that
 * a cloud variable can be added.
 * @property {function} addCloudVariable A function to call to track a new
 * cloud variable on the runtime.
 * @property {function} removeCloudVariable A function to call when
 * removing an existing cloud variable.
 * @property {function} hasCloudVariables A function to call to check that
 * the runtime has any cloud variables.
 * @property {function} getNumberOfCloudVariables A function that returns the
 * number of cloud variables in the project.
 */

/**
 * Creates and manages cloud variable limit in a project,
 * and returns two functions to be used to add a new
 * cloud variable (while checking that it can be added)
 * and remove an existing cloud variable.
 * These are to be called whenever attempting to create or delete
 * a cloud variable.
 * @param {Object} cloudOptions
 * @param {number} cloudOptions.limit Maximum number of cloud variables
 * @return {CloudDataManager} The functions to be used when adding or removing a
 * cloud variable.
 */
const cloudDataManager = cloudOptions => {
    let count = 0;

    const canAddCloudVariable = () => count < cloudOptions.limit;

    const addCloudVariable = () => {
        count++;
    };

    const removeCloudVariable = () => {
        count--;
    };

    const hasCloudVariables = () => count > 0;

    const getNumberOfCloudVariables = () => count;

    return {
        canAddCloudVariable,
        addCloudVariable,
        removeCloudVariable,
        hasCloudVariables,
        getNumberOfCloudVariables
    };
};

/**
 * Numeric ID for Runtime._step in Profiler instances.
 * @type {number}
 */
let stepProfilerId = -1;

/**
 * Numeric ID for Sequencer.stepThreads in Profiler instances.
 * @type {number}
 */
let stepThreadsProfilerId = -1;

/**
 * Numeric ID for RenderWebGL.draw in Profiler instances.
 * @type {number}
 */
let rendererDrawProfilerId = -1;

/**
 * Manages targets, scripts, and the sequencer.
 * @constructor
 */
class Runtime extends EventEmitter {
    constructor () {
        super();

        /**
         * Target management and storage.
         * @type {Array.<!Target>}
         */
        this.targets = [];

        /**
         * Targets in reverse order of execution. Shares its order with drawables.
         * @type {Array.<!Target>}
         */
        this.executableTargets = [];

        /**
         * A list of threads that are currently running in the VM.
         * Threads are added when execution starts and pruned when execution ends.
         * @type {Array.<Thread>}
         */
        this.threads = [];

        this.threadMap = new Map();

        /** @type {!Sequencer} */
        this.sequencer = new Sequencer(this);

        /**
         * Storage container for flyout blocks.
         * These will execute on `_editingTarget.`
         * @type {!Blocks}
         */
        this.flyoutBlocks = new Blocks(this, true /* force no glow */);

        /**
         * Storage container for monitor blocks.
         * These will execute on a target maybe
         * @type {!Blocks}
         */
        this.monitorBlocks = new Blocks(this, true /* force no glow */);

        /**
         * Currently known editing target for the VM.
         * @type {?Target}
         */
        this._editingTarget = null;

        /**
         * Map to look up a block primitive's implementation function by its opcode.
         * This is a two-step lookup: package name first, then primitive name.
         * @type {Object.<string, Function>}
         */
        this._primitives = {};

        /**
         * Map to look up all block information by extended opcode.
         * @type {Array.<CategoryInfo>}
         * @private
         */
        this._blockInfo = [];

        /**
         * Map to look up hat blocks' metadata.
         * Keys are opcode for hat, values are metadata objects.
         * @type {Object.<string, Object>}
         */
        this._hats = {};

        /**
         * A list of script block IDs that were glowing during the previous frame.
         * @type {!Array.<!string>}
         */
        this._scriptGlowsPreviousFrame = [];

        /**
         * Number of non-monitor threads running during the previous frame.
         * @type {number}
         */
        this._nonMonitorThreadCount = 0;

        /**
         * All threads that finished running and were removed from this.threads
         * by behaviour in Sequencer.stepThreads.
         * @type {Array<Thread>}
         */
        this._lastStepDoneThreads = null;

        /**
         * pm: The current tab manager for this runtime.
         */
        this.tabManager = new TabManager(this);

        /**
         * pm: The current modal manager for this runtime.
         */
        this.modalManager = new ModalManager(this);

        /**
         * Currently known number of clones, used to enforce clone limit.
         * @type {number}
         */
        this._cloneCounter = 0;

        /**
         * Flag to emit a targets update at the end of a step. When target data
         * changes, this flag is set to true.
         * @type {boolean}
         */
        this._refreshTargets = false;

        /**
         * Map to look up all monitor block information by opcode.
         * @type {object}
         * @private
         */
        this.monitorBlockInfo = {};

        /**
         * Ordered map of all monitors, which are MonitorReporter objects.
         */
        this._monitorState = OrderedMap({});

        /**
         * Monitor state from last tick
         */
        this._prevMonitorState = OrderedMap({});

        /**
         * Whether the project is in "turbo mode."
         * @type {Boolean}
         */
        this.turboMode = false;

        /**
         * tw: Responsible for managing the VM's many timers.
         */
        this.frameLoop = new FrameLoop(this);

        /**
         * Current length of a step.
         * Changes as mode switches, and used by the sequencer to calculate
         * WORK_TIME.
         * @type {!number}
         */
        this.currentStepTime = 1000 / 30;

        // Set an intial value for this.currentMSecs
        this.updateCurrentMSecs();

        /**
         * Whether any primitive has requested a redraw.
         * Affects whether `Sequencer.stepThreads` will yield
         * after stepping each thread.
         * Reset on every frame.
         * @type {boolean}
         */
        this.redrawRequested = false;

        // Register all given block packages.
        this._registerBlockPackages();

        // Register and initialize "IO devices", containers for processing
        // I/O related data.
        /** @type {Object.<string, Object>} */
        this.ioDevices = {
            clock: new Clock(this),
            cloud: new Cloud(this),
            keyboard: new Keyboard(this),
            mouse: new Mouse(this),
            mouseWheel: new MouseWheel(this),
            userData: new UserData(),
            video: new Video(this),
            touch: new Touch(this)
        };

        /**
         * A list of extensions, used to manage hardware connection.
         */
        this.peripheralExtensions = {};

        /**
         * A runtime profiler that records timed events for later playback to
         * diagnose Scratch performance.
         * @type {Profiler}
         */
        this.profiler = null;

        this.cloudOptions = {
            limit: 10
        };

        this.extensionRuntimeOptions = {
            javascriptUnsandboxed: false
        };

        const newCloudDataManager = cloudDataManager(this.cloudOptions);

        /**
         * Check wether the runtime has any cloud data.
         * @type {function}
         * @return {boolean} Whether or not the runtime currently has any
         * cloud variables.
         */
        this.hasCloudData = newCloudDataManager.hasCloudVariables;

        /**
         * A function which checks whether a new cloud variable can be added
         * to the runtime.
         * @type {function}
         * @return {boolean} Whether or not a new cloud variable can be added
         * to the runtime.
         */
        this.canAddCloudVariable = newCloudDataManager.canAddCloudVariable;

        /**
         * A function which returns the number of cloud variables in the runtime.
         * @returns {number}
         */
        this.getNumberOfCloudVariables = newCloudDataManager.getNumberOfCloudVariables;

        /**
         * A function that tracks a new cloud variable in the runtime,
         * updating the cloud variable limit. Calling this function will
         * emit a cloud data update event if this is the first cloud variable
         * being added.
         * @type {function}
         */
        this.addCloudVariable = this._initializeAddCloudVariable(newCloudDataManager);

        /**
         * A function which updates the runtime's cloud variable limit
         * when removing a cloud variable and emits a cloud update event
         * if the last of the cloud variables is being removed.
         * @type {function}
         */
        this.removeCloudVariable = this._initializeRemoveCloudVariable(newCloudDataManager);

        /**
         * A string representing the origin of the current project from outside of the
         * Scratch community, such as CSFirst.
         * @type {?string}
         */
        this.origin = null;

        this._stageTarget = null;

        this.addonBlocks = {};

        this.stageWidth = Runtime.STAGE_WIDTH;
        this.stageHeight = Runtime.STAGE_HEIGHT;

        this.runtimeOptions = {
            maxClones: Runtime.MAX_CLONES,
            miscLimits: true,
            fencing: true,
            dangerousOptimizations: false,
            disableOffscreenRendering: false
        };

        this.compilerOptions = {
            enabled: true,
            warpTimer: false
        };

        this.optimizationUtil = {
            sin: new Array(360),
            cos: new Array(360)
        };
        for (let i = 0; i < 360; i++) {
            this.optimizationUtil.sin[i] = Math.round(Math.sin((Math.PI * i) / 180) * 1e10) / 1e10;
            this.optimizationUtil.cos[i] = Math.round(Math.cos((Math.PI * i) / 180) * 1e10) / 1e10;
        }

        this.debug = false;

        this._lastStepTime = Date.now();
        this.interpolationEnabled = false;
        this.interpolate = interpolate;

        this._defaultStoredSettings = this._generateAllProjectOptions();

        /**
         * TW: We support a "packaged runtime" mode. This can be used when:
         *  - there will never be an editor attached such as scratch-gui or scratch-blocks
         *  - the project will never be exported with saveProjectSb3()
         *  - original costume and sound data is not needed
         * In this mode, the runtime is able to discard large amounts of data and avoid some processing
         * to make projects load faster and use less memory.
         * This is not designed to protect projects from copying as someone can still copy the data that
         * gets fed into the runtime in the first place.
         * This mode is used by the PenguinMod Packager and the TurboWarp Packager.
         */
        this.isPackaged = false;

        /**
         * PM: Legacy option. Use vm.securityManager for permission managing.
         * In the packager, the legacy Project Permission Manager can be enabled.
         * This option is used by the PenguinMod Packager.
         */
        this.isProjectPermissionManagerDisabled = true;

        /**
         * PM: Determines whether or not the project is running from a packager output.
         * Note that this is not the same as Runtime.isPackaged,
         * the packager will literally set this value to `true` when packaging regardless of settings.
         */
        this.isPackagedProject = false;

        /**
         * Contains information about the external communication methods that the scripts inside the project
         * can use to send data from inside the project to an external server.
         * Do not update this directly. Use Runtime.setExternalCommunicationMethod() instead.
         */
        this.externalCommunicationMethods = {
            cloudVariables: false,
            customExtensions: false
        };
        this.on(Runtime.HAS_CLOUD_DATA_UPDATE, enabled => {
            this.setExternalCommunicationMethod('cloudVariables', enabled);
        });

        // pm: remove listener warning
        this.setMaxListeners(50);

        /**
         * If set to true, features such as reading colors from the user's webcam will be disabled
         * when the project has access to any external communication method to protect user privacy.
         * Requires TurboWarp/scratch-render.
         * Do not update this directly. Use Runtime.setEnforcePrivacy() instead.
         */
        this.enforcePrivacy = true;

        /**
         * Internal map of opaque identifiers to the callback to run that function.
         * @type {Map<string, function>}
         */
        this.extensionButtons = new Map();

        /**
         * Contains the audio context and gain node for each extension that registers them.
         * Used to make sure the extensions respect addons or the pause button.
         * @type {Map<string, {audioContext: AudioContext, gainNode: GainNode}>}
         */
        this._extensionAudioObjects = new Map();

        /**
         * Responsible for managing custom fonts.
         */
        this.fontManager = new FontManager(this);

        this.cameraStates = {
            default: {
                pos: [0, 0],
                dir: 0,
                scale: 1
            }
        };

        // list of variable types declared by extensions
        this._extensionVariables = {};
        // lists all custom serializers
        this.serializers = {
            'pm-rendered-target': {
                serialize: target => ({ id: target.id, name: target.getName() }),
                deserialize: ({ id, name }) => this.getTargetById(id) ?? this.getSpriteTargetByName(name)
            },
            'pm-costume-asset': {
                serialize: asset => ({ id: asset.assetId, name: asset.name }),
                deserialize: ({ assetId, name }) => {
                    for (let i = 0; i < this.targets.length; i++) {
                        const assets = this.targets[i].getCostumes();
                        const found = assets.find(asset => asset.assetId === assetId || asset.name === name);
                        if (found) return found;
                    }
                }
            },
            'pm-sound-asset': {
                serialize: asset => ({ id: asset.assetId, name: asset.name }),
                deserialize: ({ assetId, name }) => {
                    for (let i = 0; i < this.targets.length; i++) {
                        const assets = this.targets[i].getSounds();
                        const found = assets.find(asset => asset.assetId === assetId || asset.name === name);
                        if (found) return found;
                    }
                }
            }
        };

        /**
         * An object to contain runtime variables from the
         * LilyMakesThings Thread Variables extension
         * @type {Object}
         */
        this.variables = Object.create(null);

        /**
         * Part of a recreation of the TurboWarp extensionStorage
         * API. The only real reason this is here is to make sure
         * that extensions that only implement TurboWarp's store
         * method don't break in PenguinMod.
         * @type {Object<string, Object>}
         */
        this.extensionStorage = ExtensionStorage();
    }

    /**
     * Width of the stage, in pixels.
     * @const {number}
     */
    static get STAGE_WIDTH () {
        // tw: stage size is set per-runtime, this is only the initial value
        return 480;
    }

    /**
     * Height of the stage, in pixels.
     * @const {number}
     */
    static get STAGE_HEIGHT () {
        // tw: stage size is set per-runtime, this is only the initial value
        return 360;
    }

    /**
     * Event name for glowing a script.
     * @const {string}
     */
    static get SCRIPT_GLOW_ON () {
        return 'SCRIPT_GLOW_ON';
    }

    /**
     * Event name for unglowing a script.
     * @const {string}
     */
    static get SCRIPT_GLOW_OFF () {
        return 'SCRIPT_GLOW_OFF';
    }

    /**
     * Event name for glowing a block.
     * @const {string}
     */
    static get BLOCK_GLOW_ON () {
        return 'BLOCK_GLOW_ON';
    }

    /**
     * Event name for unglowing a block.
     * @const {string}
     */
    static get BLOCK_GLOW_OFF () {
        return 'BLOCK_GLOW_OFF';
    }

    /**
     * Event name for a cloud data update
     * to this project.
     * @const {string}
     */
    static get HAS_CLOUD_DATA_UPDATE () {
        return 'HAS_CLOUD_DATA_UPDATE';
    }

    /**
     * Event name for turning on turbo mode.
     * @const {string}
     */
    static get TURBO_MODE_ON () {
        return 'TURBO_MODE_ON';
    }

    /**
     * Event name for turning off turbo mode.
     * @const {string}
     */
    static get TURBO_MODE_OFF () {
        return 'TURBO_MODE_OFF';
    }

    /**
     * Event name for runtime options changing.
     * @const {string}
     */
    static get RUNTIME_OPTIONS_CHANGED () {
        return 'RUNTIME_OPTIONS_CHANGED';
    }

    /**
     * Event name for compiler options changing.
     * @const {string}
     */
    static get COMPILER_OPTIONS_CHANGED () {
        return 'COMPILER_OPTIONS_CHANGED';
    }

    /**
     * Event name for framerate changing.
     * @const {string}
     */
    static get FRAMERATE_CHANGED () {
        return 'FRAMERATE_CHANGED';
    }

    /**
     * Event name for interpolation changing.
     * @const {string}
     */
    static get INTERPOLATION_CHANGED () {
        return 'INTERPOLATION_CHANGED';
    }

    /**
     * Event called before interpolation data is set.
     */
    static get BEFORE_INTERPOLATE () {
        return 'BEFORE_INTERPOLATE';
    }

    /**
     * Event called after interpolation data is set.
     */
    static get AFTER_INTERPOLATE () {
        return 'AFTER_INTERPOLATE';
    }

