media/base/time_source.h

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef MEDIA_BASE_TIME_SOURCE_H_
   6 #define MEDIA_BASE_TIME_SOURCE_H_
   7
   8 #include "base/time/time.h"
   9 #include "media/base/media_export.h"
  10
  11 namespace media {
  12
  13 // A TimeSource is capable of providing the current media time.
  14 class MEDIA_EXPORT TimeSource {
  15  public:
  16   TimeSource() {}
  17   virtual ~TimeSource() {}
  18
  19   // Signal the time source to start ticking. It is expected that values from
  20   // CurrentMediaTime() will start increasing.
  21   virtual void StartTicking() = 0;
  22
  23   // Signal the time source to stop ticking. It is expected that values from
  24   // CurrentMediaTime() will remain constant.
  25   virtual void StopTicking() = 0;
  26
  27   // Updates the current playback rate. It is expected that values from
  28   // CurrentMediaTime() will eventually reflect the new playback rate (e.g., the
  29   // media time will advance at half speed if the rate was set to 0.5f).
  30   virtual void SetPlaybackRate(float playback_rate) = 0;
  31
  32   // Sets the media time to start ticking from. Only valid to call while the
  33   // time source is not ticking.
  34   virtual void SetMediaTime(base::TimeDelta time) = 0;
  35
  36   // Returns the current media timestamp relative to the timestamp set by
  37   // SetMediaTime().
  38   //
  39   // Values returned are intended for informational purposes, such as displaying
  40   // UI with the current minute and second count. While it is guaranteed values
  41   // will never go backwards, the frequency at which they update may be low.
  42   virtual base::TimeDelta CurrentMediaTime() = 0;
  43
  44   // Converts a media timestamp into a wall clock time. If the media time is
  45   // stopped, returns a null TimeTicks.
  46   //
  47   // |media_time| values too far ahead of the current media time will return an
  48   // estimated value; as such, these values may go backwards in time slightly.
  49   //
  50   // |media_time| values behind the current media time may be significantly
  51   // incorrect if the playback rate has changed recently. The only guarantee is
  52   // that the returned time will be less than the current wall clock time.
  53   virtual base::TimeTicks GetWallClockTime(base::TimeDelta media_time) = 0;
  54 };
  55
  56 }  // namespace media
  57
  58 #endif  // MEDIA_BASE_TIME_SOURCE_H_