layers: Consolidate the ownership of layers in views::View and aura::Window.

ui/compositor/layer_owner.h

+// Copyright (c) 2012 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_LAYER_OWNER_H_
+#define UI_COMPOSITOR_LAYER_OWNER_H_
+#pragma once
+
+#include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
+#include "ui/compositor/compositor_export.h"
+#include "ui/compositor/layer.h"
+
+namespace ui {
+
+class COMPOSITOR_EXPORT LayerOwner {
+ public:
+  LayerOwner();
+  virtual ~LayerOwner();
+
+  // Releases the owning reference to its layer, and returns it.
+  // This is used when you need to animate the presentation of the owner just
+  // prior to destroying it. The Owner can be destroyed soon after calling this
+  // function, and the caller is then responsible for disposing of the layer
+  // once any animation completes. Note that layer() will remain valid until the
+  // end of ~LayerOwner().
+  Layer* AcquireLayer() WARN_UNUSED_RESULT;
+
+  ui::Layer* layer() { return layer_; }
+  const ui::Layer* layer() const { return layer_; }
+
+ protected:
+  // The LayerOwner owns its layer unless ownership is relinquished via a call
+  // to AcquireLayer(). After that moment |layer_| will still be valid but
+  // |layer_owner_| will be NULL. The reason for releasing ownership is that
+  // the client may wish to animate the layer beyond the lifetime of the owner,
+  // e.g. fading it out when it is destroyed.
+  scoped_ptr<Layer> layer_owner_;
+  Layer* layer_;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(LayerOwner);
+};
+
+}  // namespace ui
+
+#endif  // UI_COMPOSITOR_LAYER_OWNER_H_