    /**
     * Event called before any block is executed.
     */
    static get BEFORE_EXECUTE () {
        return 'BEFORE_EXECUTE';
    }

    /**
     * Event called after every block in the project has been executed.
     */
    static get AFTER_EXECUTE () {
        return 'AFTER_EXECUTE';
    }

    /**
     * Event name for stage size changing.
     * @const {string}
     */
    static get STAGE_SIZE_CHANGED () {
        return 'STAGE_SIZE_CHANGED';
    }

    /**
     * Event name when the mouse is scrolled
     * @const {string}
     */
    static get MOUSE_SCROLLED () {
        return 'MOUSE_SCROLLED';
    }

    /**
     * Event name for compiler errors.
     * @const {string}
     */
    static get COMPILE_ERROR () {
        return 'COMPILE_ERROR';
    }

    /**
     * Event name when the project is started (threads may not necessarily be
     * running).
     * @const {string}
     */
    static get PROJECT_START () {
        return 'PROJECT_START';
    }

    /**
     * Event name when the project is started (but before it runs stopAll)
     * @const {string}
     */
    static get PROJECT_START_BEFORE_RESET () {
        return 'PROJECT_START_BEFORE_RESET';
    }

    /**
     * Event name when threads start running.
     * Used by the UI to indicate running status.
     * @const {string}
     */
    static get PROJECT_RUN_START () {
        return 'PROJECT_RUN_START';
    }

    /**
     * Event name when threads stop running
     * Used by the UI to indicate not-running status.
     * @const {string}
     */
    static get PROJECT_RUN_STOP () {
        return 'PROJECT_RUN_STOP';
    }

    /**
     * Event name for project being stopped or restarted by the user.
     * Used by blocks that need to reset state.
     * @const {string}
     */
    static get PROJECT_STOP_ALL () {
        return 'PROJECT_STOP_ALL';
    }

    /**
     * Event name for target being stopped by a stop for target call.
     * Used by blocks that need to stop individual targets.
     * @const {string}
     */
    static get STOP_FOR_TARGET () {
        return 'STOP_FOR_TARGET';
    }

    /**
     * Event name for visual value report.
     * @const {string}
     */
    static get VISUAL_REPORT () {
        return 'VISUAL_REPORT';
    }

    /**
     * Event name for when a block errors.
     * @const {string}
     */
    static get BLOCK_STACK_ERROR () {
        return 'BLOCK_STACK_ERROR';
    }

    /**
     * Event name for project loaded report.
     * @const {string}
     */
    static get PROJECT_LOADED () {
        return 'PROJECT_LOADED';
    }

    /**
     * Event name for report that a change was made that can be saved
     * @const {string}
     */
    static get PROJECT_CHANGED () {
        return 'PROJECT_CHANGED';
    }

    /**
     * Event name for report that a change was made to an extension in the toolbox.
     * @const {string}
     */
    static get TOOLBOX_EXTENSIONS_NEED_UPDATE () {
        return 'TOOLBOX_EXTENSIONS_NEED_UPDATE';
    }

    /**
     * Event name for targets update report.
     * @const {string}
     */
    static get TARGETS_UPDATE () {
        return 'TARGETS_UPDATE';
    }

    /**
     * Event name for monitors update.
     * @const {string}
     */
    static get MONITORS_UPDATE () {
        return 'MONITORS_UPDATE';
    }

    /**
     * Event name for block drag update.
     * @const {string}
     */
    static get BLOCK_DRAG_UPDATE () {
        return 'BLOCK_DRAG_UPDATE';
    }

    /**
     * Event name for block drag end.
     * @const {string}
     */
    static get BLOCK_DRAG_END () {
        return 'BLOCK_DRAG_END';
    }

    /**
     * Event name for reporting that an extension was added.
     * @const {string}
     */
    static get EXTENSION_ADDED () {
        return 'EXTENSION_ADDED';
    }

    /**
     * Event name for reporting that an extension was removed
     * @const {string}
     */
    static get EXTENSION_REMOVED () {
        return 'EXTENSION_REMOVED';
    }

    /**
     * Event name for reporting that an extension as asked for a custom field to be added
     * @const {string}
     */
    static get EXTENSION_FIELD_ADDED () {
        return 'EXTENSION_FIELD_ADDED';
    }

    /**
     * Event name for updating the available set of peripheral devices.
     * This causes the peripheral connection modal to update a list of
     * available peripherals.
     * @const {string}
     */
    static get PERIPHERAL_LIST_UPDATE () {
        return 'PERIPHERAL_LIST_UPDATE';
    }

    /**
     * Event name for when the user picks a bluetooth device to connect to
     * via Companion Device Manager (CDM)
     * @const {string}
     */
    static get USER_PICKED_PERIPHERAL () {
        return 'USER_PICKED_PERIPHERAL';
    }

    /**
     * Event name for reporting that a peripheral has connected.
     * This causes the status button in the blocks menu to indicate 'connected'.
     * @const {string}
     */
    static get PERIPHERAL_CONNECTED () {
        return 'PERIPHERAL_CONNECTED';
    }

    /**
     * Event name for reporting that a peripheral has been intentionally disconnected.
     * This causes the status button in the blocks menu to indicate 'disconnected'.
     * @const {string}
     */
    static get PERIPHERAL_DISCONNECTED () {
        return 'PERIPHERAL_DISCONNECTED';
    }

    /**
     * Event name for reporting that a peripheral has encountered a request error.
     * This causes the peripheral connection modal to switch to an error state.
     * @const {string}
     */
    static get PERIPHERAL_REQUEST_ERROR () {
        return 'PERIPHERAL_REQUEST_ERROR';
    }

    /**
     * Event name for reporting that a peripheral connection has been lost.
     * This causes a 'peripheral connection lost' error alert to display.
     * @const {string}
     */
    static get PERIPHERAL_CONNECTION_LOST_ERROR () {
        return 'PERIPHERAL_CONNECTION_LOST_ERROR';
    }

    /**
     * Event name for reporting that a peripheral has not been discovered.
     * This causes the peripheral connection modal to show a timeout state.
     * @const {string}
     */
    static get PERIPHERAL_SCAN_TIMEOUT () {
        return 'PERIPHERAL_SCAN_TIMEOUT';
    }

    /**
     * Event name to indicate that the microphone is being used to stream audio.
     * @const {string}
     */
    static get MIC_LISTENING () {
        return 'MIC_LISTENING';
    }

    /**
     * Event name for reporting that blocksInfo was updated.
     * @const {string}
     */
    static get BLOCKSINFO_UPDATE () {
        return 'BLOCKSINFO_UPDATE';
    }

    /**
     * Event name when the runtime tick loop has been started.
     * @const {string}
     */
    static get RUNTIME_STARTED () {
        return 'RUNTIME_STARTED';
    }

    /**
     * Event name when the runtime tick loop has been stopped.
     * @const {string}
     */
    static get RUNTIME_STOPPED () {
        return 'RUNTIME_STOPPED';
    }

    /**
     * Event name when the runtime is paused temporarily.
     * @const {string}
     */
    static get RUNTIME_PAUSED () {
        return 'RUNTIME_PAUSED';
    }

    /**
     * Event name when the runtime is about to be paused temporarily.
     * Fires before runtime.paused = true.
     * @const {string}
     */
    static get RUNTIME_PRE_PAUSED () {
        return 'RUNTIME_PRE_PAUSED';
    }

    /**
     * Event name when the runtime is unpaused.
     * @const {string}
     */
    static get RUNTIME_UNPAUSED () {
        return 'RUNTIME_UNPAUSED';
    }

    /**
     * Event name when the runtime dispose has been called.
     * @const {string}
     */
    static get RUNTIME_DISPOSED () {
        return 'RUNTIME_DISPOSED';
    }

    /**
     * Event name when _step() has been called.
     * @const {string}
     */
    static get RUNTIME_STEP_START () {
        return 'RUNTIME_STEP_START';
    }

    /**
     * Event name when _step() has finished all processing within the function.
     * @const {string}
     */
    static get RUNTIME_STEP_END () {
        return 'RUNTIME_STEP_END';
    }

    /**
     * Event name when an editor tab is created.
     * @const {string}
     */
    static get EDITOR_TABS_NEW () {
        return 'EDITOR_TABS_NEW';
    }

    /**
     * Event name when editor tabs need to be updated.
     * @const {string}
     */
    static get EDITOR_TABS_UPDATE () {
        return 'EDITOR_TABS_UPDATE';
    }

    /**
     * Event name for reporting that a block was updated and needs to be rerendered.
     * @const {string}
     */
    static get BLOCKS_NEED_UPDATE () {
        return 'BLOCKS_NEED_UPDATE';
    }

    /**
     * Event name for camera movements.
     * @const {string}
     */
    static get CAMERA_CHANGED () {
        return 'CAMERA_CHANGED';
    }

   /**
    * Event name for the starting of hats.
    * @const {string}
    */

    static get HATS_STARTED () {
        return 'HATS_STARTED'
    }

    /**
     * Event name for thread finishing.
     * @const {string}
     */
    static get THREAD_FINISHED () {
        return 'THREAD_FINISHED'
    }

    /**
     * How rapidly we try to step threads by default, in ms.
     */
    static get THREAD_STEP_INTERVAL () {
        // tw: not used, only exists for compatibility
        return 1000 / 60;
    }

    /**
     * In compatibility mode, how rapidly we try to step threads, in ms.
     */
    static get THREAD_STEP_INTERVAL_COMPATIBILITY () {
        // tw: not used, only exists for compatibility
        return 1000 / 30;
    }

    /**
     * How many clones can be created at a time.
     * @const {number}
     */
    static get MAX_CLONES () {
        // tw: clone limit is set per-runtime in runtimeOptions, this is only the initial value
        return 300;
    }

    // -----------------------------------------------------------------------------
    // -----------------------------------------------------------------------------

    // Helper function for initializing the addCloudVariable function
    _initializeAddCloudVariable (newCloudDataManager) {
        // The addCloudVariable function
        return (() => {
            const hadCloudVarsBefore = this.hasCloudData();
            newCloudDataManager.addCloudVariable();
            if (!hadCloudVarsBefore && this.hasCloudData()) {
                this.emit(Runtime.HAS_CLOUD_DATA_UPDATE, true);
            }
        });
    }

    // Helper function for initializing the removeCloudVariable function
    _initializeRemoveCloudVariable (newCloudDataManager) {
        return (() => {
            const hadCloudVarsBefore = this.hasCloudData();
            newCloudDataManager.removeCloudVariable();
            if (hadCloudVarsBefore && !this.hasCloudData()) {
                this.emit(Runtime.HAS_CLOUD_DATA_UPDATE, false);
            }
        });
    }

    /**
     * Register default block packages with this runtime.
     * @todo Prefix opcodes with package name.
     * @private
     */
    _registerBlockPackages () {
        for (const packageName in defaultBlockPackages) {
            if (defaultBlockPackages.hasOwnProperty(packageName)) {
                // @todo pass a different runtime depending on package privilege?
                const packageObject = new (defaultBlockPackages[packageName])(this);
                // Collect primitives from package.
                if (packageObject.getPrimitives) {
                    const packagePrimitives = packageObject.getPrimitives();
                    for (const op in packagePrimitives) {
                        if (packagePrimitives.hasOwnProperty(op)) {
                            this._primitives[op] =
                                packagePrimitives[op].bind(packageObject);
                        }
                    }
                }
                // Collect hat metadata from package.
                if (packageObject.getHats) {
                    const packageHats = packageObject.getHats();
                    for (const hatName in packageHats) {
                        if (packageHats.hasOwnProperty(hatName)) {
                            this._hats[hatName] = packageHats[hatName];
                        }
                    }
                }
                // Collect monitored from package.
                if (packageObject.getMonitored) {
                    this.monitorBlockInfo = Object.assign({}, this.monitorBlockInfo, packageObject.getMonitored());
                }

                this.compilerRegisterExtension(packageName, packageObject);
            }
        }
    }

    compilerRegisterExtension (name, extensionObject) {
        this[`ext_${name}`] = extensionObject;
    }
    registerCompiledExtensionBlocks (extensionId, information) {
        if (!information) return;
        if (!information.ir) return;
        if (!information.js) return;

        // Used for extension's compiled blocks.
        // Importing the generators here avoids circular dependency issues
        const JSGenerator = require('../compiler/jsgen');
        const IRGenerator = require('../compiler/irgen');

        IRGenerator.setExtensionIr(extensionId, information.ir);
        JSGenerator.setExtensionJs(extensionId, information.js);
    }

    /**
     * Allows AudioContexts and GainNodes from an extension to respect addons and runtime pausing by default.
     * If audioContext is not supplied, recording addon + pause button will not work with the extension this way.
     * If gainNode is not supplied, recording addon + volume slider will not work with the extension this way.
     * @param {string} extensionId The extension's ID. May be used internally in the future, or by other extensions.
     * @param {AudioContext} audioContext The AudioContext being used in the extension.
     * @param {GainNode} gainNode The GainNode that is connected to the AudioContext. All other nodes in the extension should be connected to this GainNode, and this GainNode should be connected to the destination of the AudioContext.
     */
    registerExtensionAudioContext(extensionId, audioContext, gainNode) {
        if (typeof extensionId !== "string") throw new TypeError('Extension ID must be string');
        if (!extensionId) throw new Error('No extension ID specified'); // empty string

        const obj = {};
        if (audioContext) {
            obj.audioContext = audioContext;
        }
        if (gainNode) {
            obj.gainNode = gainNode;
        }
        this._extensionAudioObjects.set(extensionId, obj);
    }

    getMonitorState () {
        return this._monitorState;
    }

    /**
     * Generate an extension-specific menu ID.
     * @param {string} menuName - the name of the menu.
     * @param {string} extensionId - the ID of the extension hosting the menu.
     * @returns {string} - the constructed ID.
     * @private
     */
    _makeExtensionMenuId (menuName, extensionId) {
        return `${extensionId}_menu_${menuName}`;
    }

    /**
     * Create a context ("args") object for use with `formatMessage` on messages which might be target-specific.
     * @param {Target} [target] - the target to use as context. If a target is not provided, default to the current
     * editing target or the stage.
     */
    makeMessageContextForTarget (target) {
        const context = {};
        target = target || this.getEditingTarget() || this.getTargetForStage();
        if (target) {
            context.targetType = (target.isStage ? TargetType.STAGE : TargetType.SPRITE);
        }
    }

    /**
     * Register the primitives provided by an extension.
     * @param {ExtensionMetadata} extensionInfo - information about the extension (id, blocks, etc.)
     * @private
     */
    _registerExtensionPrimitives (extensionInfo) {
        const categoryInfo = {
            id: extensionInfo.id,
            name: maybeFormatMessage(extensionInfo.name),
            showStatusButton: extensionInfo.showStatusButton,
            blockIconURI: extensionInfo.blockIconURI,
            menuIconURI: extensionInfo.menuIconURI
        };

        if (extensionInfo.color1) {
            const color1 = Color.hexToRgb(extensionInfo.color1);
            categoryInfo.color1 = extensionInfo.color1;
            categoryInfo.color2 = extensionInfo.color2;
            if (!extensionInfo.color2) {
                const mixed = Color.mixRgb(color1, Color.RGB_BLACK, 0.1);
                categoryInfo.color2 = Color.rgbToHex(mixed);
            }
            categoryInfo.color3 = extensionInfo.color3;
            if (!extensionInfo.color3) {
                const mixed = Color.mixRgb(color1, Color.RGB_BLACK, 0.2);
                categoryInfo.color3 = Color.rgbToHex(mixed);
            }
        } else {
            categoryInfo.color1 = defaultExtensionColors[0];
            categoryInfo.color2 = defaultExtensionColors[1];
            categoryInfo.color3 = defaultExtensionColors[2];
        }

