Enable renderer side mixing behind a flag.

content/renderer/media/audio_renderer_mixer_manager.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 CONTENT_RENDERER_MEDIA_AUDIO_RENDERER_MIXER_MANAGER_H_
+#define CONTENT_RENDERER_MEDIA_AUDIO_RENDERER_MIXER_MANAGER_H_
+
+#include <map>
+
+#include "base/synchronization/lock.h"
+#include "content/common/content_export.h"
+#include "media/audio/audio_parameters.h"
+
+namespace media {
+class AudioRendererMixer;
+class AudioRendererMixerInput;
+}
+
+namespace content {
+
+// Manages sharing of an AudioRendererMixer among AudioRendererMixerInputs based
+// on their AudioParameters configuration.  Inputs with the same AudioParameters
+// configuration will share a mixer while a new AudioRendererMixer will be
+// lazily created if one with the exact AudioParameters does not exist.
+//
+// There should only be one instance of AudioRendererMixerManager per render
+// thread.
+//
+// TODO(dalecurtis): Right now we require AudioParameters to be an exact match
+// when we should be able to ignore bits per channel since we're only dealing
+// with floats.  However, bits per channel is currently used to interleave the
+// audio data by AudioDevice::AudioThreadCallback::Process for consumption via
+// the shared memory.  See http://crbug.com/114700.
+class CONTENT_EXPORT AudioRendererMixerManager {
+ public:
+  AudioRendererMixerManager();
+  ~AudioRendererMixerManager();
+
+  // Creates an AudioRendererMixerInput with the proper callbacks necessary to
+  // retrieve an AudioRendererMixer instance from AudioRendererMixerManager.
+  // Caller must ensure AudioRendererMixerManager outlives the returned input.
+  media::AudioRendererMixerInput* CreateInput();
+
+ private:
+  friend class AudioRendererMixerManagerTest;
+
+  // Returns a mixer instance based on AudioParameters; an existing one if one
+  // with the provided AudioParameters exists or a new one if not.
+  media::AudioRendererMixer* GetMixer(const media::AudioParameters& params);
+
+  // Remove a mixer instance given a mixer if the only other reference is held
+  // by AudioRendererMixerManager.  Every AudioRendererMixer owner must call
+  // this method when it's done with a mixer.
+  void RemoveMixer(const media::AudioParameters& params);
+
+  // Map of AudioParameters to <AudioRendererMixer, Count>.  Count allows
+  // AudioRendererMixerManager to keep track explicitly (v.s. RefCounted which
+  // is implicit) of the number of outstanding AudioRendererMixers.
+  struct AudioRendererMixerReference {
+    media::AudioRendererMixer* mixer;
+    int ref_count;
+  };
+  typedef std::map<media::AudioParameters, AudioRendererMixerReference,
+                   media::AudioParameters::Compare> AudioRendererMixerMap;
+  AudioRendererMixerMap mixer_map_;

[scherkus (not reviewing), 2012/07/13 23:17:38]

nit: don't bother with _map / _list to variable names -- it's a form of
hungarian notation and doesn't add much value so go with |mixers_|

[DaleCurtis, 2012/07/14 00:39:26]

Done.

+  base::Lock mixer_map_lock_;

[scherkus (not reviewing), 2012/07/13 23:17:38]

ditto here with mixers_lock_

[DaleCurtis, 2012/07/14 00:39:26]

Done.

+
+  DISALLOW_COPY_AND_ASSIGN(AudioRendererMixerManager);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_RENDERER_MEDIA_AUDIO_RENDERER_MIXER_MANAGER_H_