Explicit animation support

ui/gfx/compositor/layer_animation_preemption_strategy.h

+// Copyright (c) 2011 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_GFX_COMPOSITOR_LAYER_ANIMATION_PREEMPTION_STRATEGY_H_
+#define UI_GFX_COMPOSITOR_LAYER_ANIMATION_PREEMPTION_STRATEGY_H_
+
+#include "ui/gfx/compositor/compositor_export.h"
+#include "ui/gfx/compositor/layer_animator.h"
+
+namespace ui {
+
+class LayerAnimationDelegate;
+class LayerAnimationSequence;
+
+class COMPOSITOR_EXPORT LayerAnimationPreemptionStrategy {

[sky, 2011/10/14 16:39:52]

This seems better served by an enum. Is there a reason we need this level of
flexibility?

+ public:
+  // Do not delete these objects, they are singletons.
+  static LayerAnimationPreemptionStrategy* GetImmediatelySetNewTarget();
+  static LayerAnimationPreemptionStrategy* GetImmediatelyAnimateToNewTarget();
+  static LayerAnimationPreemptionStrategy* GetEnqueueNewAnimation();
+  static LayerAnimationPreemptionStrategy* GetBlendCurrentWithNewAnimation();
+
+  // Start the sequence |preempting|, modifying the queue and running animations
+  // as necessary.
+  virtual void Preempt(LayerAnimationDelegate* delegate,
+                       LayerAnimationSequence* preempting,
+                       LayerAnimator::RunningAnimations* running,
+                       LayerAnimator::AnimationQueue* queue) = 0;
+
+ protected:
+  virtual ~LayerAnimationPreemptionStrategy() {}
+};
+
+}  // namespace ui
+
+#endif //  UI_GFX_COMPOSITOR_LAYER_ANIMATION_PREEMPTION_STRATEGY_H_