Source: midi/scheduler.js

  1. /**
  2.  * Schedules functions to occur at a given time in the future, relative to the start time.
  3.  * Can be used to play music in realtime.
  4.  */
  5. class Scheduler {
  7.   constructor({ bpm=120 } = {}) {
  8.     this.schedule = new Map();
  9.     this.bpm = bpm;
  10.   }
  12.   /**
  13.    * Delete all scheduled callbacks
  14.    */
  15.   clear() {
  16.     this.schedule.clear();
  17.   }
  19.   /**
  20.    * Schedule a callback to run.
  21.    * Note, you may call this repeatedly with the same time parameter. It will append to any existing callbacks.
  22.    * @param time {Number} - Time in milliseconds, relative to when {@link Scheduler#start scheduler.start()} is called.
  23.    * @param callback {Function} - The code to execute at the scheduled time.
  24.    */
  25.   at(time, callback) {
  26.     if (typeof time !== 'number') throw new TypeError('time must be a number');
  27.     if (typeof callback !== 'function') throw new TypeError('callback must be a function');
  28.     const timeInBeats = time * 60000/this.bpm;
  29.     let callbacks = this.schedule.get(timeInBeats);
  30.     if (!callbacks) {
  31.       callbacks = [];
  32.       this.schedule.set(timeInBeats, callbacks);
  33.     }
  34.     callbacks.push(callback);
  35.   }
  37.   /**
  38.    * Run the scheduler and execute the callbacks as scheduled.
  39.    */
  40.   start() {
  41.     this.times = [...this.schedule.keys()].sort((a,b) => a-b);
  42.     this.startTime = new Date().getTime();
  43.     this.tick();
  44.   }
  46.   /**
  47.    * Stop the scheduler.
  48.    */
  49.   stop() {
  50.     if (this.timeout) clearTimeout(this.timeout);
  51.   }
  53.   /**
  54.    * @private
  55.    */
  56.   tick() {
  57.     if (!this.nextTime) {
  58.       this.nextSchedule = this.times.shift();
  59.       if (this.nextSchedule == null) return;
  60.       this.nextTime = this.startTime + this.nextSchedule;
  61.     }
  62.     if (this.nextTime <= new Date().getTime()) {
  63.       const callbacks = this.schedule.get(this.nextSchedule) || [];
  64.       this.nextSchedule = null;
  65.       this.nextTime = null;
  66.       callbacks.forEach(callback => callback.call(this.nextSchedule));
  67.     }
  68.     this.timeout = setTimeout(() => this.tick(), 1);
  69.   }
  71. }
  73. module.exports = Scheduler;