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"
+#include "base/time/default_tick_clock.h"
+
+namespace media {
+
+WallClockTimeSource::WallClockTimeSource()
+    : tick_clock_(new base::DefaultTickClock()),
+      ticking_(false),
+      playback_rate_(1.0f) {
+}
+
+WallClockTimeSource::~WallClockTimeSource() {
+}
+
+void WallClockTimeSource::StartTicking() {
+  DCHECK(!ticking_);
+  ticking_ = true;
+  reference_wall_ticks_ = tick_clock_->NowTicks();
+}
+
+void WallClockTimeSource::StopTicking() {
+  DCHECK(ticking_);
+  base_time_ = CurrentMediaTime();
+  ticking_ = false;
+  reference_wall_ticks_ = tick_clock_->NowTicks();
+}
+
+void WallClockTimeSource::SetPlaybackRate(float playback_rate) {
+  // Estimate current media time using old rate to use as a new base time for
+  // the new rate.
+  if (ticking_) {
+    base_time_ = CurrentMediaTime();
+    reference_wall_ticks_ = tick_clock_->NowTicks();
+  }
+
+  playback_rate_ = playback_rate;
+}
+
+void WallClockTimeSource::SetMediaTime(base::TimeDelta time) {
+  CHECK(!ticking_);
+  base_time_ = time;
+}
+
+base::TimeDelta WallClockTimeSource::CurrentMediaTime() {
+  if (!ticking_)
+    return base_time_;
+
+  base::TimeTicks now = tick_clock_->NowTicks();
+  return base_time_ +
+         base::TimeDelta::FromMicroseconds(
+             (now - reference_wall_ticks_).InMicroseconds() * playback_rate_);
+}
+
+void WallClockTimeSource::SetTickClockForTesting(
+    scoped_ptr<base::TickClock> tick_clock) {
+  tick_clock_.swap(tick_clock);
+}
+
+}  // namespace media