        // undefined will default to the regular text color
        categoryInfo.blockText = extensionInfo.blockText;

        if (extensionInfo.isDynamic) {
            categoryInfo.isDynamic = extensionInfo.isDynamic;
            categoryInfo.orderBlocks = extensionInfo.orderBlocks;
        }

        categoryInfo.tbShow = extensionInfo.tbShow || false

        this._blockInfo.push(categoryInfo);

        this._fillExtensionCategory(categoryInfo, extensionInfo);

        for (const fieldTypeName in categoryInfo.customFieldTypes) {
            if (extensionInfo.customFieldTypes.hasOwnProperty(fieldTypeName)) {
                const fieldTypeInfo = categoryInfo.customFieldTypes[fieldTypeName];

                // Emit events for custom field types from extension
                this.emit(Runtime.EXTENSION_FIELD_ADDED, {
                    name: `field_${fieldTypeInfo.extendedName}`,
                    implementation: fieldTypeInfo.fieldImplementation
                });
            }
        }

        this.emit(Runtime.EXTENSION_ADDED, categoryInfo);
    }

    /**
     * Reregister the primitives for an extension
     * @param  {ExtensionMetadata} extensionInfo - new info (results of running getInfo) for an extension
     * @private
     */
    _refreshExtensionPrimitives (extensionInfo) {
        const categoryInfo = this._blockInfo.find(info => info.id === extensionInfo.id);
        if (categoryInfo) {
            categoryInfo.name = maybeFormatMessage(extensionInfo.name);
            this._fillExtensionCategory(categoryInfo, extensionInfo);

            this.emit(Runtime.BLOCKSINFO_UPDATE, categoryInfo);
        }
    }

    _removeExtensionPrimitive(extensionId) {
        const extIdx = this._blockInfo.findIndex(ext => ext.id === extensionId);
        const info = this._blockInfo[extIdx];
        this._blockInfo.splice(extIdx, 1);
        this.emit(Runtime.EXTENSION_REMOVED);
        // cleanup blocks
        for (const target of this.targets) {
            for (const blockId in target.blocks._blocks) {
                const {opcode} = target.blocks.getBlock(blockId);
                if (info.blocks.find(block => block.json?.type === opcode)) {
                    target.blocks.deleteBlock(blockId, true);
                }
            }
        }
        this.emit(Runtime.BLOCKS_NEED_UPDATE);
    }

    /**
     * Read extension information, convert menus, blocks and custom field types
     * and store the results in the provided category object.
     * @param {CategoryInfo} categoryInfo - the category to be filled
     * @param {ExtensionMetadata} extensionInfo - the extension metadata to read
     * @private
     */
    _fillExtensionCategory (categoryInfo, extensionInfo) {
        categoryInfo.blocks = [];
        categoryInfo.customFieldTypes = {};
        categoryInfo.menus = [];
        categoryInfo.menuInfo = {};

        for (const menuName in extensionInfo.menus) {
            if (extensionInfo.menus.hasOwnProperty(menuName)) {
                const menuInfo = extensionInfo.menus[menuName];
                const convertedMenu = this._buildMenuForScratchBlocks(menuName, menuInfo, categoryInfo);
                categoryInfo.menus.push(convertedMenu);
                categoryInfo.menuInfo[menuName] = menuInfo;
            }
        }
        for (const fieldTypeName in extensionInfo.customFieldTypes) {
            if (extensionInfo.customFieldTypes.hasOwnProperty(fieldTypeName)) {
                const fieldType = extensionInfo.customFieldTypes[fieldTypeName];
                const fieldTypeInfo = this._buildCustomFieldInfo(
                    fieldTypeName,
                    fieldType,
                    extensionInfo.id,
                    categoryInfo
                );

                categoryInfo.customFieldTypes[fieldTypeName] = fieldTypeInfo;
            }
        }

        if (extensionInfo.docsURI) {
            const xml = '<button ' +
                `text="${xmlEscape(maybeFormatMessage({
                    id: 'tw.blocks.openDocs',
                    default: 'Open Documentation',
                    description: 'Button that opens site with more documentation about an extension'
                }))}" ` +
                'callbackKey="OPEN_EXTENSION_DOCS" ' +
                `callbackData="${xmlEscape(extensionInfo.docsURI)}"></button>`;
            const block = {
                info: {},
                xml
            };
            categoryInfo.blocks.push(block);
        }

        for (const blockInfo of extensionInfo.blocks) {
            try {
                const convertedBlock = this._convertForScratchBlocks(blockInfo, categoryInfo);
                categoryInfo.blocks.push(convertedBlock);
                if (convertedBlock.json) {
                    const opcode = convertedBlock.json.type;
                    if (blockInfo.blockType !== BlockType.EVENT && blockInfo.blockType !== BlockType.BUTTON) {
                        this._primitives[opcode] = convertedBlock.info.func;
                    }
                    if (blockInfo.blockType === BlockType.EVENT || blockInfo.blockType === BlockType.HAT) {
                        this._hats[opcode] = {
                            edgeActivated: blockInfo.isEdgeActivated,
                            restartExistingThreads: blockInfo.shouldRestartExistingThreads
                        };
                    }
                }
            } catch (e) {
                log.error('Error parsing block: ', {block: blockInfo, error: e});
            }
        }
    }

    /**
     * Convert the given extension menu items into the scratch-blocks style of list of pairs.
     * If the menu is dynamic (e.g. the passed in argument is a function), return the input unmodified.
     * @param {object} menuItems - an array of menu items or a function to retrieve such an array
     * @returns {object} - an array of 2 element arrays or the original input function
     * @private
     */
    _convertMenuItems (menuItems) {
        if (Array.isArray(menuItems)) {
            const extensionMessageContext = this.makeMessageContextForTarget();
            return menuItems.map(item => {
                const formattedItem = maybeFormatMessage(item, extensionMessageContext);
                switch (typeof formattedItem) {
                case 'string':
                    return [formattedItem, formattedItem];
                case 'object':
                    if (Array.isArray(item)) return item.slice(0, 2);
                    return [maybeFormatMessage(item.text, extensionMessageContext), item.value];
                default:
                    throw new Error(`Can't interpret menu item: ${JSON.stringify(item)}`);
                }
            });
        }
        return menuItems;
    }

    /**
     * Build the scratch-blocks JSON for a menu. Note that scratch-blocks treats menus as a special kind of block.
     * @param {string} menuName - the name of the menu
     * @param {object} menuInfo - a description of this menu and its items
     * @property {*} items - an array of menu items or a function to retrieve such an array
     * @property {boolean} [acceptReporters] - if true, allow dropping reporters onto this menu
     * @param {CategoryInfo} categoryInfo - the category for this block
     * @returns {object} - a JSON-esque object ready for scratch-blocks' consumption
     * @private
     */
    _buildMenuForScratchBlocks (menuName, menuInfo, categoryInfo) {
        const menuId = this._makeExtensionMenuId(menuName, categoryInfo.id);
        const menuItems = this._convertMenuItems(menuInfo.items);
        return {
            json: {
                message0: '%1',
                type: menuId,
                inputsInline: true,
                output: 'String',
                colour: menuInfo.isTypeable
                    ? '#FFFFFF'
                    : categoryInfo.color1,
                colourSecondary: menuInfo.isTypeable
                    ? '#FFFFFF'
                    : categoryInfo.color2,
                colourTertiary: menuInfo.isTypeable
                    ? '#FFFFFF'
                    : categoryInfo.color3,
                outputShape: menuInfo.acceptReporters || menuInfo.isTypeable ?
                    ScratchBlocksConstants.OUTPUT_SHAPE_ROUND : ScratchBlocksConstants.OUTPUT_SHAPE_SQUARE,
                args0: [
                    (typeof menuInfo.variableType !== 'undefined' ?
                        {
                            type: 'field_variable',
                            name: menuName,
                            variableTypes: [menuInfo.variableType === 'scalar'
                                ? Variable.SCALAR_TYPE
                                : menuInfo.variableType]
                        } : (menuInfo.isTypeable ?
                            {
                                type: menuInfo.isNumeric
                                    ? 'field_numberdropdown'
                                    : 'field_textdropdown',
                                name: menuName,
                                options: menuItems
                            } : {
                                type: 'field_dropdown',
                                name: menuName,
                                options: menuItems
                            }))
                ]
            }
        };
    }

    _buildCustomFieldInfo (fieldName, fieldInfo, extensionId, categoryInfo) {
        const extendedName = `${extensionId}_${fieldName}`;
        return {
            fieldName: fieldName,
            extendedName: extendedName,
            argumentTypeInfo: {
                shadow: {
                    type: extendedName,
                    fieldName: `field_${extendedName}`
                }
            },
            scratchBlocksDefinition: this._buildCustomFieldTypeForScratchBlocks(
                extendedName,
                fieldInfo.output,
                fieldInfo.outputShape,
                categoryInfo
            ),
            fieldImplementation: fieldInfo.implementation
        };
    }

    /**
     * Build the scratch-blocks JSON needed for a fieldType.
     * Custom field types need to be namespaced to the extension so that extensions can't interfere with each other
     * @param  {string} fieldName - The name of the field
     * @param {string} output - The output of the field
     * @param {number} outputShape - Shape of the field (from ScratchBlocksConstants)
     * @param {object} categoryInfo - The category the field belongs to (Used to set its colors)
     * @returns {object} - Object to be inserted into scratch-blocks
     */
    _buildCustomFieldTypeForScratchBlocks (fieldName, output, outputShape, categoryInfo) {
        return {
            json: {
                type: fieldName,
                message0: '%1',
                inputsInline: true,
                output: output,
                colour: categoryInfo.color1,
                colourSecondary: categoryInfo.color2,
                colourTertiary: categoryInfo.color3,
                outputShape: outputShape,
                args0: [
                    {
                        name: `field_${fieldName}`,
                        type: `field_${fieldName}`
                    }
                ]
            }
        };
    }

    /**
     * Convert ExtensionBlockMetadata into data ready for scratch-blocks.
     * @param {ExtensionBlockMetadata} blockInfo - the block info to convert
     * @param {CategoryInfo} categoryInfo - the category for this block
     * @returns {ConvertedBlockInfo} - the converted & original block information
     * @private
     */
    _convertForScratchBlocks (blockInfo, categoryInfo) {
        if (blockInfo === '---') {
            return this._convertSeparatorForScratchBlocks(blockInfo);
        }

        if (blockInfo.blockType === BlockType.LABEL)
            return this._convertLabelForScratchBlocks(blockInfo);

        if (blockInfo.blockType === BlockType.BUTTON) {
            return this._convertButtonForScratchBlocks(blockInfo);
        }

        if (blockInfo.blockType === BlockType.XML) {
            return this._convertXmlForScratchBlocks(blockInfo);
        }

        return this._convertBlockForScratchBlocks(blockInfo, categoryInfo);
    }

    /**
     * Convert ExtensionBlockMetadata into scratch-blocks JSON & XML, and generate a proxy function.
     * @param {ExtensionBlockMetadata} blockInfo - the block to convert
     * @param {CategoryInfo} categoryInfo - the category for this block
     * @returns {ConvertedBlockInfo} - the converted & original block information
     * @private
     */
    _convertBlockForScratchBlocks (blockInfo, categoryInfo) {
        const extendedOpcode = `${categoryInfo.id}_${blockInfo.opcode}`;

        const blockJSON = {
            type: extendedOpcode,
            inputsInline: true,
            category: categoryInfo.name,
            extensions: blockInfo.extensions ?? [],
            colour: blockInfo.color1 ?? categoryInfo.color1,
            colourSecondary: blockInfo.color2 ?? categoryInfo.color2,
            colourTertiary: blockInfo.color3 ?? categoryInfo.color3,
            blockText: blockInfo.blockText ?? categoryInfo.blockText,
            canDragDuplicate: blockInfo.canDragDuplicate === true
        };
        const context = {
            // TODO: store this somewhere so that we can map args appropriately after translation.
            // This maps an arg name to its relative position in the original (usually English) block text.
            // When displaying a block in another language we'll need to run a `replace` action similar to the one
            // below, but each `[ARG]` will need to be replaced with the number in this map.
            argsMap: {},
            blockJSON,
            categoryInfo,
            blockInfo,
            inputList: []
        };

        // If an icon for the extension exists, prepend it to each block, with a vertical separator.
        // We can overspecify an icon for each block, but if no icon exists on a block, fall back to
        // the category block icon.
        const iconURI = blockInfo.blockIconURI || categoryInfo.blockIconURI;

        if (iconURI) {
            if (!blockJSON.extensions.includes('scratch_extension')) {
                blockJSON.extensions.push('scratch_extension');
            }
            blockJSON.message0 = '%1 %2';
            const iconJSON = {
                type: 'field_image',
                src: iconURI,
                width: 40,
                height: 40
            };
            const separatorJSON = {
                type: 'field_vertical_separator'
            };
            blockJSON.args0 = [
                iconJSON,
                separatorJSON
            ];
        }

        let notchAccepts = blockInfo.notchAccepts ?? 'normal'

        switch (blockInfo.blockType) {
        case BlockType.COMMAND:
            blockJSON.outputShape = ScratchBlocksConstants.OUTPUT_SHAPE_SQUARE;
            blockJSON.previousStatement = notchAccepts; // null = available connection; undefined = hat
            if (!blockInfo.isTerminal) {
                blockJSON.nextStatement = notchAccepts; // null = available connection; undefined = terminal
            }
            break;
        case BlockType.REPORTER:
            blockJSON.output = blockInfo.allowDropAnywhere ? null : 'String'; // TODO: distinguish number & string here?
            blockJSON.outputShape = ScratchBlocksConstants.OUTPUT_SHAPE_ROUND;
            break;
        case BlockType.BOOLEAN:
            blockJSON.output = 'Boolean';
            blockJSON.outputShape = ScratchBlocksConstants.OUTPUT_SHAPE_HEXAGONAL;
            break;
        case BlockType.HAT:
        case BlockType.EVENT:
            if (!blockInfo.hasOwnProperty('isEdgeActivated')) {
                // if absent, this property defaults to true
                blockInfo.isEdgeActivated = true;
            }
            blockJSON.outputShape = ScratchBlocksConstants.OUTPUT_SHAPE_SQUARE;
            blockJSON.nextStatement = notchAccepts; // null = available connection; undefined = terminal
            break;
        case BlockType.CONDITIONAL:
        case BlockType.LOOP:
            blockInfo.branchCount = blockInfo.branchCount ?? 1;
            blockJSON.outputShape = ScratchBlocksConstants.OUTPUT_SHAPE_SQUARE;
            blockJSON.previousStatement = notchAccepts; // null = available connection; undefined = hat
            if (!blockInfo.isTerminal) {
                blockJSON.nextStatement = notchAccepts; // null = available connection; undefined = terminal
            }
            break;
        }

        blockInfo.branches = blockInfo.branches || Array(Math.max(blockInfo.branchCount || 0, 0)).fill({})

        const blockText = Array.isArray(blockInfo.text) ? blockInfo.text : [blockInfo.text];
        let inTextNum = 0; // text for the next block "arm" is blockText[inTextNum]
        let inBranchNum = 0; // how many branches have we placed into the JSON so far?
        let outLineNum = 0; // used for scratch-blocks `message${outLineNum}` and `args${outLineNum}`
        const convertPlaceholders = this._convertPlaceholders.bind(this, context);
        const extensionMessageContext = this.makeMessageContextForTarget();

