Implementation of 'AudioContext.getOutputTimestamp' method

media/audio/audio_io.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.
 
 #ifndef MEDIA_AUDIO_AUDIO_IO_H_
 #define MEDIA_AUDIO_AUDIO_IO_H_
 
 #include <stdint.h>
 
 #include "media/base/audio_bus.h"
@@ skipping 31 matching lines @@
 // Because we support more audio streams than physically available channels
 // a given AudioOutputStream might or might not talk directly to hardware.
 // An audio stream allocates several buffers for audio data and calls
 // AudioSourceCallback::OnMoreData() periodically to fill these buffers,
 // as the data is written to the audio device. Size of each packet is determined
 // by |samples_per_packet| specified in AudioParameters  when the stream is
 // created.
 
 namespace media {
 
+struct MEDIA_EXPORT AudioTimestamp {
+  AudioTimestamp() = default;
+  int64_t frames;

[miu, 2016/07/19 01:22:36]

Why do we need this? Client code could track this by keeping a running-sum of
AudioBus::frames(), if it's needed for a particular use case.

+  int64_t ticks;  // Obtained from base::TimeTicks::ToInternalValue().

[miu, 2016/07/19 01:22:36]

Don't use ToInternalValue(). As the header comments in base/time/time.h explain,
this is only for wire serialization. Client code should never attempt to
interpret the meaning of the TimeTicks's internal value.

naming nit: This should probably be called |reference_time| for clarity.

+};
+
+// Used in unittests.
+inline bool operator==(const AudioTimestamp& one, const AudioTimestamp& other) {
+  return one.ticks == other.ticks && one.frames == other.frames;
+}
+
 class MEDIA_EXPORT AudioOutputStream {
  public:
   // Audio sources must implement AudioSourceCallback. This interface will be
   // called in a random thread which very likely is a high priority thread. Do
   // not rely on using this thread TLS or make calls that alter the thread
   // itself such as creating Windows or initializing COM.
   class MEDIA_EXPORT AudioSourceCallback {
    public:
     virtual ~AudioSourceCallback() {}
 
     // Provide more data by fully filling |dest|.  The source will return
     // the number of frames it filled.  |total_bytes_delay| contains current
     // number of bytes of delay buffered by the AudioOutputStream.
     // |frames_skipped| contains the number of frames skipped by the consumer.
-    virtual int OnMoreData(AudioBus* dest,
-                           uint32_t total_bytes_delay,
-                           uint32_t frames_skipped) = 0;
+    // |timestamp| if provided, contains timestamp of the currently audible
+    // signal.
+    virtual int OnMoreData(
+        AudioBus* dest,
+        uint32_t total_bytes_delay,
+        uint32_t frames_skipped,

[miu, 2016/07/19 01:22:36]

This is in conflict with this change:
https://codereview.chromium.org/2101303004/

You guys need to chat and come to a consensus here. ;-)

(I've also stated some of my opinions on what this API should become, in that
code review.)

[jameswest, 2016/07/20 04:19:18]

We're in favor of changing the API to the following, which was suggested by
Yuri:

virtual int OnMoreData(
    base::TimeTicks target_playout_time,
    int prior_frames_skipped,
    AudioBus* dest);

To get |target_playout_time|, you would convert |total_bytes_delay| to time
duration and add it to base::TimeTicks::Now().

Does that work for you?

+        const AudioTimestamp& output_timestamp = AudioTimestamp()) = 0;
 
     // There was an error while playing a buffer. Audio source cannot be
     // destroyed yet. No direct action needed by the AudioStream, but it is
     // a good place to stop accumulating sound data since is is likely that
     // playback will not continue.
     virtual void OnError(AudioOutputStream* stream) = 0;
   };
 
   virtual ~AudioOutputStream() {}
 
@@ skipping 89 matching lines @@
   // Returns the Automatic Gain Control (AGC) state.
   virtual bool GetAutomaticGainControl() = 0;
 
   // Returns the current muting state for the microphone.
   virtual bool IsMuted() = 0;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_AUDIO_IO_H_