mediasource/index.js

MediaSource API as a node.js Writable stream

/*! mediasource. MIT License. Feross Aboukhadijeh <https://feross.org/opensource> */
module.exports = MediaElementWrapper

var inherits = require('inherits')
var stream = require('readable-stream')
var toArrayBuffer = require('to-arraybuffer')

var MediaSource = typeof window !== 'undefined' && window.MediaSource

var DEFAULT_BUFFER_DURATION = 60 // seconds

function MediaElementWrapper (elem, opts) {
  var self = this
  if (!(self instanceof MediaElementWrapper)) return new MediaElementWrapper(elem, opts)

  if (!MediaSource) throw new Error('web browser lacks MediaSource support')

  if (!opts) opts = {}
  self._debug = opts.debug
  self._bufferDuration = opts.bufferDuration || DEFAULT_BUFFER_DURATION
  self._elem = elem
  self._mediaSource = new MediaSource()
  self._streams = []
  self.detailedError = null

  self._errorHandler = function () {
    self._elem.removeEventListener('error', self._errorHandler)
    var streams = self._streams.slice()
    streams.forEach(function (stream) {
      stream.destroy(self._elem.error)
    })
  }
  self._elem.addEventListener('error', self._errorHandler)

  self._elem.src = window.URL.createObjectURL(self._mediaSource)
}

/*
 * `obj` can be a previous value returned by this function
 * or a string
 */
MediaElementWrapper.prototype.createWriteStream = function (obj) {
  var self = this

  return new MediaSourceStream(self, obj)
}

/*
 * Use to trigger an error on the underlying media element
 */
MediaElementWrapper.prototype.error = function (err) {
  var self = this

  // be careful not to overwrite any existing detailedError values
  if (!self.detailedError) {
    self.detailedError = err
  }
  self._dumpDebugData()
  try {
    self._mediaSource.endOfStream('decode')
  } catch (err) {}

  try {
    // Attempt to clean up object URL
    window.URL.revokeObjectURL(self._elem.src)
  } catch (err) {}
}

/*
 * When self._debug is set, dump all data to files
 */
MediaElementWrapper.prototype._dumpDebugData = function () {
  var self = this

  if (self._debug) {
    self._debug = false // prevent multiple dumps on multiple errors
    self._streams.forEach(function (stream, i) {
      downloadBuffers(stream._debugBuffers, 'mediasource-stream-' + i)
    })
  }
}

inherits(MediaSourceStream, stream.Writable)

function MediaSourceStream (wrapper, obj) {
  var self = this
  stream.Writable.call(self)

  self._wrapper = wrapper
  self._elem = wrapper._elem
  self._mediaSource = wrapper._mediaSource
  self._allStreams = wrapper._streams
  self._allStreams.push(self)
  self._bufferDuration = wrapper._bufferDuration
  self._sourceBuffer = null
  self._debugBuffers = []

  self._openHandler = function () {
    self._onSourceOpen()
  }
  self._flowHandler = function () {
    self._flow()
  }
  self._errorHandler = function (err) {
    if (!self.destroyed) {
      self.emit('error', err)
    }
  }

  if (typeof obj === 'string') {
    self._type = obj
    // Need to create a new sourceBuffer
    if (self._mediaSource.readyState === 'open') {
      self._createSourceBuffer()
    } else {
      self._mediaSource.addEventListener('sourceopen', self._openHandler)
    }
  } else if (obj._sourceBuffer === null) {
    obj.destroy()
    self._type = obj._type // The old stream was created but hasn't finished initializing
    self._mediaSource.addEventListener('sourceopen', self._openHandler)
  } else if (obj._sourceBuffer) {
    obj.destroy()
    self._type = obj._type
    self._sourceBuffer = obj._sourceBuffer // Copy over the old sourceBuffer
    self._debugBuffers = obj._debugBuffers // Copy over previous debug data
    self._sourceBuffer.addEventListener('updateend', self._flowHandler)
    self._sourceBuffer.addEventListener('error', self._errorHandler)
  } else {
    throw new Error('The argument to MediaElementWrapper.createWriteStream must be a string or a previous stream returned from that function')
  }

  self._elem.addEventListener('timeupdate', self._flowHandler)

  self.on('error', function (err) {
    self._wrapper.error(err)
  })

  self.on('finish', function () {
    if (self.destroyed) return
    self._finished = true
    if (self._allStreams.every(function (other) { return other._finished })) {
      self._wrapper._dumpDebugData()
      try {
        self._mediaSource.endOfStream()
      } catch (err) {}
    }
  })
}

