@comapeo/ipc/dist/lib/sub-channel.js

IPC wrappers for CoMapeo Core

import { EventEmitter } from 'eventemitter3';
import { extractMessageEventData } from './utils.js';
// Ideally unique ID used for identifying "global" Mapeo IPC messages
export const MAPEO_RPC_ID = '@@mapeo-rpc';
export const MANAGER_CHANNEL_ID = '@@manager';
/**
 * @typedef {Object} Events
 * @property {(message: any) => void} message
 */
/**
 * Node's built-in types for MessagePort are misleading so we opt for this limited type definition
 * that fits our usage and works in both Node and browser contexts
 * @typedef {Pick<EventTarget, 'addEventListener' | 'removeEventListener'> & { postMessage: (message: any) => void }} MessagePortLike
 */
export class SubChannel extends EventEmitter {
    #id;
    #messagePort;
    /** @type {'idle' | 'active' | 'closed'} */
    #state;
    /** @type {Array<{id: string, message: any}>} */
    #queued;
    #handleMessageEvent;
    /**
     * @param {MessagePortLike} messagePort Parent channel to add namespace to
     * @param {string} id ID for the subchannel
     */
    constructor(messagePort, id) {
        super();
        this.#id = id;
        this.#messagePort = messagePort;
        this.#state = 'idle';
        this.#queued = [];
        /**
         * @param {unknown} event
         */
        this.#handleMessageEvent = (event) => {
            const value = extractMessageEventData(event);
            if (!isRelevantEvent(value))
                return;
            const { id, message } = value;
            if (this.#id !== id)
                return;
            switch (this.#state) {
                case 'idle': {
                    this.#queued.push(value);
                    break;
                }
                case 'active': {
                    this.emit('message', message);
                    break;
                }
                case 'closed': {
                    // no-op if closed (the event listener should be removed anyway)
                    break;
                }
            }
        };
        this.#messagePort.addEventListener('message', this.#handleMessageEvent);
    }
    get id() {
        return this.#id;
    }
    /**
     * Send messages with the subchannel's ID
     * @param {any} message
     */
    postMessage(message) {
        this.#messagePort.postMessage({ id: this.#id, message });
    }
    start() {
        if (this.#state !== 'idle')
            return;
        this.#state = 'active';
        /** @type {{id: string, message: any} | undefined} */
        let event;
        while ((event = this.#queued.shift())) {
            this.#handleMessageEvent(event);
        }
    }
    close() {
        if (this.#state === 'closed')
            return;
        this.#state = 'closed';
        this.#queued = [];
        // Node types are incorrect (as of v14, Node's MessagePort should also extend [EventTarget](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget))
        this.#messagePort.removeEventListener('message', this.#handleMessageEvent);
    }
}
/**
 * @param {unknown} event
 * @returns {event is { id: string, message: any }}
 */
function isRelevantEvent(event) {
    if (!event || typeof event !== 'object')
        return false;
    if (!('id' in event && 'message' in event))
        return false;
    if (typeof event.id !== 'string')
        return false;
    return true;
}