[chromecast] Slew stream volume changes in StreamMixerAlsa.

chromecast/media/cma/backend/alsa/stream_mixer_alsa.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 CHROMECAST_MEDIA_CMA_BACKEND_ALSA_STREAM_MIXER_ALSA_H_
 #define CHROMECAST_MEDIA_CMA_BACKEND_ALSA_STREAM_MIXER_ALSA_H_
 
 #include <alsa/asoundlib.h>
 #include <stdint.h>
 
@@ skipping 75 matching lines @@
     virtual int MaxReadSize() = 0;
 
     // Pulls data from the input stream. The input stream should populate |dest|
     // with |frames| frames of data to be mixed. The mixer expects data to be
     // at a sample rate of |output_samples_per_second()|, so each input stream
     // should resample as necessary before returning. |frames| is guaranteed to
     // be no larger than the value returned by the most recent call to
     // MaxReadSize(), and |dest->frames()| shall be >= |frames|.
     virtual void GetResampledData(::media::AudioBus* dest, int frames) = 0;
 
+    // Scale |frames| frames at |src| by the current volume (smoothing as
+    // needed). Add the scaled result to |dest|.
+    virtual bool VolumeScaleAccumulate(const float* src,
+                                       int frames,
+                                       float* dest) = 0;
+
     // This is called for every InputQueue when the mixer writes data to ALSA
     // for any of its input streams.
     virtual void AfterWriteFrames(
         const MediaPipelineBackendAlsa::RenderingDelay&
             mixer_rendering_delay) = 0;
 
     // This will be called when a fatal error occurs in the mixer.
     virtual void SignalError(StreamMixerAlsaInput::MixerError error) = 0;
 
     // Notifies the input that it is being removed by the upper layers, and
@@ skipping 132 matching lines @@
   std::unique_ptr<AudioFilterInterface> pre_loopback_filter_;
   std::unique_ptr<AudioFilterInterface> post_loopback_filter_;
 
   DISALLOW_COPY_AND_ASSIGN(StreamMixerAlsa);
 };
 
 }  // namespace media
 }  // namespace chromecast
 
 #endif  // CHROMECAST_MEDIA_CMA_BACKEND_ALSA_STREAM_MIXER_ALSA_H_