Introduce media::TimeSource.

media/base/wall_clock_time_source.cc

+// 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.
+
+#include "media/base/wall_clock_time_source.h"
+
+#include "base/logging.h"
+
+namespace media {
+
+WallClockTimeSource::WallClockTimeSource()
+    : ticking_(false), playback_rate_(1.0f) {
+}
+
+WallClockTimeSource::~WallClockTimeSource() {
+}
+
+void WallClockTimeSource::StartTicking() {
+  DCHECK(!ticking_);
+  ticking_ = true;
+  reference_wall_ticks_ = base::TimeTicks::Now();
+}
+
+void WallClockTimeSource::StopTicking() {
+  DCHECK(ticking_);
+  ticking_ = false;
+  base::TimeTicks now = base::TimeTicks::Now();
+  base_timestamp_ += base::TimeDelta::FromMicroseconds(
+      (now - reference_wall_ticks_).InMicroseconds() * playback_rate_);
+  reference_wall_ticks_ = now;
+}
+
+void WallClockTimeSource::SetPlaybackRate(float playback_rate) {
+  base::TimeTicks now = base::TimeTicks::Now();
+
+  // Estimate current media time using old rate to use as a new base timestamp
+  // for the new rate.
+  if (ticking_) {
+    base_timestamp_ += base::TimeDelta::FromMicroseconds(
+        (now - reference_wall_ticks_).InMicroseconds() * playback_rate_);
+    reference_wall_ticks_ = now;
+  }
+
+  playback_rate_ = playback_rate;
+}
+
+void WallClockTimeSource::SetMediaTimestamp(base::TimeDelta timestamp) {
+  CHECK(!ticking_);
+  base_timestamp_ = timestamp;
+}
+
+base::TimeDelta WallClockTimeSource::CurrentMediaTimestamp() {
+  if (!ticking_)
+    return base_timestamp_;
+
+  base::TimeTicks now = base::TimeTicks::Now();
+  return base_timestamp_ +
+         base::TimeDelta::FromMicroseconds(
+             (now - reference_wall_ticks_).InMicroseconds() * playback_rate_);

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

nit: This appears 3 times and deserve a helper function, e.g

TimeDelta ElapsedTime() {
  base::TimeDelta::FromMicroseconds(
    (base::TimeTicks::Now() - reference_wall_ticks_).InMicroseconds() *
playback_rate_);
}

Since TimeDelta supports multiplication and division with integers, can we
always store 1,000,000 * playback_rate_ so that we can do:
(now - reference_wall_ticks_) * playback_rate_ / 1000000 ?

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

Done.

+}
+
+}  // namespace media