Web Audio API doesn't provide a comprehensive API for scheduling things in the time. For example, it is hard or impossible to cancel events once they have been scheduled ; there is no way to schedule a custom event ; there is no way to schedule repeating events ; or change tempo.

WAAClock(audioContext) adds a very thin layer allowing you to play with the time in a more easy way.

Schedule custom events

// prints 'wow!' at context.currentTime = 13
var event = clock.callbackAtTime(function() { console.log('wow!') }, 13)
// prints 'wow!' in 13 seconds
var event = clock.setTimeout(function() { console.log('wow!') }, 13)

Set events to repeat periodically

var event = clock.callbackAtTime(function() { console.log('wow!') }, 3).repeat(2)

Cancel an event

// Start an oscillator node at context.currentTime = 13
var event = clock.callbackAtTime(function() { oscNode.start(13) }, 13)
// ... but change your mind and cancel that
event.clear()

Change the tempo of a group of events

var event1 = clock.callbackAtTime(function() { console.log('wow!') }, 1).repeat(2)
  , event2 = clock.callbackAtTime(function() { console.log('what?') }, 2).repeat(2)

// in 10 seconds, the tempo will be multiplied by 2
clock.setTimeout(function() {
  clock.timeStretch([event1, event2], 0.5)
}, 10)

note : this library uses current web audio API specification. Some older browsers still use prefixed / deprecated function names. You can use Chris Wilson's AudioContext-MonkeyPatch if you want to support those older browsers as well.

First download the latest stable release of WAAClock from dist/, then create an AudioContext and a WAAClock :

var context = window.AudioContext ? new AudioContext() : new webkitAudioContext()
  , clock = new WAAClock(context)
clock.start()

For complete examples, check-out this simple repetitive pattern or a basic sequencer.

##WAAClock(context, opts)

WAAClock handles all the scheduling work. It is the only object you need to create directly.

Because Web Audio API events cannot be cancelled, WAAClock simply queues all events, and schedules them only at the last moment. In fact, each event has a tolerance zone [t1, t2] in which it should be executed. Each event is scheduled as soon as the clock enters its tolerance zone. On the other hand, if the event hasn't been scheduled when the clock gets out of the tolerance zone, the event will be dropped. Once the event has been scheduled, it cannot be cancelled anymore, and will be executed. Therefore, you should set the tolerance wisely : a too tight upper bound (lateTolerance), and the event can be dropped abusively, a too loose lower bound (earlyTolerance), and the event will be scheduled too early.

You can set the default tolerance with the options lateTolerance and earlyTolerance. You can also set the tolerance on a "per-event" basis, by calling the tolerance method of the event.

###start()

Starts the clock.

###stop()

Stops the clock.

###callbackAtTime(func, time)

Schedules func to run at time in seconds, and returns an Event object.

###setTimeout(func, delay)

Schedules func to run after delay seconds, and returns an Event object.

###timeStretch(events, ratio)

Stretch time and repeat time of events by ratio, keeping their relative distance. In fact this is equivalent to changing the tempo.

##Event

Every scheduling method returns an event object. All methods from Event return the calling event, so that you can chain them.

###time

The time at which the event is scheduled.

###schedule(time)

Reschedule an event, time is the absolute time as given by context.currentTime.

###tolerance(late, early)

Sets the event's tolerance. See WAAClock for a detailed explanation.

###repeat(time)

Sets the event to repeat every time seconds. If you want to remove the repeat you can pass null. Note that even if an event is dropped because it expired, subsequent "repeats" of the event will still be executed.

###clear()

Cancels the event execution. This will work only if the event hasn't been scheduled yet (see WAAClock for more infos).

This message is emitted when the event has been executed. You can listen to it by calling on :

event.on('executed', function() { console.log('yeaay! :)') })

This message is emitted when the clock fell out of the event tolerance zone. You can listen to it by calling on :

event.on('expired', function() { console.log('oooh :(!') })

Tests are written with mocha. Just install mocha globally and run mocha from the root directory. Integration with node-web-audio-api is tested manually running node test/node-web-audio-api-test.js.

Released under MIT license

###0.4.0

• made WAAClock.start method public, and start needs to be called explicitely
• WAAClock.stop method
• removed web audio API monkey-patching
• removed support for old web audio API functions

###0.3.2

• bug fix

###0.3.1

• made schedule method of Event public.

###0.3.0

• added support for node-web-audio-api

###0.2.0

• changed the tick method from setInterval to ScriptProcessorNode
• added event's toleranceEarly and toleranceLate
• removed clock tickTime and lookAheadTime options
• added support for old Web Audio API names

###0.1.2

• added callbackAtTime
• bug fixes