        // alternate between a block "arm" with text on it and an open slot for a substack
        while (inTextNum < blockText.length || inBranchNum < blockInfo.branches.length) {
            if (inTextNum < blockText.length) {
                context.outLineNum = outLineNum;
                const lineText = maybeFormatMessage(blockText[inTextNum], extensionMessageContext);
                const convertedText = lineText.replace(/\[(.+?)]/g, convertPlaceholders);
                if (blockJSON[`message${outLineNum}`]) {
                    blockJSON[`message${outLineNum}`] += convertedText;
                } else {
                    blockJSON[`message${outLineNum}`] = convertedText;
                }
                ++inTextNum;
                ++outLineNum;
            }
            if (inBranchNum < blockInfo.branches.length) {
                blockJSON[`message${outLineNum}`] = '%1';
                blockJSON[`args${outLineNum}`] = [{
                    type: 'input_statement',
                    name: `SUBSTACK${inBranchNum > 0 ? inBranchNum + 1 : ''}`,
                    check: blockInfo.branches[inBranchNum].accepts ?? 'normal'
                }];
                ++inBranchNum;
                ++outLineNum;
            }
        }

        if (blockInfo.blockType === BlockType.REPORTER || blockInfo.blockType === BlockType.BOOLEAN) {
            if (!blockInfo.disableMonitor && context.inputList.length === 0) {
                blockJSON.checkboxInFlyout = true;
            }
        }
        if ((blockInfo.blockType === BlockType.LOOP || ('branchIndicator' in blockInfo || 'branchIconURI' in blockInfo)) && blockInfo.branchIndicator !== "") {
            // Add icon to the bottom right of a loop block
            blockJSON[`lastDummyAlign${outLineNum}`] = 'RIGHT';
            blockJSON[`message${outLineNum}`] = '%1';
            blockJSON[`args${outLineNum}`] = [{
                type: 'field_image',
                src: blockInfo.branchIndicator ?? blockInfo.branchIconURI ?? './static/blocks-media/repeat.svg',
                width: blockInfo.branchIndicatorWidth ?? 24,
                height: blockInfo.branchIndicatorHeight ?? 24,
                alt: '*', // TODO remove this since we don't use collapsed blocks in scratch
                flip_rtl: true
            }];
            ++outLineNum;
        }

        if (Array.isArray(blockInfo.alignments)) {
            let idx = 0;
            // i love for (const of)
            for (const alignment of blockInfo.alignments) {
                if (typeof alignment === "string") {
                    blockJSON[`lastDummyAlign${idx}`] = alignment.toUpperCase();
                }
                idx++;
            }
        }

        const blockShape = blockInfo.blockShape;
        if (typeof blockShape === 'number') blockJSON.outputShape = blockShape;
        else if (typeof blockShape === 'string') {
            // assume we are handling a custom shape...
            // if it doesnt exist it will default to a round reporter
            if (!blockShape.startsWith('custom-')) blockJSON.outputShape = 'custom-' + blockShape;
            else blockJSON.outputShape = blockShape;
        }
        if (blockInfo.forceOutputType) {
            blockJSON.output = blockInfo.forceOutputType;
        }

        const mutation = blockInfo.isDynamic
            ? `<mutation blockInfo="${xmlEscape.escapeAttribute(JSON.stringify(blockInfo))}"/>`
            : '';
        const inputs = context.inputList.join('');
        const blockId = blockInfo.isSpriteSpecific ? `id="${xmlEscape.escapeAttribute(this.vm.editingTarget.id + '_' + extendedOpcode)}"` : '';
        const blockXML = `<block type="${xmlEscape.escapeAttribute(extendedOpcode)}" ${blockId}>${mutation}${inputs}</block>`;

        return {
            info: context.blockInfo,
            json: context.blockJSON,
            xml: blockXML
        };
    }

    /**
     * Generate a separator between blocks categories or sub-categories.
     * @param {ExtensionBlockMetadata} blockInfo - the block to convert
     * @param {CategoryInfo} categoryInfo - the category for this block
     * @returns {ConvertedBlockInfo} - the converted & original block information
     * @private
     */
    _convertSeparatorForScratchBlocks (blockInfo) {
        return {
            info: blockInfo,
            xml: '<sep gap="36"/>'
        };
    }

    /**
     * Generate generate the xml for a toolbox lable.
     * @param {ExtensionBlockMetadata} blockInfo - the block to convert
     * @param {CategoryInfo} categoryInfo - the category for this block
     * @returns {ConvertedBlockInfo} - the converted & original block information
     * @private
     */
    _convertLabelForScratchBlocks (blockInfo) {
        const text = xmlEscape.escapeAttribute(blockInfo.text);
        return {
            info: blockInfo,
            xml: `<label text="${text}"></label>`
        };
    }

    /**
     * Convert a button for scratch-blocks. A button has no opcode but specifies a callback name in the `func` field.
     * @param {ExtensionBlockMetadata} buttonInfo - the button to convert
     * @property {string} func - the callback name
     * @param {CategoryInfo} categoryInfo - the category for this button
     * @returns {ConvertedBlockInfo} - the converted & original button information
     * @private
     */
    _convertButtonForScratchBlocks (buttonInfo) {
        const extensionMessageContext = this.makeMessageContextForTarget();
        const buttonText = xmlEscape.escapeAttribute(maybeFormatMessage(buttonInfo.text, extensionMessageContext));
        const callback = xmlEscape.escapeAttribute(buttonInfo.opcode
            ? buttonInfo.opcode
            : buttonInfo.func);

        return {
            info: buttonInfo,
            xml: `<button text="${buttonText}" callbackKey="${callback}"></button>`
        };
    }

    _convertXmlForScratchBlocks (xmlInfo) {
        return {
            info: xmlInfo,
            xml: xmlInfo.xml
        };
    }

    /**
     * Helper for _convertPlaceholdes which handles inline images which are a specialized case of block "arguments".
     * @param {object} argInfo Metadata about the inline image as specified by the extension
     * @return {object} JSON blob for a scratch-blocks image field.
     * @private
     */
    _constructInlineImageJson (argInfo) {
        if (!argInfo.dataURI) {
            log.warn('Missing data URI in extension block with argument type IMAGE');
        }
        return {
            type: 'field_image',
            src: argInfo.dataURI || '',
            // TODO these probably shouldn't be hardcoded...?
            width: argInfo.width ?? 24,
            height: argInfo.height ?? 24,
            // Whether or not the inline image should be flipped horizontally
            // in RTL languages. Defaults to false, indicating that the
            // image will not be flipped.
            flip_rtl: argInfo.flipRTL || false
        };
    }

    /**
     * Helper for _convertPlaceholdes which handles variable dropdowns
     * which are a specialized case of block "arguments".
     * @param {object} argInfo Metadata about the variable dropdown
     * @return {object} JSON blob for a scratch-blocks variable field.
     * @private
     */
    _constructVariableDropdown(argInfo, placeholder) {
        // console.log(argInfo, placeholder);
        const isList = argInfo.type === 'list';
        const isBroadcast = argInfo.type === 'broadcast';
        return {
            type: 'field_variable',
            name: placeholder,
            variableTypes: isList ? ['list'] : (isBroadcast ? ['broadcast_msg'] : ['']),
            variable: isBroadcast ? 'message1' : null
        };
    }

    /**
     * Helper for _convertForScratchBlocks which handles linearization of argument placeholders. Called as a callback
     * from string#replace. In addition to the return value the JSON and XML items in the context will be filled.
     * @param {object} context - information shared with _convertForScratchBlocks about the block, etc.
     * @param {string} match - the overall string matched by the placeholder regex, including brackets: '[FOO]'.
     * @param {string} placeholder - the name of the placeholder being matched: 'FOO'.
     * @return {string} scratch-blocks placeholder for the argument: '%1'.
     * @private
     */
    _convertPlaceholders (context, match, placeholder) {
        // Determine whether the argument type is one of the known standard field types
        const argInfo = context.blockInfo.arguments[placeholder] || {};
        let argTypeInfo = ArgumentTypeMap[argInfo.type] || {};

        // Field type not a standard field type, see if extension has registered custom field type
        if (!ArgumentTypeMap[argInfo.type] && context.categoryInfo.customFieldTypes[argInfo.type]) {
            argTypeInfo = context.categoryInfo.customFieldTypes[argInfo.type].argumentTypeInfo;
        }

        // Start to construct the scratch-blocks style JSON defining how the block should be
        // laid out
        let argJSON;

        // Most field types are inputs (slots on the block that can have other blocks plugged into them)
        // check if this is not one of those cases. E.g. an inline image on a block.
        if (argTypeInfo.fieldType === 'field_image') {
            argJSON = this._constructInlineImageJson(argInfo);
        } else if (argTypeInfo.fieldType === 'field_variable') {
            argJSON = this._constructVariableDropdown(argInfo, placeholder);
        } else if (argTypeInfo.fieldType === 'field_vertical_separator') {
            argJSON = {
                type: 'field_vertical_separator',
            };
        } else if (argTypeInfo.fieldType === 'field_customInput') {
            argJSON = {
                type: 'field_customInput',
                name: placeholder,
                id: argInfo.id,
                value: argInfo.defaultValue
            };
        } else {
            // Construct input value

            // Layout a block argument (e.g. an input slot on the block)
            argJSON = {
                type: 'input_value',
                name: placeholder
            };

            const defaultValue =
                typeof argInfo.defaultValue === 'undefined' ? '' :
                    xmlEscape.escapeAttribute(maybeFormatMessage(
                        argInfo.defaultValue, this.makeMessageContextForTarget()).toString());

            if (argTypeInfo.check || argInfo.check) {
                // Right now the only type of 'check' we have specifies that the
                // input slot on the block accepts Boolean reporters, so it should be
                // shaped like a hexagon
                argJSON.check = argInfo.check || argTypeInfo.check;
            }
            const argShape = argTypeInfo.shape || argInfo.shape;
            if (argShape) {
                if (typeof argShape === 'number') argJSON.shape = argShape;
                else {
                    // assume we are handling a custom shape...
                    // if it doesnt exist it will default to a null input
                    if (!argShape.startsWith('custom-')) argJSON.shape = 'custom-' + argShape;
                    else argJSON.shape = argShape;
                }
            }

            let valueName;
            let shadowType;
            let blockType;
            let fieldName;
            let variableID;
            let variableName;
            let variableType;
            if (argInfo.menu) {
                const menuInfo = context.categoryInfo.menuInfo[argInfo.menu];
                if (menuInfo.acceptReporters || menuInfo.isTypeable) {
                    valueName = placeholder;
                    shadowType = this._makeExtensionMenuId(argInfo.menu, context.categoryInfo.id);
                    fieldName = argInfo.menu;
                } else if (typeof menuInfo.variableType !== 'undefined') {
                    const args = Object.keys(context.blockInfo.arguments);
                    const blockText = context.blockInfo.text.toString();
                    const isVariableGetter = args.length === 1 && blockText.length === args[0].length + 2;
                    if (isVariableGetter) {
                        context.blockJSON.extensions ??= [];
                        context.blockJSON.extensions.push('contextMenu_getVariableBlockAnyType');
                    }
                    argJSON.type = isVariableGetter
                        ? 'field_variable_getter'
                        : 'field_variable';
                    argJSON.variableTypes = [menuInfo.variableType === 'scalar'
                        ? Variable.SCALAR_TYPE
                        : menuInfo.variableType];
                    argJSON.variableType = argJSON.variableTypes[0];
                    valueName = null;
                    shadowType = null;
                    fieldName = placeholder;
                    variableType = menuInfo.variableType === 'scalar'
                        ? Variable.SCALAR_TYPE
                        : menuInfo.variableType
                    const defaultVar = argInfo.defaultValue ?? [];
                    variableID = defaultVar[0];
                    variableName = defaultVar[1];
                } else {
                    argJSON.type = 'field_dropdown';
                    argJSON.options = this._convertMenuItems(menuInfo.items);
                    valueName = null;
                    shadowType = null;
                    fieldName = placeholder;
                }
            } else {
                valueName = placeholder;
                shadowType = (argTypeInfo.shadow && argTypeInfo.shadow.type) || null;
                fieldName = (argTypeInfo.shadow && argTypeInfo.shadow.fieldName) || null;
            }
            // TODO: Allow fillIn to work with non-shadow.
            if (argInfo.fillIn || argInfo.fillInGlobal/* && argInfo.fillInShadow*/) {
                shadowType = argInfo.fillInGlobal || `${context.categoryInfo.id}_${argInfo.fillIn}`;
            }/* else if (argInfo.fillIn) {
                blockType = `${context.categoryInfo.id}_${argInfo.fillIn}`;
            }*/

            // <value> is the ScratchBlocks name for a block input.
            if (valueName) {
                context.inputList.push(`<value name="${xmlEscape.escapeAttribute(placeholder)}">`);
            }

            // The <shadow> is a placeholder for a reporter and is visible when there's no reporter in this input.
            // Boolean inputs don't need to specify a shadow in the XML.
            if (shadowType) {
                context.inputList.push(`<shadow type="${xmlEscape.escapeAttribute(shadowType)}">`);
            }

            // TODO: This doesnt seem to work properly with fillIn. Default to shadow for now.
            if (blockType) {
                context.inputList.push(`<block type="${xmlEscape.escapeAttribute(blockType)}">`);
            }

            if (shadowType === 'polygon') {
                // eslint-disable-next-line max-len
                context.inputList.push(`<mutation expanded="false" points="${argInfo.nodes}" color="${context.blockJSON.colour}" midle="[0,0]" scale="${argInfo.defaultSize || 30}"/>`);
            }

            // A <field> displays a dynamic value: a user-editable text field, a drop-down menu, etc.
            // Leave out the field if defaultValue or fieldName are not specified
            if (fieldName && !variableID) {
                if ((defaultValue) || ((argInfo.type === "string") && (!argInfo.menu))) {
                    context.inputList.push(`<field name="${fieldName}">${defaultValue}</field>`);
                }
            }

            if (variableID) {
                // eslint-disable-next-line max-len
                context.inputList.push(`<field name="${fieldName}" id="${variableID}" variableType="${variableType}">${variableName}</field>`);
            }

            if (blockType) {
                context.inputList.push('</block>');
            }

            if (shadowType) {
                context.inputList.push('</shadow>');
            }

            if (valueName) {
                context.inputList.push('</value>');
            }
        }

        const argsName = `args${context.outLineNum}`;
        const blockArgs = (context.blockJSON[argsName] = context.blockJSON[argsName] || []);
        if (argJSON) blockArgs.push(argJSON);
        const argNum = blockArgs.length;
        context.argsMap[placeholder] = argNum;

        return `%${argNum}`;
    }

    /**
     * @returns {Array.<object>} scratch-blocks XML for each category of extension blocks, in category order.
     * @param {?Target} [target] - the active editing target (optional)
     * @property {string} id - the category / extension ID
     * @property {string} xml - the XML text for this category, starting with `<category>` and ending with `</category>`
     */
    getBlocksXML (target) {
        return this._blockInfo.map(categoryInfo => {
            const {name, color1, color2} = categoryInfo;
            let orderBlocks = categoryInfo.orderBlocks;
            // Filter out blocks that aren't supposed to be shown on this target, as determined by the block info's
            // `hideFromPalette` and `filter` properties.
            const paletteBlocks = categoryInfo.blocks.filter(block => {
                let blockFilterIncludesTarget = true;
                // If an editing target is not passed, include all blocks
                // If the block info doesn't include a `filter` property, always include it
                if (target && block.info.filter) {
                    blockFilterIncludesTarget = block.info.filter.includes(
                        target.isStage ? TargetType.STAGE : TargetType.SPRITE
                    );
                }
                // If the block info's `hideFromPalette` is true, then filter out this block
                return blockFilterIncludesTarget && !block.info.hideFromPalette;
            });

            orderBlocks = orderBlocks
                ? orderBlocks
                : blocks => blocks;

            const colorXML = `colour="${xmlEscape(color1)}" secondaryColour="${xmlEscape(color2)}"`;

