media/base/clock.h

   1 // Copyright (c) 2012 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_CLOCK_H_
   6 #define MEDIA_BASE_CLOCK_H_
   7
   8 #include "base/basictypes.h"
   9 #include "base/time/time.h"
  10 #include "media/base/media_export.h"
  11
  12 namespace base {
  13 class TickClock;
  14 }  // namespace base
  15
  16 namespace media {
  17
  18 // A clock represents a single source of time to allow audio and video streams
  19 // to synchronize with each other.  Clock essentially tracks the media time with
  20 // respect to some other source of time, whether that may be the monotonic
  21 // system clock or updates via SetTime(). Clock uses linear interpolation to
  22 // calculate the current media time since the last time SetTime() was called.
  23 //
  24 // Clocks start off paused with a playback rate of 1.0f and a media time of 0.
  25 //
  26 // Clock is not thread-safe and must be externally locked.
  27 //
  28 // TODO(scherkus): Clock will some day be responsible for executing callbacks
  29 // given a media time.  This will be used primarily by video renderers.  For now
  30 // we'll keep using a poll-and-sleep solution.
  31 //
  32 // TODO(miu): Rename media::Clock to avoid confusion (and tripping up the media
  33 // PRESUBMIT script on future changes).
  34 class MEDIA_EXPORT Clock {
  35  public:
  36   explicit Clock(base::TickClock* clock);
  37   ~Clock();
  38
  39   // Returns true if the clock is running.
  40   bool IsPlaying() const;
  41
  42   // Starts the clock and returns the current media time, which will increase
  43   // with respect to the current playback rate.
  44   base::TimeDelta Play();
  45
  46   // Stops the clock and returns the current media time, which will remain
  47   // constant until Play() is called.
  48   base::TimeDelta Pause();
  49
  50   // Sets a new playback rate.  The rate at which the media time will increase
  51   // will now change.
  52   void SetPlaybackRate(float playback_rate);
  53
  54   // Forcefully sets the media time to |current_time|. The second parameter is
  55   // the |max_time| that the clock should progress after a call to Play(). This
  56   // value is often the time of the end of the last frame buffered and decoded.
  57   //
  58   // These values are clamped to the duration of the video, which is initially
  59   // set to 0 (before SetDuration() is called).
  60   void SetTime(base::TimeDelta current_time, base::TimeDelta max_time);
  61
  62   // Sets the |max_time| to be returned by a call to Elapsed().
  63   void SetMaxTime(base::TimeDelta max_time);
  64
  65   // Returns the current elapsed media time. Returns 0 if SetDuration() has
  66   // never been called.
  67   base::TimeDelta Elapsed();
  68
  69   // Sets the duration of the video. Clock expects the duration will be set
  70   // exactly once.
  71   void SetDuration(base::TimeDelta duration);
  72
  73   // Returns the duration of the clock, or 0 if not set.
  74   base::TimeDelta Duration() const;
  75
  76  private:
  77   // Updates the reference points based on the current calculated time.
  78   void UpdateReferencePoints();
  79
  80   // Updates the reference points based on the given |current_time|.
  81   void UpdateReferencePoints(base::TimeDelta current_time);
  82
  83   // Returns the time elapsed based on the current reference points, ignoring
  84   // the |max_time_| cap.
  85   base::TimeDelta EstimatedElapsedTime();
  86
  87   // Translates |time| into the current media time, based on the perspective of
  88   // the monotonically-increasing system clock.
  89   base::TimeDelta ElapsedViaProvidedTime(const base::TimeTicks& time) const;
  90
  91   base::TimeDelta ClampToValidTimeRange(base::TimeDelta time) const;
  92
  93   base::TickClock* const clock_;
  94
  95   // Whether the clock is running.
  96   bool playing_;
  97
  98   // Whether the clock is stalled because it has reached the |max_time_|
  99   // allowed.
 100   bool underflow_;
 101
 102   // The monotonic system clock time when this Clock last started playing or had
 103   // its time set via SetTime().
 104   base::TimeTicks reference_;
 105
 106   // Current accumulated amount of media time.  The remaining portion must be
 107   // calculated by comparing the system time to the reference time.
 108   base::TimeDelta media_time_;
 109
 110   // Current playback rate.
 111   float playback_rate_;
 112
 113   // The maximum time that can be returned by calls to Elapsed().
 114   base::TimeDelta max_time_;
 115
 116   // Duration of the media.
 117   base::TimeDelta duration_;
 118
 119   DISALLOW_COPY_AND_ASSIGN(Clock);
 120 };
 121
 122 }  // namespace media
 123
 124 #endif  // MEDIA_BASE_CLOCK_H_