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

media/audio/audio_output_controller.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_AUDIO_AUDIO_OUTPUT_CONTROLLER_H_
 #define MEDIA_AUDIO_AUDIO_OUTPUT_CONTROLLER_H_
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 136 matching lines @@
                            AudioBuffersState buffers_state) OVERRIDE;
   virtual void OnError(AudioOutputStream* stream, int code) OVERRIDE;
   virtual void WaitTillDataReady() OVERRIDE;
 
   // AudioDeviceListener implementation.  When called AudioOutputController will
   // shutdown the existing |stream_|, transition to the kRecreating state,
   // create a new stream, and then transition back to an equivalent state prior
   // 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 {
     kEmpty,
     kCreated,
     kPlaying,
     kStarting,
     kPausedWhenStarting,
     kPaused,
     kClosed,
@@ skipping 22 matching lines @@
   void DoSetVolume(double volume);
   void DoReportError(int code);
 
   // Helper method that starts physical stream.
   void StartStream();
 
   // Helper method that stops, closes, and NULLs |*stream_|.
   // 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.
   double volume_;
 
   // |state_| is written on the audio manager thread and is read on the
   // hardware audio thread. These operations need to be locked. But lock
   // is not required for reading on the audio manager thread.
   State state_;
 
   // The |lock_| must be acquired whenever we access |state_| from a thread
   // other than the audio manager thread.
   base::Lock lock_;
 
   // SyncReader is used only in low latency mode for synchronous reading.
   SyncReader* sync_reader_;
 
   // The message loop of audio manager thread that this object runs on.
   scoped_refptr<base::MessageLoopProxy> message_loop_;
 
   // When starting stream we wait for data to become available.
   // 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.
   // Also, if we're shutting down, we do not want to poll for more data.
   base::WeakPtrFactory<AudioOutputController> weak_this_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioOutputController);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_AUDIO_OUTPUT_CONTROLLER_H_