compositor: Add rAF-like functionality for the UI.

ui/compositor/animation_frame_task.h

+// 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.
+
+#ifndef UI_COMPOSITOR_ANIMATION_FRAME_TASK_H_
+#define UI_COMPOSITOR_ANIMATION_FRAME_TASK_H_
+
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/time/time.h"
+#include "ui/compositor/compositor_export.h"
+
+namespace ui {
+
+enum AnimationFrameTaskStatus {
+  ANIMATION_FRAME_TASK_CONTINUE,
+  ANIMATION_FRAME_TASK_END,
+};
+
+class COMPOSITOR_EXPORT AnimationFrameTask {
+ public:
+  virtual ~AnimationFrameTask() {}
+
+  // Updates the task. Returns ANIMATION_FRAME_TASK_CONTINUE if the task has not
+  // completed, and should be triggered again at the next tick. Returns
+  // ANIMATION_FRAME_TASK_END if the task is complete. When returning
+  // ANIMATION_FRAME_TASK_END, the corresponding ScopedAnimationFrameTask handle
+  // for the task must be destroyed.
+  virtual AnimationFrameTaskStatus Progress(base::TimeTicks timestamp) = 0;
+};
+
+// A handle for an AnimationFrameTask. Destroying this handle removes the task
+// from the compositor. This handle can be destroyed at any time.
+class COMPOSITOR_EXPORT ScopedAnimationFrameTask {
+ public:
+  ScopedAnimationFrameTask(AnimationFrameTask* task,
+                           const base::Closure& destroy_callback);
+  ~ScopedAnimationFrameTask();
+
+ private:
+  AnimationFrameTask* task_;
+  base::Closure destroy_callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedAnimationFrameTask);
+};
+
+class COMPOSITOR_EXPORT AnimationFrameTaskProvider {
+ public:
+  virtual ~AnimationFrameTaskProvider() {}
+
+  // Adds |task| to the set of tasks that should be run at each animation
+  // step. Note that there is no guaranteed order among the tasks in the set.
+  // Destroying the returned handle removes the task from the set. Note that the
+  // caller must ensure that |task| outlives the returned handle.
+  virtual scoped_ptr<ScopedAnimationFrameTask> RequestAnimationFrameTask(

[piman, 2014/07/31 01:17:03]

I would prefer if ScopedAnimationFrameTask was not part of the API.
Instead you can make RemoveAnimationFrameTask part of the API, and have
ScopedAnimationFrameTask explicitly call it. No need for closures any more,
simpler code overall, I think.

+      AnimationFrameTask* task) = 0;
+};
+
+}  // namespace ui
+
+#endif  // UI_COMPOSITOR_ANIMATION_FRAME_TASK_H_