            // Use a menu icon if there is one. Otherwise, use the block icon. If there's no icon,
            // the category menu will show its default colored circle.
            let menuIconURI = '';
            if (categoryInfo.menuIconURI) {
                menuIconURI = categoryInfo.menuIconURI;
            } else if (categoryInfo.blockIconURI) {
                menuIconURI = categoryInfo.blockIconURI;
            }
            const menuIconXML = menuIconURI ?
                `iconURI="${xmlEscape(menuIconURI)}"` : '';

            let statusButtonXML = '';
            if (categoryInfo.showStatusButton) {
                statusButtonXML = 'showStatusButton="true"';
            }

            let xml = `<category name="${xmlEscape.escapeAttribute(name)}"`;
            xml += ` id="${xmlEscape.escapeAttribute(categoryInfo.id)}"`;
            xml += ` options="extensionControls"`;
            xml += ` ${statusButtonXML}`;
            xml += ` ${colorXML}`;
            xml += ` ${menuIconXML}>`;
            xml += orderBlocks(paletteBlocks.map(block => block.xml)).join('');
            xml += '</category>';

            return {
                id: categoryInfo.id,
                xml
            };
        });
    }

    /**
     * @returns {Array.<string>} - an array containing the scratch-blocks JSON information for each dynamic block.
     */
    getBlocksJSON () {
        return this._blockInfo.reduce(
            (result, categoryInfo) => result.concat(categoryInfo.blocks.map(blockInfo => blockInfo.json)), []);
    }

    /**
     * Get a scratch link socket.
     * @param {string} type Either BLE or BT
     * @returns {ScratchLinkSocket} The scratch link socket.
     */
    getScratchLinkSocket (type) {
        const factory = this._linkSocketFactory || this._defaultScratchLinkSocketFactory;
        return factory(type);
    }

    /**
     * Configure how ScratchLink sockets are created. Factory must consume a "type" parameter
     * either BT or BLE.
     * @param {Function} factory The new factory for creating ScratchLink sockets.
     */
    configureScratchLinkSocketFactory (factory) {
        this._linkSocketFactory = factory;
    }

    /**
     * The default scratch link socket creator, using websockets to the installed device manager.
     * @param {string} type Either BLE or BT
     * @returns {ScratchLinkSocket} The new scratch link socket (a WebSocket object)
     */
    _defaultScratchLinkSocketFactory (type) {
        return new ScratchLinkWebSocket(type);
    }

    /**
     * Register an extension that communications with a hardware peripheral by id,
     * to have access to it and its peripheral functions in the future.
     * @param {string} extensionId - the id of the extension.
     * @param {object} extension - the extension to register.
     */
    registerPeripheralExtension (extensionId, extension) {
        this.peripheralExtensions[extensionId] = extension;
    }

    /**
     * Tell the specified extension to scan for a peripheral.
     * @param {string} extensionId - the id of the extension.
     */
    scanForPeripheral (extensionId) {
        if (this.peripheralExtensions[extensionId]) {
            this.peripheralExtensions[extensionId].scan();
        }
    }

    /**
     * Connect to the extension's specified peripheral.
     * @param {string} extensionId - the id of the extension.
     * @param {number} peripheralId - the id of the peripheral.
     */
    connectPeripheral (extensionId, peripheralId) {
        if (this.peripheralExtensions[extensionId]) {
            this.peripheralExtensions[extensionId].connect(peripheralId);
        }
    }

    /**
     * Disconnect from the extension's connected peripheral.
     * @param {string} extensionId - the id of the extension.
     */
    disconnectPeripheral (extensionId) {
        if (this.peripheralExtensions[extensionId]) {
            this.peripheralExtensions[extensionId].disconnect();
        }
    }

    /**
     * Returns whether the extension has a currently connected peripheral.
     * @param {string} extensionId - the id of the extension.
     * @return {boolean} - whether the extension has a connected peripheral.
     */
    getPeripheralIsConnected (extensionId) {
        let isConnected = false;
        if (this.peripheralExtensions[extensionId]) {
            isConnected = this.peripheralExtensions[extensionId].isConnected();
        }
        return isConnected;
    }

    /**
     * Emit an event to indicate that the microphone is being used to stream audio.
     * @param {boolean} listening - true if the microphone is currently listening.
     */
    emitMicListening (listening) {
        this.emit(Runtime.MIC_LISTENING, listening);
    }

    /**
     * Retrieve the function associated with the given opcode.
     * @param {!string} opcode The opcode to look up.
     * @return {Function} The function which implements the opcode.
     */
    getOpcodeFunction (opcode) {
        return this._primitives[opcode];
    }

    /**
     * Return whether an opcode represents a hat block.
     * @param {!string} opcode The opcode to look up.
     * @return {boolean} True if the op is known to be a hat.
     */
    getIsHat (opcode) {
        return this._hats.hasOwnProperty(opcode);
    }

    /**
     * Return whether an opcode represents an edge-activated hat block.
     * @param {!string} opcode The opcode to look up.
     * @return {boolean} True if the op is known to be a edge-activated hat.
     */
    getIsEdgeActivatedHat (opcode) {
        return this._hats.hasOwnProperty(opcode) &&
            this._hats[opcode].edgeActivated;
    }


    /**
     * Attach the audio engine
     * @param {!AudioEngine} audioEngine The audio engine to attach
     */
    attachAudioEngine (audioEngine) {
        this.audioEngine = audioEngine;
    }

    /**
     * Attach the renderer
     * @param {!RenderWebGL} renderer The renderer to attach
     */
    attachRenderer (renderer) {
        this.renderer = renderer;
        this.renderer.setLayerGroupOrdering(StageLayering.LAYER_GROUPS);
        this.renderer.offscreenTouching = !this.runtimeOptions.fencing;
        this.renderer.renderOffscreen = this.runtimeOptions.disableOffscreenRendering;
        this.updatePrivacy();
    }

    /**
     * registers a custom serializer to allow saving custom data into standard variables
     * @param {Function} validate validates if a given chunk of data is correctly for this serializer
     * @param {Function} serialize the function to be ran on serialized data in variables.
     * @param {Function} deserialize the function to be ran on serialized data in variables
     */
    registerSerializer (id, serialize, deserialize) {
        if (typeof serialize !== 'function') {
            throw new TypeError('serialize must be of type function');
        }
        if (typeof deserialize !== 'function') {
            throw new TypeError('deserialize must be of type function');
        }
        this.serializers[id] = {
            serialize,
            deserialize
        };
    }

    /**
     * Register a variable type
     * @param {string} type the type name of this variable
     * @param {ObjectConstructor} varClass the class to handle the data this variable contains
     */
    registerVariable (type, varClass) {
        this._extensionVariables[type] = varClass;
    }

    /**
     * Remove a variable type
     * @param {string} type the type name of this variable
     */
    unregisterVariable (type) {
        const variable = this._extensionVariables[type];
        if (!variable) throw new Error(`can not remove a variable type that does not exist. removing ${type}`);
        delete this._extensionVariables[type];
    }

    /**
     * create a new variable instance
     * @param {string} type variable type
     * @param  {...any} args the arguments to pass down to the variable
     * @returns {Variable|InstanceType} the new variable instance
     */
    newVariableInstance (type, ...args) {
        if (coreVariableTypes.includes(type)) {
            args.splice(2, 0, type);
            return new Variable(...args);
        }
        const variable = this._extensionVariables[type];
        // return a fake variable, this is because of loading variables from the sb3 parser
        if (!variable) return {
            type,
            value: args,
            mustRecreate: true
        };
        return new variable(this, ...args);
    }

    /**
     * Set the bitmap adapter for the VM/runtime, which converts scratch 2
     * bitmaps to scratch 3 bitmaps. (Scratch 3 bitmaps are all bitmap resolution 2)
     * @param {!function} bitmapAdapter The adapter to attach
     */
    attachV2BitmapAdapter (bitmapAdapter) {
        this.v2BitmapAdapter = bitmapAdapter;
    }

    /**
     * Attach the storage module
     * @param {!ScratchStorage} storage The storage module to attach
     */
    attachStorage (storage) {
        this.storage = storage;

        if (this.isPackaged) {
            // In packaged runtime mode, generating real asset IDs is a waste of time.
            // We do still want to preserve every asset having a unique ID.
            const originalCreateAsset = storage.createAsset;
            let assetIdCounter = 0;
            // eslint-disable-next-line no-unused-vars
            storage.createAsset = function packagedCreateAsset (assetType, dataFormat, data, assetId, generateId) {
                if (!assetId) {
                    assetId = (++assetIdCounter).toString();
                }
                return originalCreateAsset.call(
                    this,
                    assetType,
                    dataFormat,
                    data,
                    assetId,
                    // Never generate real asset ID
                    false
                );
            };
        }
    }

    // -----------------------------------------------------------------------------
    // -----------------------------------------------------------------------------

    /**
     * Create a thread and push it to the list of threads.
     * @param {!string} id ID of block that starts the stack.
     * @param {!Target} target Target to run thread on.
     * @param {?object} opts optional arguments
     * @param {?boolean} opts.stackClick true if the script was activated by clicking on the stack
     * @param {?boolean} opts.updateMonitor true if the script should update a monitor value
     * @param {?Target} opts.targetBlockLocation where the blocks are located
     * @return {!Thread} The newly created thread.
     */
    _pushThread (id, target, opts) {
        const thread = new Thread(id);
        thread.target = target;
        thread.stackClick = Boolean(opts && opts.stackClick);
        thread.updateMonitor = Boolean(opts && opts.updateMonitor);
        thread.blockContainer = thread.updateMonitor ?
            this.monitorBlocks :
            ((opts && opts.targetBlockLocation) || target.blocks);

        thread.pushStack(id);
        this.threads.push(thread);
        if (!thread.stackClick && !thread.updateMonitor) {
            this.threadMap.set(thread.getId(), thread);
        }

        // tw: compile new threads. Do not attempt to compile monitor threads.
        if (!(opts && opts.updateMonitor) && this.compilerOptions.enabled) {
            thread.tryCompile();
        }

        return thread;
    }

    /**
     * Stop a thread: stop running it immediately, and remove it from the thread list later.
     * @param {!Thread} thread Thread object to remove from actives
     */
    _stopThread (thread) {
        // Mark the thread for later removal
        thread.isKilled = true;
        // Inform sequencer to stop executing that thread.
        this.sequencer.retireThread(thread);
    }

    /**
     * Restart a thread in place, maintaining its position in the list of threads.
     * This is used by `startHats` to and is necessary to ensure 2.0-like execution order.
     * Test project: https://scratch.mit.edu/projects/130183108/
     * @param {!Thread} thread Thread object to restart.
     * @return {Thread} The restarted thread.
     */
    _restartThread (thread) {
        const newThread = new Thread(thread.topBlock);
        newThread.target = thread.target;
        newThread.stackClick = thread.stackClick;
        newThread.updateMonitor = thread.updateMonitor;
        newThread.blockContainer = thread.blockContainer;
        newThread.pushStack(thread.topBlock);
        // tw: when a thread is restarted, we have to check whether the previous script was attempted to be compiled.
        if (thread.triedToCompile && this.compilerOptions.enabled) {
            newThread.tryCompile();
        }
        if (!newThread.stackClick && !newThread.updateMonitor) {
            this.threadMap.set(newThread.getId(), newThread);
        }
        const i = this.threads.indexOf(thread);
        if (i > -1) {
            this.threads[i] = newThread;
            return newThread;
        }
        this.threads.push(thread);
        return thread;
    }

    emitCompileError (target, error) {
        this.emit(Runtime.COMPILE_ERROR, target, error);
    }

    /**
     * Return whether a thread is currently active/running.
     * @param {?Thread} thread Thread object to check.
     * @return {boolean} True if the thread is active/running.
     */
    isActiveThread (thread) {
        return (
            (
                thread.stack.length > 0 &&
                thread.status !== Thread.STATUS_DONE) &&
            this.threads.indexOf(thread) > -1);
    }

    /**
     * Return whether a thread is waiting for more information or done.
     * @param {?Thread} thread Thread object to check.
     * @return {boolean} True if the thread is waiting
     */
    isWaitingThread (thread) {
        return (
            thread.status === Thread.STATUS_PROMISE_WAIT ||
            thread.status === Thread.STATUS_YIELD_TICK ||
            !this.isActiveThread(thread)
        );
    }

    /**
     * Toggle a script.
     * @param {!string} topBlockId ID of block that starts the script.
     * @param {?object} opts optional arguments to toggle script
     * @param {?string} opts.target target ID for target to run script on. If not supplied, uses editing target.
     * @param {?boolean} opts.stackClick true if the user activated the stack by clicking, false if not. This
     *     determines whether we show a visual report when turning on the script.
     */
    toggleScript (topBlockId, opts) {
        opts = Object.assign({
            target: this._editingTarget,
            stackClick: false
        }, opts);
        // Remove any existing thread.
        for (let i = 0; i < this.threads.length; i++) {
            // Toggling a script that's already running turns it off
            if (this.threads[i].topBlock === topBlockId && this.threads[i].status !== Thread.STATUS_DONE) {
                const blockContainer = opts.target.blocks;
                const opcode = blockContainer.getOpcode(blockContainer.getBlock(topBlockId));

                if (this.getIsEdgeActivatedHat(opcode) && this.threads[i].stackClick !== opts.stackClick) {
                    // Allow edge activated hat thread stack click to coexist with
                    // edge activated hat thread that runs every frame
                    continue;
                }
                this._stopThread(this.threads[i]);
                return;
            }
        }
        // Otherwise add it.
        this._pushThread(topBlockId, opts.target, opts);
    }

    /**
     * Enqueue a script that when finished will update the monitor for the block.
     * @param {!string} topBlockId ID of block that starts the script.
     * @param {?Target} optTarget target Target to run script on. If not supplied, uses editing target.
     */
    addMonitorScript (topBlockId, optTarget) {
        if (!optTarget) optTarget = this._editingTarget;
        for (let i = 0; i < this.threads.length; i++) {
            // Don't re-add the script if it's already running
            if (this.threads[i].topBlock === topBlockId && this.threads[i].status !== Thread.STATUS_DONE &&
                    this.threads[i].updateMonitor) {
                return;
            }
        }
        // Otherwise add it.
        this._pushThread(topBlockId, optTarget, {updateMonitor: true});
    }

    /**
     * Run a function `f` for all scripts in a workspace.
     * `f` will be called with two parameters:
     *  - the top block ID of the script.
     *  - the target that owns the script.
     * @param {!Function} f Function to call for each script.
     * @param {Target=} optTarget Optionally, a target to restrict to.
     */
    allScriptsDo (f, optTarget) {
        let targets = this.executableTargets;
        if (optTarget) {
            targets = [optTarget];
        }
        for (let t = targets.length - 1; t >= 0; t--) {
            const target = targets[t];
            const scripts = target.blocks.getScripts();
            for (let j = 0; j < scripts.length; j++) {
                const topBlockId = scripts[j];
                f(topBlockId, target);
            }
        }
    }

    allScriptsByOpcodeDo (opcode, f, optTarget) {
        let targets = this.executableTargets;
        if (optTarget) {
            targets = [optTarget];
        }
        for (let t = targets.length - 1; t >= 0; t--) {
            const target = targets[t];
            const scripts = BlocksRuntimeCache.getScripts(target.blocks, opcode);
            for (let j = 0; j < scripts.length; j++) {
                f(scripts[j], target);
            }
        }
    }

