scheduler.js

/*
handlers: {
  start(time) {},
  stop(time) {},
  schedule(from, to, currentTime) {}
}
context: AudioContext
*/
const createScheduler = handlers => context => {
  let lastSchedule = context.currentTime
  // Context time during the last "play", used to calculate how
  // long scheduler has been playing
  let playStartContextTime = 0
  let playFrom = 0
  let playing = false

  // At least this much time needs to be scheduled
  // before any scheduling will be done
  const schedulingThreashold = 0.1
  // How far into the future to schedule
  const extraSchedulingTime = 1

  const resolveNextFrame = () => new Promise(r => requestAnimationFrame(r))

  const beginScheduling = async () => {
    const attemptScheduling = () => {
      const timeToSchedule =
        context.currentTime + extraSchedulingTime - lastSchedule
      if (timeToSchedule > schedulingThreashold) {
        const from = playFrom + lastSchedule - playStartContextTime
        const to = from + timeToSchedule
        lastSchedule = context.currentTime + extraSchedulingTime
        if (handlers.schedule) handlers.schedule(from, to, currentTime())
      }
    }
    playing = true
    while (playing) {
      await resolveNextFrame()
      attemptScheduling()
    }
  }

  const stopScheduling = () => {
    playing = false
  }

  const currentTime = () => {
    return playFrom + context.currentTime - playStartContextTime
  }

  const checkpoint = time => {
    lastSchedule = context.currentTime
    playFrom = time
    playStartContextTime = context.currentTime
  }

  const play = (time = currentTime()) => {
    checkpoint(time)
    if (handlers.stop) handlers.stop(time)
    if (handlers.start) handlers.start(time)
    beginScheduling()
    if (context.state === 'suspended') {
      context.resume()
    }
  }
  const pause = (time = currentTime()) => {
    checkpoint(time)
    if (handlers.stop) handlers.stop(currentTime())
    stopScheduling()
    if (context.state === 'running') context.suspend()
  }
  const stop = () => {
    pause(0)
  }

  return {
    play,
    pause,
    stop,
    currentTime,
  }
}

export default createScheduler