Implemented a proper clock for audio/video synchronization.

media/base/clock.h

+// Copyright (c) 2009 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.
+//
+// A clock represent a single source of time to allow audio and video streams
+// to synchronize with each other.  Clocks essentially track the media time with
+// respect to some other source of time, whether that may be the system clock,
+// audio hardware or some other OS-level API.
+//
+// Clocks start off paused with a playback rate of 1.0f and a media time of 0.
+//
+// TODO(scherkus): Clocks will some day be responsible for executing callbacks
+// given a media time.  This will be used primarily by video renderers.  For now
+// we'll keep using a poll-and-sleep solution.
+
+#ifndef MEDIA_BASE_CLOCK_H_
+#define MEDIA_BASE_CLOCK_H_
+
+#include "base/time.h"
+
+namespace media {
+
+class Clock {
+ public:
+  // Starts the clock and returns the current media time, which will increase
+  // with respect to the current playback rate.
+  virtual base::TimeDelta Play() = 0;
+
+  // Stops the clock and returns the current media time, which will remain
+  // constant until Play() is called.
+  virtual base::TimeDelta Pause() = 0;
+
+  // Sets a new playback rate.  The rate at which the media time will increase
+  // will now change.
+  virtual void SetPlaybackRate(float playback_rate) = 0;
+
+  // Forcefully sets the media time to the given time.  This should only be used
+  // where a discontinuity in the media is found (i.e., seeking).
+  virtual void SetTime(const base::TimeDelta& time) = 0;
+
+  // Returns the current elapsed media time.
+  virtual base::TimeDelta Elapsed() const = 0;
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_CLOCK_H_