Audio software mixer.

media/audio/audio_output_mixer.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.
+
+// AudioOutputMixer is a class that implements browser-side audio mixer.
+// AudioOutputMixer implements both AudioOutputDispatcher and
+// AudioSourceCallback interfaces.
+
+#ifndef MEDIA_AUDIO_AUDIO_OUTPUT_MIXER_H_
+#define MEDIA_AUDIO_AUDIO_OUTPUT_MIXER_H_
+
+#include <map>
+
+#include "base/basictypes.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/synchronization/lock.h"
+#include "base/timer.h"
+#include "media/audio/audio_io.h"
+#include "media/audio/audio_manager.h"
+#include "media/audio/audio_output_dispatcher.h"
+#include "media/audio/audio_parameters.h"
+
+class MEDIA_EXPORT AudioOutputMixer
+    : public AudioOutputDispatcher,
+      public AudioOutputStream::AudioSourceCallback {
+ public:
+  AudioOutputMixer(AudioManager* audio_manager,
+                   const AudioParameters& params,
+                   const base::TimeDelta& close_delay);
+
+  // AudioOutputDispatcher interface.
+  virtual bool OpenStream() OVERRIDE;
+  virtual bool StartStream(AudioOutputStream::AudioSourceCallback* callback,
+                           AudioOutputProxy* stream_proxy) OVERRIDE;
+  virtual void StopStream(AudioOutputProxy* stream_proxy) OVERRIDE;
+  virtual void StreamVolumeSet(AudioOutputProxy* stream_proxy,
+                               double volume) OVERRIDE;
+  virtual void CloseStream(AudioOutputProxy* stream_proxy) OVERRIDE;
+  virtual void Shutdown() OVERRIDE;
+
+  // AudioSourceCallback interface.
+  virtual uint32 OnMoreData(AudioOutputStream* stream,
+                            uint8* dest,
+                            uint32 max_size,
+                            AudioBuffersState buffers_state) OVERRIDE;
+  virtual void OnError(AudioOutputStream* stream, int code) OVERRIDE;
+  virtual void WaitTillDataReady() OVERRIDE;
+
+ private:
+  friend class base::RefCountedThreadSafe<AudioOutputMixer>;
+  virtual ~AudioOutputMixer();
+
+  // Called by |close_timer_|. Closes physical stream.
+  void ClosePhysicalStream();
+
+  // The |lock_| must be acquired whenever we modify |proxies_| in the audio
+  // manager thread or accessing it in the hardware audio thread. Read in the
+  // audio manager thread is safe.
+  base::Lock lock_;
+
+  // List of audio output proxies.
+  // For every proxy we store aux structure containing data necessary for
+  // mixing.
+  struct ProxyData {
+    int pending_bytes;
+  };
+  typedef std::map<AudioOutputProxy*, ProxyData> ProxyMap;

[vrk (LEFT CHROMIUM), 2012/04/06 23:37:05]

This is closer to what I was thinking, but now that I see the changes to
AudioOutputProxy, I have a bit of a different suggestion:

Instead of referencing AudioOutputProxy at all, you should create a struct or
class with the data that you need. I'm thinking something like

struct AudioSource {
  int stream_id
  AudioSourceCallback* audio_source_cb_;
  int pending_bytes;
  double volume;
}

Then you can have a map of ids -> AudioSource.

The ID can come from the stream_id_ used in AudioDevice and AudioRendererHost.
It will take some wiring through... again, I would recommend adding this to a
design doc and getting consensus on it instead of coding it first!

The main benefit of this is AudioOutputProxy can continue to live on one thread
instead of living on multiple threads. The audio threading story is complicated
enough, and I think it's important to avoid adding complexity.

[enal1, 2012/04/16 22:01:35]

Done.

+  ProxyMap proxies_;
+
+  // Physical stream for this mixer.
+  scoped_ptr<AudioOutputStream> physical_stream_;
+
+  // Temporary buffer used when mixing. Allocated in the constructor
+  // to avoid constant allocation/deallocation in the callback.
+  scoped_array<uint8> mixer_data_;
+
+  // Used to post delayed tasks to ourselves that we cancel inside Shutdown().
+  base::WeakPtrFactory<AudioOutputMixer> weak_this_;
+  base::DelayTimer<AudioOutputMixer> close_timer_;
+
+  DISALLOW_COPY_AND_ASSIGN(AudioOutputMixer);
+};
+
+#endif  // MEDIA_AUDIO_AUDIO_OUTPUT_MIXER_H_