midi-clock.js

var watch = require('observ/watch')

module.exports = function (tempo, listener) {
  var pos = null
  var length = 25
  var destroyed = false
  var releases = []

  if (typeof tempo === 'function') {
    releases.push(watch(tempo, setTempo))
  } else if (typeof tempo === 'number') {
    setTempo(tempo)
  }

  tick()

  return function () {
    destroyed = true
    while (releases.length) {
      releases.pop()()
    }
  }

  // scoped

  function setTempo (value) {
    length = 60000 / (value * 24)
  }

  function tick () {
    if (destroyed) return false
    if (!pos) {
      pos = window.performance.now()
    }
    pos += length
    var diff = pos - window.performance.now()
    listener()
    setTimeout(tick, diff)
  }
}