Supporting a playbackTime attribute on AudioprocessingEvent.

LayoutTests/webaudio/scriptprocessornode.html

Index: LayoutTests/webaudio/scriptprocessornode.html
diff --git a/LayoutTests/webaudio/scriptprocessornode.html b/LayoutTests/webaudio/scriptprocessornode.html
index 6919d74e08466f354c283ef861411e35bf348495..2f2b8de8b4132b270bdbcdd7b0383cca724c1087 100644
--- a/LayoutTests/webaudio/scriptprocessornode.html
+++ b/LayoutTests/webaudio/scriptprocessornode.html
@@ -15,6 +15,7 @@ description("Tests ScriptProcessorNode.");
 
 var sampleRate = 44100.0;
 var outputChannels = 6;
+var playbackTime = 0.0;
 
 // For the current implementation of ScriptProcessorNode, when it works with OfflineAudioContext (which runs much faster
 // than real-time) the event.inputBuffer might be overwrite again before onaudioprocess ever get chance to be called.
@@ -39,6 +40,11 @@ function createBuffer(context, length) {
 }
 
 function processAudioData(event) {
+    playbackTime = event.playbackTime;
+    var expectedTime = context.currentTime + (bufferSize / context.sampleRate);
+    var precision = 0.0000001;
+    shouldBeCloseTo("playbackTime", expectedTime, precision, true);

[Raymond Toy, 2014/03/27 18:52:20]

Is there a reason playbackTime this is not EXACTLY equal expectedTime?

And this is not a particularly good test since it's just testing that you
computed what you thought you did.

Ideally, we would want to look at the real delay caused by the script processor
node and compare that with the value returned by playbackTime.  Unfortunately, I
don't know how to do this because ScriptProcessorNodes don't work so well in an
OfflineAudioContext.

[KhNo, 2014/03/28 04:41:37]

The reason that is not exactly same with expectedTime and playbackTime is from
two thread operation.

The delta gap is from diff context.currentTime when audio thread fire
onaudioprocess() and context.currentTime when JS thread read.

When JS thread try to read context.currentTime to check on first line of
eventhandler, a few frameToProcess has been passed to destination already, so it
has a little gap.

I tried to make test case. But that was the best I could except the naming
"precision". I think there is no perfect condition to check pass or fail on JS.
A "precision" should be changed to "gap" and be added some comment about this
gap.
Do you have any good naming or proper way I missed.

+
     buffer = event.outputBuffer;
     if (buffer.numberOfChannels != outputChannels)
         testFailed("numberOfOutputChannels doesn't match!");
@@ -49,7 +55,7 @@ function processAudioData(event) {
     buffer = event.inputBuffer;
     var bufferDataL = buffer.getChannelData(0);
     var bufferDataR = buffer.getChannelData(1);
-    
+
     var success = true;
     // Go through every sample and make sure it's all -1 for the left-channel, and all +1 for the right-channel.
     for (var i = 0; i < buffer.length; ++i) {