    /**
     * Start all relevant hats.
     * @param {!string} requestedHatOpcode Opcode of hats to start.
     * @param {object=} optMatchFields Optionally, fields to match on the hat.
     * @param {Target=} optTarget Optionally, a target to restrict to.
     * @return {Array.<Thread>} List of threads started by this function.
     */
    startHats (requestedHatOpcode, optMatchFields, optTarget) {
        if (!this._hats.hasOwnProperty(requestedHatOpcode)) {
            // No known hat with this opcode.
            return;
        }
        const instance = this;
        const newThreads = [];
        // Look up metadata for the relevant hat.
        const hatMeta = instance._hats[requestedHatOpcode];

        for (const opts in optMatchFields) {
            if (!optMatchFields.hasOwnProperty(opts)) continue;
            optMatchFields[opts] = Cast.toString(optMatchFields[opts]).toUpperCase();
        }

        // tw: By assuming that all new threads will not interfere with eachother, we can optimize the loops
        // inside the allScriptsByOpcodeDo callback below.
        const startingThreadListLength = this.threads.length;

        // Consider all scripts, looking for hats with opcode `requestedHatOpcode`.
        this.allScriptsByOpcodeDo(requestedHatOpcode, (script, target) => {
            const {
                blockId: topBlockId,
                fieldsOfInputs: hatFields
            } = script;

            // Match any requested fields.
            // For example: ensures that broadcasts match.
            // This needs to happen before the block is evaluated
            // (i.e., before the predicate can be run) because "broadcast and wait"
            // needs to have a precise collection of started threads.
            for (const matchField in optMatchFields) {
                if (hatFields[matchField].value !== optMatchFields[matchField]) {
                    // Field mismatch.
                    return;
                }
            }

            if (hatMeta.restartExistingThreads) {
                // If `restartExistingThreads` is true, we should stop
                // any existing threads starting with the top block.
                const existingThread = this.threadMap.get(Thread.getIdFromTargetAndBlock(target, topBlockId));
                if (existingThread) {
                    newThreads.push(this._restartThread(existingThread));
                    return;
                }
            } else {
                // If `restartExistingThreads` is false, we should
                // give up if any threads with the top block are running.
                for (let j = 0; j < startingThreadListLength; j++) {
                    if (this.threads[j].target === target &&
                        this.threads[j].topBlock === topBlockId &&
                        // stack click threads and hat threads can coexist
                        !this.threads[j].stackClick &&
                        this.threads[j].status !== Thread.STATUS_DONE) {
                        // Some thread is already running.
                        return;
                    }
                }
            }
            // Start the thread with this top block.
            newThreads.push(this._pushThread(topBlockId, target));
        }, optTarget);
        // For compatibility with Scratch 2, edge triggered hats need to be processed before
        // threads are stepped. See ScratchRuntime.as for original implementation
        newThreads.forEach(thread => {
            // just incase, pause any new threads that appear while we are paused
            if (this.paused) thread.pause();
            if (thread.isCompiled) {
                if (thread.executableHat) {
                    // It is quite likely that we are currently executing a block, so make sure
                    // that we leave the compiler's state intact at the end.
                    compilerExecute.saveGlobalState();
                    compilerExecute(thread);
                    compilerExecute.restoreGlobalState();
                }
            } else {
                execute(this.sequencer, thread);
                thread.goToNextBlock();
            }
        });
        this.emit(Runtime.HATS_STARTED, requestedHatOpcode, optMatchFields, optTarget, newThreads);
        return newThreads;
    }


    /**
     * Dispose all targets. Return to clean state.
     */
    dispose () {
        this.stopAll();
        // Deleting each target's variable's monitors.
        this.targets.forEach(target => {
            if (target.isOriginal) target.deleteMonitors();
        });

        this.targets.map(this.disposeTarget, this);
        // tw: explicitly emit a MONITORS_UPDATE instead of relying on implicit behavior of _step()
        const emptyMonitorState = OrderedMap({});
        if (!emptyMonitorState.equals(this._monitorState)) {
            this._monitorState = emptyMonitorState;
            this.emit(Runtime.MONITORS_UPDATE, this._monitorState);
        }
        this.emit(Runtime.RUNTIME_DISPOSED);
        this.ioDevices.clock.resetProjectTimer();
        this.fontManager.clear();
        // @todo clear out extensions? turboMode? etc.

        // *********** Cloud *******************

        // If the runtime currently has cloud data,
        // emit a has cloud data update event resetting
        // it to false
        if (this.hasCloudData()) {
            this.emit(Runtime.HAS_CLOUD_DATA_UPDATE, false);
        }

        this.ioDevices.cloud.clear();

        // Reset runtime cloud data info
        const newCloudDataManager = cloudDataManager(this.cloudOptions);
        this.hasCloudData = newCloudDataManager.hasCloudVariables;
        this.canAddCloudVariable = newCloudDataManager.canAddCloudVariable;
        this.getNumberOfCloudVariables = newCloudDataManager.getNumberOfCloudVariables;
        this.addCloudVariable = this._initializeAddCloudVariable(newCloudDataManager);
        this.removeCloudVariable = this._initializeRemoveCloudVariable(newCloudDataManager);
    }

    /**
     * Add a target to the runtime. This tracks the sprite pane
     * ordering of the target. The target still needs to be put
     * into the correct execution order after calling this function.
     * @param {Target} target target to add
     */
    addTarget (target) {
        for (const varId in target.variables) {
            const variable = target.variables[varId];
            if (variable.mustRecreate) {
                // variable must be fully created now as we couldnt earlier
                const newVar = this.newVariableInstance(variable.type, ...variable.value);
                // variable type doesnt exist, remove variable entirely
                if (newVar.mustRecreate) {
                    delete target.variables[varId];
                    continue;
                }
                target.variables[varId] = newVar;
            }
        }
        this.targets.push(target);
        this.executableTargets.push(target);
        if (target.isStage && !this._stageTarget) {
            this._stageTarget = target;
        }
    }

    /**
     * Move a target in the execution order by a relative amount.
     *
     * A positve number will make the target execute earlier. A negative number
     * will make the target execute later in the order.
     *
     * @param {Target} executableTarget target to move
     * @param {number} delta number of positions to move target by
     * @returns {number} new position in execution order
     */
    moveExecutable (executableTarget, delta) {
        const oldIndex = this.executableTargets.indexOf(executableTarget);
        this.executableTargets.splice(oldIndex, 1);
        let newIndex = oldIndex + delta;
        if (newIndex > this.executableTargets.length) {
            newIndex = this.executableTargets.length;
        }
        if (newIndex <= 0) {
            if (this.executableTargets.length > 0 && this.executableTargets[0].isStage) {
                newIndex = 1;
            } else {
                newIndex = 0;
            }
        }
        this.executableTargets.splice(newIndex, 0, executableTarget);
        return newIndex;
    }

    /**
     * Set a target to execute at a specific position in the execution order.
     *
     * Infinity will set the target to execute first. 0 will set the target to
     * execute last (before the stage).
     *
     * @param {Target} executableTarget target to move
     * @param {number} newIndex position in execution order to place the target
     * @returns {number} new position in the execution order
     */
    setExecutablePosition (executableTarget, newIndex) {
        const oldIndex = this.executableTargets.indexOf(executableTarget);
        return this.moveExecutable(executableTarget, newIndex - oldIndex);
    }

    /**
     * Remove a target from the execution set.
     * @param {Target} executableTarget target to remove
     */
    removeExecutable (executableTarget) {
        const oldIndex = this.executableTargets.indexOf(executableTarget);
        if (oldIndex > -1) {
            this.executableTargets.splice(oldIndex, 1);
        }
    }

    /**
     * Dispose of a target.
     * @param {!Target} disposingTarget Target to dispose of.
     */
    disposeTarget (disposingTarget) {
        this.targets = this.targets.filter(target => {
            if (disposingTarget !== target) return true;
            // Allow target to do dispose actions.
            target.dispose();
            // Remove from list of targets.
            return false;
        });
        if (this._stageTarget === disposingTarget) {
            this._stageTarget = null;
        }
    }

    /**
     * Stop any threads acting on the target.
     * @param {!Target} target Target to stop threads for.
     * @param {Thread=} optThreadException Optional thread to skip.
     */
    stopForTarget (target, optThreadException) {
        // Emit stop event to allow blocks to clean up any state.
        this.emit(Runtime.STOP_FOR_TARGET, target, optThreadException);

        // Stop any threads on the target.
        for (let i = 0; i < this.threads.length; i++) {
            if (this.threads[i] === optThreadException) {
                continue;
            }
            if (this.threads[i].target === target) {
                this._stopThread(this.threads[i]);
            }
        }
    }

    /**
     * Start all threads that start with the green flag.
     */
    greenFlag() {
        this.emit(Runtime.PROJECT_START_BEFORE_RESET);
        this.stopAll();
        this.emit(Runtime.PROJECT_START);
        this.updateCurrentMSecs();
        this.ioDevices.clock.resetProjectTimer();
        this.targets.forEach(target => target.clearEdgeActivatedValues());
        // Inform all targets of the green flag.
        for (let i = 0; i < this.targets.length; i++) {
            this.targets[i].onGreenFlag();
        }
        this.startHats('event_whenflagclicked');
    }

    /**
     * Pause running scripts
     */
    pause() {
        if (this.paused) return;
        this.emit(Runtime.RUNTIME_PRE_PAUSED);
        this.paused = true;
        // pause all audio contexts (that includes exts with their own AC or gain node)
        this.audioEngine.audioContext.suspend();
        for (const audioData of this._extensionAudioObjects.values()) {
            if (audioData.audioContext) {
                audioData.audioContext.suspend();
            }
        }

        this.ioDevices.clock.pause();
        for (const thread of this.threads) {
            thread.pause();
        }
        this.emit(Runtime.RUNTIME_PAUSED);
    }

    /**
     * Unpause running scripts
     */
    play() {
        if (!this.paused) return;
        this.paused = false;
        // resume all audio contexts (that includes exts with their own AC or gain node)
        this.audioEngine.audioContext.resume();
        for (const audioData of this._extensionAudioObjects.values()) {
            if (audioData.audioContext) {
                audioData.audioContext.resume();
            }
        }

        this.ioDevices.clock.resume();
        for (const thread of this.threads) {
            thread.play();
        }
        this.emit(Runtime.RUNTIME_UNPAUSED);
    }

    /**
     * Stop "everything."
     */
    stopAll () {
        // unpause everything before we destroy all the threads
        this.play();
        // Emit stop event to allow blocks to clean up any state.
        this.emit(Runtime.PROJECT_STOP_ALL);
        // clear runtime variables
        this.variables = Object.create(null);

        // Dispose all clones.
        const newTargets = [];
        for (let i = 0; i < this.targets.length; i++) {
            this.targets[i].onStopAll();
            if (this.targets[i].hasOwnProperty('isOriginal') &&
                !this.targets[i].isOriginal) {
                this.targets[i].dispose();
            } else {
                newTargets.push(this.targets[i]);
            }
        }
        this.targets = newTargets;
        // Dispose of the active thread.
        if (this.sequencer.activeThread !== null) {
            this._stopThread(this.sequencer.activeThread);
        }
        // Remove all remaining threads from executing in the next tick.
        this.threads.forEach(v => v.status !== Thread.STATUS_DONE && this.emit(Runtime.THREAD_FINISHED, v))
        this.threads = [];
        this.threadMap.clear();
    }

    _renderInterpolatedPositions () {
        const frameStarted = this._lastStepTime;
        const now = Date.now();
        const timeSinceStart = now - frameStarted;
        const progressInFrame = Math.min(1, Math.max(0, timeSinceStart / this.currentStepTime));

        interpolate.interpolate(this, progressInFrame);

        if (this.renderer) {
            this.renderer.draw();
        }
    }

    updateThreadMap () {
        this.threadMap.clear();
        for (const thread of this.threads) {
            if (!thread.stackClick && !thread.updateMonitor) {
                this.threadMap.set(thread.getId(), thread);
            }
        }
    }

    /**
     * Repeatedly run `sequencer.stepThreads` and filter out
     * inactive threads after each iteration.
     */
    _step () {
        // pm: RUNTIME_STEP_START runs before BEFORE_EXECUTE
        // this runs before any processing of this new step
        this.frameLoop._stepCounter++;
        this.emit(Runtime.RUNTIME_STEP_START);

        if (this.interpolationEnabled) {
            interpolate.setupInitialState(this);
        }

        if (this.profiler !== null) {
            if (stepProfilerId === -1) {
                stepProfilerId = this.profiler.idByName('Runtime._step');
            }
            this.profiler.start(stepProfilerId);
        }

        // Clean up threads that were told to stop during or since the last step
        this.threads = this.threads.filter(thread => !thread.isKilled);
        this.updateThreadMap();

        // Find all edge-activated hats, and add them to threads to be evaluated.
        for (const hatType in this._hats) {
            if (!this._hats.hasOwnProperty(hatType)) continue;
            const hat = this._hats[hatType];
            if (hat.edgeActivated) {
                this.startHats(hatType);
            }
        }
        this.redrawRequested = false;
        this._pushMonitors();
        if (this.profiler !== null) {
            if (stepThreadsProfilerId === -1) {
                stepThreadsProfilerId = this.profiler.idByName('Sequencer.stepThreads');
            }
            this.profiler.start(stepThreadsProfilerId);
        }
        this.emit(Runtime.BEFORE_EXECUTE);
        const doneThreads = this.sequencer.stepThreads();
        if (this.profiler !== null) {
            this.profiler.stop();
        }
        this.emit(Runtime.AFTER_EXECUTE);
        this._updateGlows(doneThreads);
        // Add done threads so that even if a thread finishes within 1 frame, the green
        // flag will still indicate that a script ran.
        this._emitProjectRunStatus(
            this.threads.length + doneThreads.length -
                this._getMonitorThreadCount([...this.threads, ...doneThreads]));
        // Store threads that completed this iteration for testing and other
        // internal purposes.
        this._lastStepDoneThreads = doneThreads;
        if (this.renderer) {
            // @todo: Only render when this.redrawRequested or clones rendered.
            if (this.profiler !== null) {
                if (rendererDrawProfilerId === -1) {
                    rendererDrawProfilerId = this.profiler.idByName('RenderWebGL.draw');
                }
                this.profiler.start(rendererDrawProfilerId);
            }
            // tw: do not draw if document is hidden or a rAF loop is running
            // Checking for the animation frame loop is more reliable than using
            // interpolationEnabled in some edge cases
            if (!document.hidden && !this.frameLoop._interpolationAnimation) {
                this.renderer.draw();
            }
            if (this.profiler !== null) {
                this.profiler.stop();
            }
        }

        if (this._refreshTargets) {
            this.emit(Runtime.TARGETS_UPDATE, false /* Don't emit project changed */);
            this._refreshTargets = false;
        }

        let forceUpd = false;
        // if a custom type set _monitorUpToDate to false on an existing instance, we need to report that update to the gui
        if (this._monitorState.some(item =>
            typeof item.get('value') === 'object' &&
            '_monitorUpToDate' in item.get('value') &&
            !item.get('value')._monitorUpToDate
        )) {
            const old = this._monitorState;
            // make a new instance so redux detects this as different later on
            this._monitorState = this._monitorState.toOrderedMap();
            if (!(old !== this._monitorState)) // why wont redux just accept the fucking value
                throw new Error('Expected OrderedMap.toOrderedMap() to produce a truly unique value');
            forceUpd = true;
        }
        if (!this._prevMonitorState.equals(this._monitorState) || forceUpd) {
            this.emit(Runtime.MONITORS_UPDATE, this._monitorState);
            this._prevMonitorState = this._monitorState;
        }

        if (this.profiler !== null) {
            this.profiler.stop();
            this.profiler.reportFrames();
        }

        if (this.interpolationEnabled) {
            this._lastStepTime = Date.now();
        }

