Introduce media::TimeSource.

media/base/time_source.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_BASE_TIME_SOURCE_H_
+#define MEDIA_BASE_TIME_SOURCE_H_
+
+#include "base/basictypes.h"
+#include "base/time/time.h"
+#include "media/base/media_export.h"
+
+namespace media {
+
+// A TimeSource is capable of providing the current media time.
+class MEDIA_EXPORT TimeSource {
+ public:
+  // Signal the time source to start ticking. It is expected that values from
+  // CurrentMediaTimestamp() will start increasing.
+  virtual void StartTicking() = 0;
+
+  // Signal the time source to start ticking. It is expected that values from

[xhwang, 2014/07/12 06:36:44]

s/start/stop

[scherkus (not reviewing), 2014/07/17 23:52:56]

Done.

+  // CurrentMediaTimestamp() will remain constant.
+  virtual void StopTicking() = 0;
+
+  // Updates the current playback rate.
+  virtual void SetPlaybackRate(float playback_rate) = 0;

[xhwang, 2014/07/12 06:36:44]

Can you add comments explaining how current media time increases based on start
timestamp and playback rate?

[scherkus (not reviewing), 2014/07/17 23:52:56]

Done.

Let me know if what I wrote is what you had in mind.

+
+  // Sets the media timestamp to start ticking from. Only valid to call while
+  // the time source is not ticking.
+  virtual void SetMediaTimestamp(base::TimeDelta timestamp) = 0;
+
+  // Returns the current media timestamp.
+  virtual base::TimeDelta CurrentMediaTimestamp() = 0;

[xhwang, 2014/07/12 06:36:44]

const function?

Also, can we just have SetMediaTime() and CurrentMediaTime() since this is a
"time" source?

[scherkus (not reviewing), 2014/07/17 23:52:56]

Done for the "time" bit.

I don't think const makes as much sense for this interface as some of the
underlying bits aren't const (e.g., base::TimeTicks set of functions)

+
+ protected:
+  TimeSource() {}
+  virtual ~TimeSource() {}
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_TIME_SOURCE_H_