Add a simple scheduler for animation frames

mc/scheduler/timer.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 "mc/scheduler/timer.h"
+
+#include <cstdlib>
+
+#include "base/bind.h"
+#include "base/tracked_objects.h"
+
+namespace mc {
+
+// We're willing to slop around 1/4 of a tick duration to avoid trashing our
+// client with irregular ticks.
+static const int64 kTickSlop = 4;
+
+Timer::Client::~Client() {
+}
+
+Timer::Timer(Client* client,
+             scoped_refptr<base::SingleThreadTaskRunner> task_runner)
+    : client_(client),
+      task_runner_(task_runner),
+      enabled_(false),
+      weak_factory_(this) {
+  DCHECK(client_);
+}
+
+Timer::~Timer() {
+}
+
+void Timer::SetEnabled(bool enabled) {
+  enabled_ = enabled;
+
+  if (enabled_ && current_target_.is_null())
+    ScheduleNextTick(base::TimeTicks::Now());
+}
+
+void Timer::SetInterval(const TimeInterval& interval) {
+  interval_ = interval;
+
+  // We don't have a tick scheduled, so there's no need to reschedule it.
+  if (current_target_.is_null())
+    return;
+
+  base::TimeTicks now = base::TimeTicks::Now();
+
+  base::TimeTicks new_target = NextTickTarget(now);
+  base::TimeDelta delta = base::TimeDelta::FromInternalValue(
+      std::abs((new_target - current_target_).ToInternalValue()));
+
+  if (delta * kTickSlop < interval_.duration)
+    return;
+
+  current_target_ = base::TimeTicks();
+  weak_factory_.InvalidateWeakPtrs();
+  PostTickTask(now, new_target);
+}
+
+base::TimeTicks Timer::NextTickTarget(base::TimeTicks now) {
+  base::TimeTicks target = interval_.NextAfter(now);
+
+  // If we're targeting a time that's too soon since the last tick, we push out
+  // the target to the next tick.
+  if ((target - last_tick_) * kTickSlop < interval_.duration)
+    target += interval_.duration;
+
+  return target;
+}
+
+void Timer::ScheduleNextTick(base::TimeTicks now) {
+  PostTickTask(now, NextTickTarget(now));
+}
+
+void Timer::PostTickTask(base::TimeTicks now, base::TimeTicks target) {
+  DCHECK(current_target_.is_null());
+  current_target_ = target;
+  task_runner_->PostDelayedTask(
+      FROM_HERE, base::Bind(&Timer::OnTimerFired, weak_factory_.GetWeakPtr()),
+      current_target_ - now);
+}
+
+void Timer::OnTimerFired() {
+  current_target_ = base::TimeTicks();
+  if (!enabled_)
+    return;
+  base::TimeTicks now = base::TimeTicks::Now();
+  ScheduleNextTick(now);
+  last_tick_ = now;
+  client_->OnTimerTick(now);
+  // We might be deleted here.
+}
+}