Refactor VideoRendererImpl to use VideoFrameScheduler.

media/filters/video_frame_scheduler.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 MEDIA_FILTERS_VIDEO_FRAME_SCHEDULER_H_
 #define MEDIA_FILTERS_VIDEO_FRAME_SCHEDULER_H_
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
+#include "base/time/time.h"
 #include "media/base/media_export.h"
 
 namespace media {
 
 class VideoFrame;
 
 // Defines an abstract video frame scheduler that is capable of managing the
 // display of video frames at explicit times.
 class MEDIA_EXPORT VideoFrameScheduler {
  public:
   VideoFrameScheduler() {}
   virtual ~VideoFrameScheduler() {}
 
   enum Reason {
     DISPLAYED,  // Frame was displayed.
     DROPPED,    // Frame was dropped.
-    RESET,      // Scheduler was reset before frame was scheduled for display.
   };
   typedef base::Callback<void(const scoped_refptr<VideoFrame>&, Reason)> DoneCB;
 
   // Schedule |frame| to be displayed at |wall_ticks|, firing |done_cb| when
   // the scheduler has finished with the frame.
+  //
+  // To avoid reentrancy issues, |done_cb| is run on a separate calling stack.
   virtual void ScheduleVideoFrame(const scoped_refptr<VideoFrame>& frame,
                                   base::TimeTicks wall_ticks,
                                   const DoneCB& done_cb) = 0;
 
-  // Causes the scheduler to release all previously scheduled frames. Frames
-  // will be returned as RESET.
+  // Causes the scheduler to cancel any previously scheduled frames.
+  //
+  // There is no guarantee that |done_cb|'s for previously scheduled frames
+  // will not be run. Clients should implement callback tracking/cancellation
+  // if they are sensitive to old callbacks being run.
   virtual void Reset() = 0;

[scherkus (not reviewing), 2014/04/25 02:04:47]

I do like this style of API as it makes less guarantees, however I noticed
during testing that when going from a high playback rate to a low playback rate
that we end up showing a few duplicate frames.

it's a bit of a tricky problem where the real answer might be to re-introduce
the RESET parameter and make SetPlaybackRate() async :(

I need to think about this more ...

[xhwang, 2014/04/25 05:44:47]

Note to myself: How does the caller know which frame has been displayed/dropped
or not?

 };
 
 }  // namespace media
 
 #endif  // MEDIA_FILTERS_VIDEO_FRAME_SCHEDULER_H_