        // pm: RUNTIME_STEP_END runs after AFTER_EXECUTE
        this.emit(Runtime.RUNTIME_STEP_END);
    }

    /**
     * Get the number of threads in the given array that are monitor threads (threads
     * that update monitor values, and don't count as running a script).
     * @param {!Array.<Thread>} threads The set of threads to look through.
     * @return {number} The number of monitor threads in threads.
     */
    _getMonitorThreadCount (threads) {
        let count = 0;
        threads.forEach(thread => {
            if (thread.updateMonitor) count++;
        });
        return count;
    }

    /**
     * Queue monitor blocks to sequencer to be run.
     */
    _pushMonitors () {
        this.monitorBlocks.runAllMonitored(this);
    }

    /**
     * Set the current editing target known by the runtime.
     * @param {!Target} editingTarget New editing target.
     */
    setEditingTarget (editingTarget) {
        const oldEditingTarget = this._editingTarget;
        this._editingTarget = editingTarget;
        // Script glows must be cleared.
        this._scriptGlowsPreviousFrame = [];
        this._updateGlows();

        if (oldEditingTarget !== this._editingTarget) {
            this.requestToolboxExtensionsUpdate();
        }
    }

    /**
     * Set whether we are in 30 TPS compatibility mode.
     * @param {boolean} compatibilityModeOn True iff in compatibility mode.
     */
    setCompatibilityMode (compatibilityModeOn) {
        // tw: "compatibility mode" is replaced with a generic framerate setter,
        // but this method is kept for compatibility
        if (compatibilityModeOn) {
            this.setFramerate(30);
        } else {
            this.setFramerate(60);
        }
    }

    /**
     * tw: Change runtime target frames per second
     * @param {number} framerate Target frames per second
     */
    setFramerate (framerate) {
        // Setting framerate to anything greater than this is unnecessary and can break the sequencer
        // Additionally, the JS spec says intervals can't run more than once every 4ms (250/s) anyways
        if (framerate > 250) framerate = 250;
        // Convert negative framerates to 1FPS
        // Note that 0 is a special value which means "matching device screen refresh rate"
        if (framerate < 0) framerate = 1;
        this.frameLoop.setFramerate(framerate);
        this.emit(Runtime.FRAMERATE_CHANGED, framerate);
    }

    /**
     * tw: Enable or disable interpolation.
     * @param {boolean} interpolationEnabled True if interpolation should be enabled.
     */
    setInterpolation (interpolationEnabled) {
        this.interpolationEnabled = interpolationEnabled;
        this.frameLoop.setInterpolation(this.interpolationEnabled);
        this.emit(Runtime.INTERPOLATION_CHANGED, interpolationEnabled);
    }

    /**
     * tw: Update runtime options
     * @param {*} runtimeOptions New options
     */
    setRuntimeOptions (runtimeOptions) {
        this.runtimeOptions = Object.assign({}, this.runtimeOptions, runtimeOptions);
        this.emit(Runtime.RUNTIME_OPTIONS_CHANGED, this.runtimeOptions);
        if (this.renderer) {
            this.renderer.offscreenTouching = !this.runtimeOptions.fencing;
            // if these miss match then update (do full rerender as the state drastically changes output)
            if (this.runtimeOptions.disableOffscreenRendering === this.renderer.renderOffscreen) {
                this.renderer.setRenderOffscreen(!this.runtimeOptions.disableOffscreenRendering);
            }
        }
    }

    /**
     * tw: Update compiler options
     * @param {*} compilerOptions New options
     */
    setCompilerOptions (compilerOptions) {
        this.compilerOptions = Object.assign({}, this.compilerOptions, compilerOptions);
        this.resetAllCaches();
        this.emit(Runtime.COMPILER_OPTIONS_CHANGED, this.compilerOptions);
    }

    /**
     * Change width and height of stage. This will also inform the renderer of the new stage size.
     * @param {number} width New stage width
     * @param {number} height New stage height
     */
    setStageSize (width, height) {
        width = Math.round(Math.max(1, width));
        height = Math.round(Math.max(1, height));
        if (this.stageWidth !== width || this.stageHeight !== height) {
            const deltaX = width - this.stageWidth;
            const deltaY = height - this.stageHeight;
            // Preserve monitor location relative to the center of the stage
            if (this._monitorState.size > 0) {
                const offsetX = deltaX / 2;
                const offsetY = deltaY / 2;
                for (const monitor of this._monitorState.valueSeq()) {
                    const newMonitor = monitor
                        .set('x', monitor.get('x') + offsetX)
                        .set('y', monitor.get('y') + offsetY);
                    this.requestUpdateMonitor(newMonitor);
                }
                this.emit(Runtime.MONITORS_UPDATE, this._monitorState);
            }

            this.stageWidth = width;
            this.stageHeight = height;
            if (this.renderer) {
                this.renderer.setStageSize(
                    -width / 2,
                    width / 2,
                    -height / 2,
                    height / 2
                );
            }
            this.emit(Runtime.STAGE_SIZE_CHANGED, width, height);
        }
    }

    // eslint-disable-next-line no-unused-vars
    setInEditor (inEditor) {
        // no-op
    }

    /**
     * TW: Enable "packaged runtime" mode. This is a one-way operation.
     */
    convertToPackagedRuntime () {
        if (this.storage) {
            throw new Error('convertToPackagedRuntime must be called before attachStorage');
        }

        this.isPackaged = true;
    }

    /**
     * tw: Reset the cache of all block containers.
     */
    resetAllCaches () {
        for (const target of this.targets) {
            if (target.isOriginal) {
                target.blocks.resetCache();
            }
        }
        this.flyoutBlocks.resetCache();
        this.monitorBlocks.resetCache();
    }

    /**
     * Add an "addon block"
     * @param {object} options Options object
     * @param {string} options.procedureCode The ID of the block
     * @param {function} options.callback The callback, called with (args, BlockUtility). May return a promise.
     * @param {string[]} options.arguments Names of the arguments accepted
     * @param {boolean} [hidden] True to not include this block in the block palette
     */
    addAddonBlock (options) {
        const procedureCode = options.procedureCode;
        const names = options.arguments;
        const ids = options.arguments.map((_, i) => `arg${i}`);
        const defaults = options.arguments.map(() => '');
        this.addonBlocks[procedureCode] = {
            namesIdsDefaults: [names, ids, defaults],
            ...options
        };

        if (!options.hidden) {
            const ID = 'a-b';
            let blockInfo = this._blockInfo.find(i => i.id === ID);
            if (!blockInfo) {
                // eslint-disable-next-line max-len
                const ICON = '<svg xmlns="http://www.w3.org/2000/svg" width="48" height="48"><path d="M14.92 1.053A13.835 13.835 0 0 0 1.052 14.919v18.162a13.835 13.835 0 0 0 13.866 13.866h18.162a13.835 13.835 0 0 0 13.866-13.866V14.919A13.835 13.835 0 0 0 33.081 1.053zm16.6 12.746L41.72 24 31.52 34.201l-3.276-3.275L35.17 24l-6.926-6.926Zm-15.116.073 3.278 3.278L12.83 24l6.926 6.926L16.48 34.2 6.28 24Z" style="fill:#29beb8;fill-opacity:1;stroke:none;stroke-width:1.51371;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1"/></svg>';
                blockInfo = {
                    id: ID,
                    name: 'Addons',
                    color1: '#29beb8',
                    color2: '#3aa8a4',
                    color3: '#3aa8a4',
                    menuIconURI: `data:image/svg+xml;,${encodeURIComponent(ICON)}`,
                    blocks: [],
                    customFieldTypes: {},
                    menus: []
                };
                this._blockInfo.unshift(blockInfo);
            }
            blockInfo.blocks.push({
                info: {},
                xml:
                   '<block type="procedures_call" gap="16"><mutation generateshadows="true" warp="false"' +
                    ` proccode="${xmlEscape(procedureCode)}"` +
                    ` argumentnames="${xmlEscape(JSON.stringify(names))}"` +
                    ` argumentids="${xmlEscape(JSON.stringify(ids))}"` +
                    ` argumentdefaults="${xmlEscape(JSON.stringify(defaults))}"` +
                    '></mutation></block>'
            });
        }

        this.resetAllCaches();
    }

    getAddonBlock (procedureCode) {
        if (Object.prototype.hasOwnProperty.call(this.addonBlocks, procedureCode)) {
            return this.addonBlocks[procedureCode];
        }
        return null;
    }

    findProjectOptionsComment () {
        const target = this.getTargetForStage();
        const comments = target.comments;
        for (const comment of Object.values(comments)) {
            if (comment.text.includes(COMMENT_CONFIG_MAGIC)) {
                return comment;
            }
        }
        return null;
    }

    parseProjectOptions (optComment) {
        const comment = optComment ?? this.findProjectOptionsComment();
        if (!comment) return;
        const lineWithMagic = comment.text.split('\n').find(i => i.endsWith(COMMENT_CONFIG_MAGIC));
        if (!lineWithMagic) {
            log.warn('Config comment does not contain valid line');
            return;
        }

        const jsonText = lineWithMagic.slice(0, lineWithMagic.length - COMMENT_CONFIG_MAGIC.length);
        let parsed;
        try {
            parsed = ExtendedJSON.parse(jsonText);
            if (!parsed || typeof parsed !== 'object') {
                throw new Error('Invalid object');
            }
        } catch (e) {
            log.warn('Config comment has invalid JSON', e);
            return;
        }

        if (typeof parsed.framerate === 'number') {
            this.setFramerate(parsed.framerate);
        }
        if (parsed.turbo) {
            this.turboMode = true;
            this.emit(Runtime.TURBO_MODE_ON);
        }
        if (parsed.interpolation) {
            this.setInterpolation(true);
        }
        if (parsed.runtimeOptions) {
            this.setRuntimeOptions(parsed.runtimeOptions);
        }
        if (typeof parsed.hq === 'boolean' && this.renderer) {
            this.renderer.setUseHighQualityRender(parsed.hq);
        }
        const storedWidth = +parsed.width || this.stageWidth;
        const storedHeight = +parsed.height || this.stageHeight;
        if (storedWidth !== this.stageWidth || storedHeight !== this.stageHeight) {
            this.setStageSize(storedWidth, storedHeight);
        }
    }

    _generateAllProjectOptions () {
        return {
            framerate: this.frameLoop.framerate,
            runtimeOptions: this.runtimeOptions,
            interpolation: this.interpolationEnabled,
            turbo: this.turboMode,
            hq: this.renderer ? this.renderer.useHighQualityRender : true,
            width: this.stageWidth,
            height: this.stageHeight
        };
    }

    generateDifferingProjectOptions () {
        const difference = (oldObject, newObject) => {
            const result = {};
            for (const key of Object.keys(newObject)) {
                const newValue = newObject[key];
                const oldValue = oldObject[key];
                if (typeof newValue === 'object' && newValue) {
                    const valueDiffering = difference(oldValue, newValue);
                    if (Object.keys(valueDiffering).length > 0) {
                        result[key] = valueDiffering;
                    }
                } else if (newValue !== oldValue) {
                    result[key] = newValue;
                }
            }
            return result;
        };
        return difference(this._defaultStoredSettings, this._generateAllProjectOptions());
    }

    storeProjectOptions () {
        const options = this.generateDifferingProjectOptions();
        // TODO: translate
        const text = `Configuration for https://penguinmod.com/\nYou can move, resize, and minimize this comment, but don't edit it by hand. This comment can be deleted to remove the stored settings.\n${ExtendedJSON.stringify(options)}${COMMENT_CONFIG_MAGIC}`;
        const existingComment = this.findProjectOptionsComment();
        if (existingComment) {
            existingComment.text = text;
        } else {
            const target = this.getTargetForStage();
            // TODO: smarter position logic
            target.createComment(uid(), null, text, 50, 50, 350, 170, false);
        }
        this.emitProjectChanged();
    }

    /**
     * Eagerly (re)compile all scripts within this project.
     */
    precompile () {
        this.allScriptsDo((topBlockId, target) => {
            const topBlock = target.blocks.getBlock(topBlockId);
            if (this.getIsHat(topBlock.opcode)) {
                const thread = new Thread(topBlockId);
                thread.target = target;
                thread.blockContainer = target.blocks;
                thread.tryCompile();
            }
        });
    }

    enableDebug () {
        this.resetAllCaches();
        this.debug = true;
    }

    /**
     * Emit glows/glow clears for scripts after a single tick.
     * Looks at `this.threads` and notices which have turned on/off new glows.
     * @param {Array.<Thread>=} optExtraThreads Optional list of inactive threads.
     */
    _updateGlows (optExtraThreads) {
        const searchThreads = [];
        searchThreads.push.apply(searchThreads, this.threads);
        if (optExtraThreads) {
            searchThreads.push.apply(searchThreads, optExtraThreads);
        }
        // Set of scripts that request a glow this frame.
        const requestedGlowsThisFrame = [];
        // Final set of scripts glowing during this frame.
        const finalScriptGlows = [];
        // Find all scripts that should be glowing.
        for (let i = 0; i < searchThreads.length; i++) {
            const thread = searchThreads[i];
            const target = thread.target;
            if (target === this._editingTarget) {
                const blockForThread = thread.blockGlowInFrame;
                if (thread.requestScriptGlowInFrame || thread.stackClick) {
                    let script = target.blocks.getTopLevelScript(blockForThread);
                    if (!script) {
                        // Attempt to find in flyout blocks.
                        script = this.flyoutBlocks.getTopLevelScript(
                            blockForThread
                        );
                    }
                    if (script) {
                        requestedGlowsThisFrame.push(script);
                    }
                }
            }
        }
        // Compare to previous frame.
        for (let j = 0; j < this._scriptGlowsPreviousFrame.length; j++) {
            const previousFrameGlow = this._scriptGlowsPreviousFrame[j];
            if (requestedGlowsThisFrame.indexOf(previousFrameGlow) < 0) {
                // Glow turned off.
                this.glowScript(previousFrameGlow, false);
            } else {
                // Still glowing.
                finalScriptGlows.push(previousFrameGlow);
            }
        }
        for (let k = 0; k < requestedGlowsThisFrame.length; k++) {
            const currentFrameGlow = requestedGlowsThisFrame[k];
            if (this._scriptGlowsPreviousFrame.indexOf(currentFrameGlow) < 0) {
                // Glow turned on.
                this.glowScript(currentFrameGlow, true);
                finalScriptGlows.push(currentFrameGlow);
            }
        }
        this._scriptGlowsPreviousFrame = finalScriptGlows;
    }

    /**
     * Emit run start/stop after each tick. Emits when `this.threads.length` goes
     * between non-zero and zero
     *
     * @param {number} nonMonitorThreadCount The new nonMonitorThreadCount
     */
    _emitProjectRunStatus (nonMonitorThreadCount) {
        if (this._nonMonitorThreadCount === 0 && nonMonitorThreadCount > 0) {
            this.emit(Runtime.PROJECT_RUN_START);
        }
        if (this._nonMonitorThreadCount > 0 && nonMonitorThreadCount === 0) {
            this.emit(Runtime.PROJECT_RUN_STOP);
        }
        this._nonMonitorThreadCount = nonMonitorThreadCount;
    }

    /**
     * "Quiet" a script's glow: stop the VM from generating glow/unglow events
     * about that script. Use when a script has just been deleted, but we may
     * still be tracking glow data about it.
     * @param {!string} scriptBlockId Id of top-level block in script to quiet.
     */
    quietGlow (scriptBlockId) {
        const index = this._scriptGlowsPreviousFrame.indexOf(scriptBlockId);
        if (index > -1) {
            this._scriptGlowsPreviousFrame.splice(index, 1);
        }
    }

