Handle audio device changes on Mac (Take 2).

media/audio/mac/audio_manager_mac.h

Index: media/audio/mac/audio_manager_mac.h
diff --git a/media/audio/mac/audio_manager_mac.h b/media/audio/mac/audio_manager_mac.h
index d655d4b11ed9d54fac49daf85f6dd13de0916fd9..a3f76f480f8294e4d4cd2752bbd327b4eea896c7 100644
--- a/media/audio/mac/audio_manager_mac.h
+++ b/media/audio/mac/audio_manager_mac.h
@@ -6,11 +6,14 @@
 #define MEDIA_AUDIO_MAC_AUDIO_MANAGER_MAC_H_
 
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "media/audio/audio_manager_base.h"
 
 namespace media {
 
+class AudioDeviceListenerMac;
+
 // Mac OS X implementation of the AudioManager singleton. This class is internal
 // to the audio output and only internal users can call methods not exposed by
 // the AudioManager class.
@@ -37,7 +40,16 @@ class MEDIA_EXPORT AudioManagerMac : public AudioManagerBase {
  protected:
   virtual ~AudioManagerMac();
 
+  // Implementation of AudioManager.
+  virtual void InitializeOnAudioThread() OVERRIDE;

[scherkus (not reviewing), 2012/10/31 23:17:22]

silliness alert!

Initialize() is an empty virtual provided by AMB + called by
AudioManager::Create()

Destruct is handled by the OS-specific AM impl

Seeing as GetMessageLoop() is guaranteed to be valid inside of an AMImpl how
about we move Initialize to be OS-specific AM impl and remove the PostTask()
from AM::Create?

[DaleCurtis, 2012/11/01 00:16:33]

I don't know how that would work? You can't post a task to the message loop on
another thread using "this" during construction. That was the original design
and that was why we had to move InitializeOnAudioThread() to AM::Create().

[scherkus (not reviewing), 2012/11/01 00:47:48]

I believe you're referring to https://chromiumcodereview.appspot.com/11260002/

In that case you were posting a task from inside the non-concrete
AudioManagerBase ctor but as a result introduced a race as the subclass' ctor
hadn't executed yet.

If you post a task using |this| inside of a concrete ctor (e.g., AMLinux, AMWin)
it's functionally the same as executing it post-concrete-ctor as
AudioManager::Create().

+
  private:
+  void DestructOnAudioThread();
+
+  // Listens for output device changes.
+  base::Closure listener_cb_;
+  scoped_ptr<AudioDeviceListenerMac> output_device_listener_;
+
   DISALLOW_COPY_AND_ASSIGN(AudioManagerMac);
 };