Tab Audio Capture: Browser-side connect/disconnect functionality.

media/audio/audio_output_controller.h

Index: media/audio/audio_output_controller.h
diff --git a/media/audio/audio_output_controller.h b/media/audio/audio_output_controller.h
index 762a9483d0fb51d1bdeed0cfa08a042508bab368..1e8be2d3c7f831fe878d5138186cf73921e21c69 100644
--- a/media/audio/audio_output_controller.h
+++ b/media/audio/audio_output_controller.h
@@ -154,6 +154,21 @@ class MEDIA_EXPORT AudioOutputController
   // to being called.
   virtual void OnDeviceChange() OVERRIDE;
 
+  // Accessor to audio output parameters.
+  const AudioParameters& params() const { return params_; }
+
+  // Stops the normal audio output stream and creates an AudioSourceCallback to
+  // provide audio data to some other destination.  The given object will always
+  // return data when OnMoreData() is invoked, even if the underlying
+  // implementation is paused/stopped.  AudioOutputController retains ownership
+  // of the returned object.
+  AudioOutputStream::AudioSourceCallback* Divert();

[DaleCurtis, 2012/12/05 23:35:14]

Hmm, this is odd since AudioOutputController is generally the target callback.
Why not have Divert(AudioSourceCallback* callback) and Revert() instead? Why
make the caller manage the original callback?

[miu, 2012/12/11 02:30:45]

It's a pull model, not a push model (the AudioSourceCallback interface).  In
other words, some outside entity needs AudioOutputController to provide an
interface whereby it can invoke OnMoreData() to pull audio data out.
I took a look, and I think this is actually a weirdness in the existing code
organization: AudioOutputController implements the AudioSourceCallback
interface.  However, you might ask, "What objects call those methods?"  As it
turns out, only one object: AOC::stream_, which is private/internal to AOC.

Therefore, I removed ASC from AOC's inheritance hierarchy, and added a private
helper class (AOC::Glue) that implements ASC.  Doing this also solved another
major problem: AOC::Divert() is virtually non-blocking across these
timing-sensitive threads.  Two birds with one stone!  :-)

+
+  // Restores normal audio output behavior for the current playback state.
+  // |asc| is the pointer returned by the previous call to Divert() and becomes
+  // invalid once this method is called.
+  void Revert(AudioOutputStream::AudioSourceCallback* asc);
+
  protected:
     // Internal state of the source.
   enum State {
@@ -196,10 +211,14 @@ class MEDIA_EXPORT AudioOutputController
   // Signals event when done if it is not NULL.
   void DoStopCloseAndClearStream(base::WaitableEvent *done);
 
-  AudioManager* audio_manager_;
+  AudioManager* const audio_manager_;
+  const AudioParameters params_;
 
   // |handler_| may be called only if |state_| is not kClosed.
   EventHandler* handler_;
+
+  // AudioManager-owned stream.  Note that this could be NULL during state
+  // kCreated, kPlaying, etc.; if audio is currently being diverted.
   AudioOutputStream* stream_;
 
   // The current volume of the audio stream.
@@ -224,8 +243,6 @@ class MEDIA_EXPORT AudioOutputController
   // Number of times left.
   int number_polling_attempts_left_;
 
-  AudioParameters params_;
-
   // Used to post delayed tasks to ourselves that we can cancel.
   // We don't want the tasks to hold onto a reference as it will slow down
   // shutdown and force it to wait for the most delayed task.