    /**
     * Emit feedback for block glowing (used in the sequencer).
     * @param {?string} blockId ID for the block to update glow
     * @param {boolean} isGlowing True to turn on glow; false to turn off.
     */
    glowBlock (blockId, isGlowing) {
        if (isGlowing) {
            this.emit(Runtime.BLOCK_GLOW_ON, {id: blockId});
        } else {
            this.emit(Runtime.BLOCK_GLOW_OFF, {id: blockId});
        }
    }

    /**
     * Emit feedback for script glowing.
     * @param {?string} topBlockId ID for the top block to update glow
     * @param {boolean} isGlowing True to turn on glow; false to turn off.
     */
    glowScript (topBlockId, isGlowing) {
        if (isGlowing) {
            this.emit(Runtime.SCRIPT_GLOW_ON, {id: topBlockId});
        } else {
            this.emit(Runtime.SCRIPT_GLOW_OFF, {id: topBlockId});
        }
    }

    /**
     * Emit whether blocks are being dragged over gui
     * @param {boolean} areBlocksOverGui True if blocks are dragged out of blocks workspace, false otherwise
     */
    emitBlockDragUpdate (areBlocksOverGui) {
        this.emit(Runtime.BLOCK_DRAG_UPDATE, areBlocksOverGui);
    }

    /**
     * Emit event to indicate that the block drag has ended with the blocks outside the blocks workspace
     * @param {Array.<object>} blocks The set of blocks dragged to the GUI
     * @param {string} topBlockId The original id of the top block being dragged
     */
    emitBlockEndDrag (blocks, topBlockId) {
        this.emit(Runtime.BLOCK_DRAG_END, blocks, topBlockId);
    }

    /**
     * Emit value for reporter to show in the blocks.
     * @param {string} blockId ID for the block.
     * @param {string} value Value to show associated with the block.
     */
    visualReport (blockId, value) {
        this.emit(Runtime.VISUAL_REPORT, {id: blockId, value});
    }

    /**
     * Add a monitor to the state. If the monitor already exists in the state,
     * updates those properties that are defined in the given monitor record.
     * @param {!MonitorRecord} monitor Monitor to add.
     */
    requestAddMonitor (monitor) {
        const id = monitor.get('id');
        if (!this.requestUpdateMonitor(monitor)) { // update monitor if it exists in the state
            // if the monitor did not exist in the state, add it
            this._monitorState = this._monitorState.set(id, monitor);
        }
    }

    /**
     * Update a monitor in the state and report success/failure of update.
     * @param {!Map} monitor Monitor values to update. Values on the monitor with overwrite
     *     values on the old monitor with the same ID. If a value isn't defined on the new monitor,
     *     the old monitor will keep its old value.
     * @return {boolean} true if monitor exists in the state and was updated, false if it did not exist.
     */
    requestUpdateMonitor (monitor) {
        const id = monitor.get('id');
        if (this._monitorState.has(id)) {
            this._monitorState =
                // Use mergeWith here to prevent undefined values from overwriting existing ones
                this._monitorState.set(id, this._monitorState.get(id).mergeWith((prev, next) => {
                    if (typeof next === 'undefined' || next === null) {
                        return prev;
                    }
                    return next;
                }, monitor));
            return true;
        }
        return false;
    }

    /**
     * Removes a monitor from the state. Does nothing if the monitor already does
     * not exist in the state.
     * @param {!string} monitorId ID of the monitor to remove.
     */
    requestRemoveMonitor (monitorId) {
        this._monitorState = this._monitorState.delete(monitorId);
    }

    /**
     * Hides a monitor and returns success/failure of action.
     * @param {!string} monitorId ID of the monitor to hide.
     * @return {boolean} true if monitor exists and was updated, false otherwise
     */
    requestHideMonitor (monitorId) {
        return this.requestUpdateMonitor(new Map([
            ['id', monitorId],
            ['visible', false]
        ]));
    }

    /**
     * Shows a monitor and returns success/failure of action.
     * not exist in the state.
     * @param {!string} monitorId ID of the monitor to show.
     * @return {boolean} true if monitor exists and was updated, false otherwise
     */
    requestShowMonitor (monitorId) {
        return this.requestUpdateMonitor(new Map([
            ['id', monitorId],
            ['visible', true]
        ]));
    }

    /**
     * Removes all monitors with the given target ID from the state. Does nothing if
     * the monitor already does not exist in the state.
     * @param {!string} targetId Remove all monitors with given target ID.
     */
    requestRemoveMonitorByTargetId (targetId) {
        this._monitorState = this._monitorState.filterNot(value => value.targetId === targetId);
    }

    /**
     * Get a target by its id.
     * @param {string} targetId Id of target to find.
     * @return {?Target} The target, if found.
     */
    getTargetById (targetId) {
        for (let i = 0; i < this.targets.length; i++) {
            const target = this.targets[i];
            if (target.id === targetId) {
                return target;
            }
        }
    }

    /**
     * Get the first original (non-clone-block-created) sprite given a name.
     * @param {string} spriteName Name of sprite to look for.
     * @return {?Target} Target representing a sprite of the given name.
     */
    getSpriteTargetByName (spriteName) {
        const json = validateJSON(spriteName);
        if (json.id) return this.getTargetById(json.id);
        for (let i = 0; i < this.targets.length; i++) {
            const target = this.targets[i];
            if (target.isStage) {
                continue;
            }
            if (target.sprite && target.sprite.name === spriteName) {
                return target;
            }
        }
    }

    /**
     * Get a target by its drawable id.
     * @param {number} drawableID drawable id of target to find
     * @return {?Target} The target, if found
     */
    getTargetByDrawableId (drawableID) {
        for (let i = 0; i < this.targets.length; i++) {
            const target = this.targets[i];
            if (target.drawableID === drawableID) return target;
        }
    }

    /**
     * Update the clone counter to track how many clones are created.
     * @param {number} changeAmount How many clones have been created/destroyed.
     */
    changeCloneCounter (changeAmount) {
        this._cloneCounter += changeAmount;
    }

    /**
     * Return whether there are clones available.
     * @return {boolean} True until the number of clones hits runtimeOptions.maxClones
     */
    clonesAvailable () {
        return this._cloneCounter < this.runtimeOptions.maxClones;
    }

    /**
     * Report that the project has loaded in the Virtual Machine.
     * and also handle the parsing of custom values to allow for
     * minimal code when making cross-target refences
     */
    emitProjectLoaded () {
        for (const target of this.targets) {
            for (const varId in target.variables) {
                const variable = target.variables[varId];
                if (variable.type === Variable.LIST_TYPE) {
                    for (const idx in variable.value) {
                        const item = variable.value[idx];
                        if (item.customType) {
                            const {deserialize} = this.serializers[item.typeId];
                            variable.value[idx] = deserialize(item.serialized, target);
                        }
                    }
                }
                if (variable.value?.customType) {
                    const customData = variable.value;
                    const {deserialize} = this.serializers[customData.typeId];
                    variable.value = deserialize(customData.serialized, target);
                }
            }
        }
        this.emit(Runtime.PROJECT_LOADED);
    }

    /**
     * Report that the project has changed in a way that would affect serialization
     */
    emitProjectChanged () {
        this.emit(Runtime.PROJECT_CHANGED);
    }

    /**
     * Report that a new target has been created, possibly by cloning an existing target.
     * @param {Target} newTarget - the newly created target.
     * @param {Target} [sourceTarget] - the target used as a source for the new clone, if any.
     * @fires Runtime#targetWasCreated
     */
    fireTargetWasCreated (newTarget, sourceTarget) {
        this.emit('targetWasCreated', newTarget, sourceTarget);
    }

    /**
     * Report that a clone target is being removed.
     * @param {Target} target - the target being removed
     * @fires Runtime#targetWasRemoved
     */
    fireTargetWasRemoved (target) {
        this.emit('targetWasRemoved', target);
    }

    /**
     * Get the branch for a particular C-shaped block, and it's target.
     * @param {?string} id ID for block to get the branch for.
     * @param {?string} branchId Which branch to select (e.g. for if-else).
     * @return {?string} ID of block in the branch.
     */
    getBranchAndTarget (id, branchId) {
        for (const target of this.targets) {
            const result = target.blocks.getBranch(id, branchId);
            if (result) {
                return [result, target];
            }
        }
        return null;
    }

    /**
     * gets a screen, if no screen can be found it will create one
     * @param {string} screen the screen to get
     * @returns {Object} the screen state object
     */
    getCamera(screen) {
        if (typeof this.cameraStates[screen] !== 'object') {
            this.cameraStates[screen] = {
                pos: [0, 0],
                dir: 0,
                scale: 1
            };
        }
        return this.cameraStates[screen];
    }

    /**
     * assign new camera state options
     * @param {string} screen the screen
     * @param {Object} state the state to apply to the screen
     * @param {boolean} silent if we should emit an event because of this change
     */
    updateCamera(screen, state, silent) {
        if (state.dir) state.dir = MathUtil.wrapClamp(state.dir, -179, 180);
        if (typeof this.cameraStates[screen] !== 'object') {
            this.cameraStates[screen] = {
                pos: [0, 0],
                dir: 0,
                scale: 1
            };
        }
        this.cameraStates[screen] = state =
            Object.assign(this.cameraStates[screen], state);
        if (!silent ?? state.silent) this.emitCameraChanged(screen);
    }
    emitCameraChanged(screen) {
        for (let i = 0; i < this.targets.length; i++)
            if (this.targets[i].cameraBound === screen)
                this.targets[i].cameraUpdateEvent();
        this.emit(Runtime.CAMERA_CHANGED, screen);
        this.requestRedraw();
    }

    /**
     * Get a target representing the Scratch stage, if one exists.
     * @return {?Target} The target, if found.
     */
    getTargetForStage () {
        if (this._stageTarget) {
            return this._stageTarget;
        }
        for (let i = 0; i < this.targets.length; i++) {
            const target = this.targets[i];
            if (target.isStage) {
                this._stageTarget = target;
                return target;
            }
        }
    }

    /**
     * Get the editing target.
     * @return {?Target} The editing target.
     */
    getEditingTarget () {
        return this._editingTarget;
    }

    getAllVarNamesOfType (varType) {
        let varNames = [];
        for (const target of this.targets) {
            const targetVarNames = target.getAllVariableNamesInScopeByType(varType, true);
            varNames = varNames.concat(targetVarNames);
        }
        return varNames;
    }

    /**
     * Get the label or label function for an opcode
     * @param {string} extendedOpcode - the opcode you want a label for
     * @return {object} - object with label and category
     * @property {string} category - the category for this opcode
     * @property {Function} [labelFn] - function to generate the label for this opcode
     * @property {string} [label] - the label for this opcode if `labelFn` is absent
     */
    getLabelForOpcode (extendedOpcode) {
        const [category, opcode] = StringUtil.splitFirst(extendedOpcode, '_');
        if (!(category && opcode)) return;

        const categoryInfo = this._blockInfo.find(ci => ci.id === category);
        if (!categoryInfo) return;

        const block = categoryInfo.blocks.find(b => b.info.opcode === opcode);
        if (!block) return;

        const categoryInstance = this[`ext_${category}`];
        let labelFn = block.info.labelFn ? categoryInstance[block.info.labelFn] : undefined;

        // TODO: we may want to format the label in a locale-specific way.
        return {
            category: block.info.color1 ?? categoryInfo.color1 ?? "extension",
            label: block.info.label ?? `${categoryInfo.name}: ${block.info.text}`,
            labelFn,
        };
    }

    /**
     * Create a new global variable avoiding conflicts with other variable names.
     * @param {string} variableName The desired variable name for the new global variable.
     * This can be turned into a fresh name as necessary.
     * @param {string} optVarId An optional ID to use for the variable. A new one will be generated
     * if a falsey value for this parameter is provided.
     * @param {string} optVarType The type of the variable to create. Defaults to Variable.SCALAR_TYPE.
     * @return {Variable} The new variable that was created.
     */
    createNewGlobalVariable (variableName, optVarId, optVarType) {
        const varType = (typeof optVarType === 'string') ? optVarType : Variable.SCALAR_TYPE;
        const allVariableNames = this.getAllVarNamesOfType(varType);
        const newName = StringUtil.unusedName(variableName, allVariableNames);

        const variable = this.newVariableInstance(varType, optVarId || uid(), newName);
        const stage = this.getTargetForStage();
        stage.variables[variable.id] = variable;
        return variable;
    }

    /**
     * Tell the runtime to request a redraw.
     * Use after a clone/sprite has completed some visible operation on the stage.
     */
    requestRedraw () {
        this.redrawRequested = true;
    }

    /**
     * Emit a targets update at the end of the step if the provided target is
     * the original sprite
     * @param {!Target} target Target requesting the targets update
     */
    requestTargetsUpdate (target) {
        if (!target.isOriginal) return;
        this._refreshTargets = true;
    }

    /**
     * Emit an event that indicates that the blocks on the workspace need updating.
     */
    requestBlocksUpdate () {
        this.emit(Runtime.BLOCKS_NEED_UPDATE);
    }

    /**
     * Emit an event that indicates that the toolbox extension blocks need updating.
     */
    requestToolboxExtensionsUpdate () {
        this.emit(Runtime.TOOLBOX_EXTENSIONS_NEED_UPDATE);
    }

    /**
     * Set up timers to repeatedly step in a browser.
     */
    start () {
        // Do not start if we are already running
        if (this.frameLoop.running) return;
        this.frameLoop.start();
        this.emit(Runtime.RUNTIME_STARTED);
    }

    /**
     * tw: Stop the tick loop
     * Note: This only stops the loop. It will not stop any threads the next time the VM starts
     */
    stop () {
        if (!this.frameLoop.running) {
            return;
        }
        this.frameLoop.stop();
        this.emit(Runtime.RUNTIME_STOPPED);
    }

    /**
     * Turn on profiling.
     * @param {Profiler/FrameCallback} onFrame A callback handle passed a
     * profiling frame when the profiler reports its collected data.
     */
    enableProfiling (onFrame) {
        if (Profiler.available()) {
            this.profiler = new Profiler(onFrame);
        }
    }

    /**
     * Turn off profiling.
     */
    disableProfiling () {
        this.profiler = null;
    }

    /**
     * Update a millisecond timestamp value that is saved on the Runtime.
     * This value is helpful in certain instances for compatibility with Scratch 2,
     * which sometimes uses a `currentMSecs` timestamp value in Interpreter.as
     */
    updateCurrentMSecs () {
        this.currentMSecs = Date.now();
    }

    updatePrivacy () {
        const enforceRestrictions = (
            this.enforcePrivacy &&
            Object.values(this.externalCommunicationMethods).some(i => i)
        );
        if (this.renderer && this.renderer.setPrivateSkinAccess) {
            this.renderer.setPrivateSkinAccess(!enforceRestrictions);
        }
    }

    /**
     * @param {boolean} enabled True if restrictions should be enforced to protect user privacy.
     */
    setEnforcePrivacy (enabled) {
        this.enforcePrivacy = enabled;
        this.updatePrivacy();
    }

    /**
     * @param {string} method Name of the method in Runtime.externalCommunicationMethods
     * @param {boolean} enabled True if the feature is enabled.
     */
    setExternalCommunicationMethod (method, enabled) {
        if (!Object.prototype.hasOwnProperty.call(this.externalCommunicationMethods, method)) {
            throw new Error(`Unknown method: ${method}`);
        }
        this.externalCommunicationMethods[method] = enabled;
        this.updatePrivacy();
    }
}

/**
 * Event fired after a new target has been created, possibly by cloning an existing target.
 *
 * @event Runtime#targetWasCreated
 * @param {Target} newTarget - the newly created target.
 * @param {Target} [sourceTarget] - the target used as a source for the new clone, if any.
 */

module.exports = Runtime;