MediaSourceStream.prototype._onSourceOpen = function () {
  var self = this
  if (self.destroyed) return

  self._mediaSource.removeEventListener('sourceopen', self._openHandler)
  self._createSourceBuffer()
}

MediaSourceStream.prototype.destroy = function (err) {
  var self = this
  if (self.destroyed) return
  self.destroyed = true

  // Remove from allStreams
  self._allStreams.splice(self._allStreams.indexOf(self), 1)

  self._mediaSource.removeEventListener('sourceopen', self._openHandler)
  self._elem.removeEventListener('timeupdate', self._flowHandler)
  if (self._sourceBuffer) {
    self._sourceBuffer.removeEventListener('updateend', self._flowHandler)
    self._sourceBuffer.removeEventListener('error', self._errorHandler)
    if (self._mediaSource.readyState === 'open') {
      self._sourceBuffer.abort()
    }
  }

  if (err) self.emit('error', err)
  self.emit('close')
}

MediaSourceStream.prototype._createSourceBuffer = function () {
  var self = this
  if (self.destroyed) return

  if (MediaSource.isTypeSupported(self._type)) {
    self._sourceBuffer = self._mediaSource.addSourceBuffer(self._type)
    self._sourceBuffer.addEventListener('updateend', self._flowHandler)
    self._sourceBuffer.addEventListener('error', self._errorHandler)
    if (self._cb) {
      var cb = self._cb
      self._cb = null
      cb()
    }
  } else {
    self.destroy(new Error('The provided type is not supported'))
  }
}

MediaSourceStream.prototype._write = function (chunk, encoding, cb) {
  var self = this
  if (self.destroyed) return
  if (!self._sourceBuffer) {
    self._cb = function (err) {
      if (err) return cb(err)
      self._write(chunk, encoding, cb)
    }
    return
  }

  if (self._sourceBuffer.updating) {
    return cb(new Error('Cannot append buffer while source buffer updating'))
  }

  var arr = toArrayBuffer(chunk)
  if (self._wrapper._debug) {
    self._debugBuffers.push(arr)
  }

  try {
    self._sourceBuffer.appendBuffer(arr)
  } catch (err) {
    // appendBuffer can throw for a number of reasons, most notably when the data
    // being appended is invalid or if appendBuffer is called after another error
    // already occurred on the media element. In Chrome, there may be useful debugging
    // info in chrome://media-internals
    self.destroy(err)
    return
  }
  self._cb = cb
}

MediaSourceStream.prototype._flow = function () {
  var self = this

  if (self.destroyed || !self._sourceBuffer || self._sourceBuffer.updating) {
    return
  }

  if (self._mediaSource.readyState === 'open') {
    // check buffer size
    if (self._getBufferDuration() > self._bufferDuration) {
      return
    }
  }

  if (self._cb) {
    var cb = self._cb
    self._cb = null
    cb()
  }
}

// TODO: if zero actually works in all browsers, remove the logic associated with this below
var EPSILON = 0

MediaSourceStream.prototype._getBufferDuration = function () {
  var self = this

  var buffered = self._sourceBuffer.buffered
  var currentTime = self._elem.currentTime
  var bufferEnd = -1 // end of the buffer
  // This is a little over complex because some browsers seem to separate the
  // buffered region into multiple sections with slight gaps.
  for (var i = 0; i < buffered.length; i++) {
    var start = buffered.start(i)
    var end = buffered.end(i) + EPSILON

    if (start > currentTime) {
      // Reached past the joined buffer
      break
    } else if (bufferEnd >= 0 || currentTime <= end) {
      // Found the start/continuation of the joined buffer
      bufferEnd = end
    }
  }

  var bufferedTime = bufferEnd - currentTime
  if (bufferedTime < 0) {
    bufferedTime = 0
  }

  return bufferedTime
}

function downloadBuffers (bufs, name) {
  var a = document.createElement('a')
  a.href = window.URL.createObjectURL(new window.Blob(bufs))
  a.download = name
  a.click()
}