Move audio frame memory handling inside AudioMixer.

webrtc/api/audio/audio_mixer.h

Index: webrtc/api/audio/audio_mixer.h
diff --git a/webrtc/api/audio/audio_mixer.h b/webrtc/api/audio/audio_mixer.h
index 960adbbd430b8cb24403a5736aa9f0d039a43a6d..5e36abf73c8bea1f6256b6832cd253a362d7a7c3 100644
--- a/webrtc/api/audio/audio_mixer.h
+++ b/webrtc/api/audio/audio_mixer.h
@@ -35,21 +35,15 @@ class AudioMixer : public rtc::RefCountInterface {
       kError,   // The audio_frame will not be used.
     };
 
-    struct AudioFrameWithInfo {
-      AudioFrame* audio_frame;
-      AudioFrameInfo audio_frame_info;
-    };
-
-    // The implementation of GetAudioFrameWithInfo should update
-    // audio_frame with new audio every time it's called. Implementing
-    // classes are allowed to return the same AudioFrame pointer on
-    // different calls. The pointer must stay valid until the next
-    // mixing call or until this audio source is disconnected from the
-    // mixer. The mixer may modify the contents of the passed
-    // AudioFrame pointer at any time until the next call to
-    // GetAudioFrameWithInfo, or until the source is removed from the
-    // mixer.
-    virtual AudioFrameWithInfo GetAudioFrameWithInfo(int sample_rate_hz) = 0;
+    // The AudioFrame pointer should be treated as only valid for the
+    // duration of this call. The passed AudioFrame is strictly an
+    // output parameter and its fields can have any values when being
+    // passed to GetAudioFrameWithInfo. Implementing classes may
+    // return sound with either one or two interleaved
+    // channels. Implementing classes are responsible for filling all
+    // fields of the provided AudioFrame with meaningful values.
+    virtual AudioFrameInfo GetAudioFrameWithInfo(int sample_rate_hz,
+                                                 AudioFrame* audio_frame) = 0;

[kwiberg-webrtc, 2016/10/14 19:59:32]

You never say what the method is supposed to *do*, or what |sample_rate_hz|
means... and I think the rest can be said with less words. Something like this?

  "Overwrites |audio_frame| with new audio (either 1 or 2 interleaved channels)
of the given sample rate."

(Note: I'm just guessing, so you need to fact check in case this suggestion
sounds good.)

Should the comment additionally be explicit about how much new audio we're
talking about? Is it 10 ms?

[aleloi, 2016/10/17 11:06:39]

I've updated the comment. I guess that it's clear and we don't need to add that
implementing classes shouldn't store the audio frame pointer somewhere between
calls?

 
     // A way for a mixer implementation to distinguish participants.
     virtual int Ssrc() = 0;