Add AudioTrackRecorder for audio component of MediaStream recording.

content/renderer/media/audio_track_recorder.h

+// Copyright 2015 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_TRACK_RECORDER_H_
+#define CONTENT_RENDERER_MEDIA_AUDIO_TRACK_RECORDER_H_
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/threading/thread.h"
+#include "base/threading/thread_checker.h"
+#include "base/time/time.h"
+#include "content/public/renderer/media_stream_audio_sink.h"
+#include "third_party/WebKit/public/platform/WebMediaStreamTrack.h"
+
+namespace media {
+class AudioBus;
+}  // namespace media
+
+namespace content {
+
+// AudioTrackRecorder is a MediaStreamAudioSink that encodes the audio buses
+// received from a Stream Audio Track. The class is constructed on a
+// single thread (the main Render thread) but can recieve MediaStreamAudioSink-
+// related calls on a different "live audio" thread (referred to internally as
+// the "capture thread"). It owns an internal thread to use for encoding, on
+// which lives an AudioEncoder (a private nested class of ATR) with its own
+// threading subtleties, see the implementation file.
+class CONTENT_EXPORT AudioTrackRecorder
+    : NON_EXPORTED_BASE(public MediaStreamAudioSink) {
+ public:
+  using OnEncodedAudioCB =
+      base::Callback<void(const media::AudioParameters& params,
+                          scoped_ptr<std::string> encoded_data,

[minyue, 2015/11/11 13:40:10]

Em.. why using a std::string for encoded_data?

[ajose, 2015/11/12 00:10:41]

I believe this choice was made to simplify data movement across threads, as
std::string owns its memory (as compared to C-style strings or
base::StringPiece, which depend on external ownership). This choice was also
made in VideoTrackRecorder. Do you have suggestions for an alternative?

[minyue, 2015/11/12 16:52:31]

Yes, indeed, it is quite often used in VPx. Good, that gives me a lot of
confidence. Thanks.

+                          base::TimeTicks capture_time)>;
+
+  AudioTrackRecorder(const blink::WebMediaStreamTrack& track,
+                     const OnEncodedAudioCB& on_encoded_audio_cb);
+  ~AudioTrackRecorder() override;
+
+  // Implement MediaStreamAudioSink.
+  void OnSetFormat(const media::AudioParameters& params) override;
+  void OnData(const media::AudioBus& audio_bus,
+              base::TimeTicks capture_time) override;
+
+ private:
+  friend class AudioTrackRecorderTest;
+  class AudioParameters;
+
+  // Forward declaration of nested class for handling encoding.
+  // See the implementation file for details.
+  class AudioEncoder;
+
+  // Used to check that we are destroyed on the same thread we were created on.
+  base::ThreadChecker main_render_thread_checker_;
+
+  // Used to check that MediaStreamAudioSink's methods are called on the
+  // capture audio thread.
+  base::ThreadChecker capture_thread_checker_;
+
+  // We need to hold on to the Blink track to remove ourselves on destruction.
+  const blink::WebMediaStreamTrack track_;
+
+  // Thin wrapper around OpusEncoder.
+  // |encoder_| should be initialized before |encoder_thread_| such that
+  // |encoder_thread_| is destructed first. This, combined with all
+  // AudioEncoder work (aside from construction and destruction) happening on
+  // |encoder_thread_|, should allow us to be sure that all AudioEncoder work is
+  // done by the time we destroy it on ATR's thread.
+  const scoped_refptr<AudioEncoder> encoder_;
+  // The thread on which |encoder_| works.
+  base::Thread encoder_thread_;
+
+  DISALLOW_COPY_AND_ASSIGN(AudioTrackRecorder);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_RENDERER_MEDIA_AUDIO_TRACK_RECORDER_H_