sup-ytsync/index.js

// ==UserScript==
// @name         sup-ytsync
// @namespace    https://forgejo.sup39.dev/sup39/sup-ytsync/
// @version      2024-02-03
// @description  A script to sync Youtube video progress with others via MQTT
// @author       sup39
// @match        https://www.youtube.com/watch*
// @grant        none
// ==/UserScript==

(() => {
  'use strict';

  const url = '...';
  const username = '...';
  const password = '...';
  const roomId = '...';
  const clientId = `ytsync-${username}-${+new Date()}`;
  // topic = ${topicPrefixBase}/${roomId}/${videoId}/${commander.clientId}/${command}
  const topicPrefixBase = 'sup-ytsync';

  const mqttScriptUrl = 'https://unpkg.com/mqtt@5.3.4/dist/mqtt.min.js';

  /**
   * @param {HTMLVideoElement} element
   * @param {VideoCommandBroadcaster} commandBroadcaster
   */
  function Video(element, commandBroadcaster) {
    /** @type {Set<string>} */
    const queuedCommands = new Set();

    for (const [eventName, command] of (/**@type{const}*/([
      ['play', 'play'],
      ['pause', 'pause'],
      ['seeked', 'seek'],
    ]))) {
      element.addEventListener(eventName, () => {
        // ignore queued command
        if (queuedCommands.delete(command)) return;
        // ignore seeked event when playing
        if (eventName === 'seeked' && !element.paused) return;
        // callback
        const {currentTime} = element;
        commandBroadcaster(command, {currentTime});
      });
    }

    return {
      /** @param {number} t */
      seek(t) {
        queuedCommands.add('seek');
        element.currentTime = t;
      },
      play() {
        queuedCommands.add('play');
        element.play();
      },
      pause() {
        queuedCommands.add('pause');
        element.pause();
      },
      get isPaused() {
        return element.paused;
      },
    };
  }

  const videoId = new URLSearchParams(window.location.search).get('v');
  if (videoId == null) return;

  const videoElement = document.querySelector('video');
  if (videoElement == null) return;

  /** @returns {Promise<void>} */
  const loadMqttScript = () => new Promise(resolve => {
    if (typeof mqtt === 'object') return resolve();
    document.head.appendChild(
      Object.assign(document.createElement('script'), {
        src: mqttScriptUrl,
        async: true,
      }),
    ).addEventListener('load', () => resolve());
  });

  const startYtsync = () => loadMqttScript().then(() => {
    const topicPrefix = `${topicPrefixBase}/${roomId}/${videoId}/`;

    console.info('Connecting to MQTT server:', url);
    const client = mqtt.connect(url, {username, password, clientId});
    client.on('error', e => console.warn('Error occurs on MQTT connection:', e));

    /** @param {Buffer} payload */
    function parseTime(payload) {
      if (payload.byteLength !== 8) {
        throw new Error('Invalid payload');
      }
      return payload.readDoubleBE(0);
    }

    client.on('connect', () => {
      console.info('Connected to MQTT server:', url);

      const video = Video(videoElement, (command, state) => {
        const {currentTime: t} = state;
        console.debug(`Broadcast video sync command [${command}]`, t);
        const payload = new Uint8Array(8);
        new DataView(payload.buffer).setFloat64(0, t);
        client.publish(
          `${topicPrefix}${clientId}/${command}`,
          // FIXME
          /**@type{Buffer}*/(/**@type{unknown}*/(payload)),
        );
      });

      client.on('message', (topic, payload) => {
        if (!topic.startsWith(topicPrefix)) {
          return console.warn('Unexpected MQTT message', {topic, payload});
        }
        const [commander, command=''] = topic.slice(topicPrefix.length).split('/');
        // ignore command sent by self
        if (commander === clientId) return;

        try {
          if (command === 'play') {
            const t = parseTime(payload);
            console.debug(`Received command [${command}]`, t);
            video.seek(t);
            video.play();
          } else if (command === 'pause') {
            const t = parseTime(payload);
            console.debug(`Received command [${command}]`, t);
            video.seek(t);
            video.pause();
          } else if (command === 'seek') {
            /**
             * (seek while playing) pause -> play -> seeked
             *  => no need to update current time
             * (seek when paused) pause -> seeked
             *  => need to update current time
             */
            if (video.isPaused) {
              const t = parseTime(payload);
              console.debug(`Received command [${command}]`, t);
              video.seek(t);
            }
          } else {
            return console.warn('Unknown command', {command, payload});
          }
        } catch (e) {
          console.warn(e, {topic, payload});
        }
      });

      client.subscribe(topicPrefix+'+/+');
    });
  });

  /**
   * @param {string} selector
   * @param {Element} root
   * @returns {Promise<Element>}
   */
  // https://stackoverflow.com/a/61511955
  const untilElementLoaded = (selector, root=document.body) => new Promise(resolve => {
    // resolve immediately if element already exists
    const elm = root.querySelector(selector);
    if (elm != null) return resolve(elm);
    // observe root until element presents
    const observer = new MutationObserver(() => {
      const elm = root.querySelector(selector);
      if (elm != null) {
        observer.disconnect();
        resolve(elm);
      }
    });
    observer.observe(root, {
      childList: true,
      subtree: true
    });
  });

  // TODO better UI
  untilElementLoaded('#title h1').then(elm => elm.appendChild(
    Object.assign(document.createElement('button'), {
      textContent: 'Start sup-ytsync'
    }),
  ).addEventListener('click', startYtsync, {once: true}));
})();

/**
 * The MIT License (MIT)
 *
 * Copyright (c) 2024 sup39
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */