Allow media track switching.

media/renderers/audio_renderer_impl.h

Index: media/renderers/audio_renderer_impl.h
diff --git a/media/renderers/audio_renderer_impl.h b/media/renderers/audio_renderer_impl.h
index f7915d31a607bdc512ac96704a1ddc7085c8d5cf..7f1e16d83f5222dd5df5739b343d3c08626b46a3 100644
--- a/media/renderers/audio_renderer_impl.h
+++ b/media/renderers/audio_renderer_impl.h
@@ -36,6 +36,7 @@
 #include "media/base/time_source.h"
 #include "media/filters/audio_renderer_algorithm.h"
 #include "media/filters/decoder_stream.h"
+#include "media/renderers/default_renderer_factory.h"
 
 namespace base {
 class SingleThreadTaskRunner;
@@ -62,7 +63,7 @@ class MEDIA_EXPORT AudioRendererImpl
   AudioRendererImpl(
       const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
       AudioRendererSink* sink,
-      ScopedVector<AudioDecoder> decoders,
+      const CreateAudioDecodersCB& create_audio_decoders_cb,
       const scoped_refptr<MediaLog>& media_log);
   ~AudioRendererImpl() override;
 
@@ -96,16 +97,18 @@ class MEDIA_EXPORT AudioRendererImpl
   // Important detail: being in kPlaying doesn't imply that audio is being
   // rendered. Rather, it means that the renderer is ready to go. The actual
   // rendering of audio is controlled via Start/StopRendering().
+  // Audio renderer can be reinitialized completely by calling Initialize again
+  // when it is in a kFlushed and stopped (StopTicking) state.

[xhwang, 2017/03/27 19:13:53]

AudioRenderer API only says it can be reinitialized in kFlushed state. Why do we
need "stopped state" here?

[servolk, 2017/03/27 22:34:12]

AFAIR this must be a left over from some earlier attempts to make it work. Now
that we are going to stop the sink automatically in
AudioRendererImpl::Initialize when it's called from kFlushed state, this should
no longer be necessary, so I've removed the stopped state from the comment.

   //
   //   kUninitialized
-  //         | Initialize()
-  //         |
-  //         V
-  //    kInitializing
-  //         | Decoders initialized
-  //         |
-  //         V            Decoders reset
-  //      kFlushed <------------------ kFlushing
+  //  +----> | Initialize()
+  //  |      |
+  //  |      V
+  //  | kInitializing
+  //  |      | Decoders initialized
+  //  |      |
+  //  |      V            Decoders reset
+  //  +-  kFlushed <------------------ kFlushing
   //         | StartPlaying()             ^
   //         |                            |
   //         |                            | Flush()
@@ -253,6 +256,8 @@ class MEDIA_EXPORT AudioRendererImpl
   // Simple state tracking variable.
   State state_;
 
+  CreateAudioDecodersCB create_audio_decoders_cb_;

[xhwang, 2017/03/27 19:13:53]

FYI, we do have a DecoderFactory interface:

https://cs.chromium.org/chromium/src/media/base/decoder_factory.h

I wonder whether it's a good idea to consolidate that with this callback (in a
later CL)

[servolk, 2017/03/27 22:34:13]

Yes, I saw that, but for now the DefaultRendererFactory doesn't implement the
DecoderFactory interface, so it was easier to get a callback rather than a full
DecoderFactory. Besides, here we only need to create audio decoders (and in
video renderer we need to create only video decoders), but in the DecoderFactory
interface these two methods are tied together. I agree that we should probably
revisit this design later.

[xhwang, 2017/03/29 00:16:47]

Could you please add a TODO?

[servolk, 2017/03/29 01:49:33]

Done.

+
   BufferingState buffering_state_;
 
   // Keep track of whether or not the sink is playing and whether we should be