Calculate the signal level on the media stream local audio track

content/renderer/media/media_stream_audio_level_calculator.h

+// Copyright 2014 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_MEDIA_STREAM_AUDIO_LEVEL_CALCULATOR_H_
+#define CONTENT_RENDERER_MEDIA_MEDIA_STREAM_AUDIO_LEVEL_CALCULATOR_H_
+
+#include "base/threading/thread_checker.h"
+
+namespace content {
+
+// This class is used by the WebRtcLocalAudioTrack to calculate the  level of
+// the audio signal. And the audio level will be eventually used by the volume
+// animation UI.
+// The algorithm used by this class is the same as how it is done in
+// third_party/webrtc/voice_engine/level_indicator.cc.
+class MediaStreamAudioLevelCalculator {
+ public:
+  MediaStreamAudioLevelCalculator();
+  ~MediaStreamAudioLevelCalculator();
+
+  // Calculates the signal level of the audio data.
+  // Returns the level of bars that the volume animation UI uses for
+  // presenting the energy level of the audio data.
+  int Calculate(const int16* audio_data, int number_of_channels,
+                int number_of_frames);
+
+ private:
+  // Used to DCHECK that the constructor and Calculate() are always called on
+  // the same audio thread. Note that the destructor can be called on a new
+  // audio thread, this is because the WebRtcAudioCapturer will creates a new

[tommi (sloooow) - chröme, 2014/03/02 10:19:53]

s/creates/create

[no longer working on chromium, 2014/03/03 13:07:13]

The comment has been modified.

+  // source with a new audio thread when the format of audio changes.

[tommi (sloooow) - chröme, 2014/03/02 10:19:53]

I think this comment assumes that the reader knows everything about how this
code hangs together.  It's not clear how WebRtcAudioCapturer and this class are
related but it looks like this class is now depending on implementation details
of WebRtcAudioCapturer.  Can this object sometimes be deleted on the thread on
which it is used and sometimes not?

Thanks for adding the documentation but I guess I'm actually more confused :)

Assuming I'm still generally parsing the comment correctly though; if the object
is created and used on the same thread, why do we need to pass ownership over to
a different thread just to delete it?

[no longer working on chromium, 2014/03/03 13:07:13]

No, either the main render thread or the new audio thread.
We don't really pass the ownership to another thread.
The track uses a scoped_ptr for the MediaStreamAudioLevelCalculator, and create
the instance in OnSetFormat() which happens on the audio thread. So if
OnSetFormat() only happen once, the object gets deleted when the track goes
away, which happens on the main render thread.
If OnSetFormat() is called more than once, the old instance will go out of scope
and new instance is created, and this happens on a new audio thread.

I changed the comment and hide the implementation details, PTAL.

+  base::ThreadChecker thread_checker_;
+
+  int counter_;
+  int max_amplitude_;
+  int level_;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_RENDERER_MEDIA_MEDIA_STREAM_AUDIO_LEVEL_CALCULATOR_H_