ui: Cache the output of View::OnPaint when the View isn't invalid.

ui/compositor/paint_cache.h

+// Copyright 2015 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_PAINT_CACHE_H_
+#define UI_COMPOSITOR_PAINT_CACHE_H_
+
+#include "base/memory/scoped_ptr.h"
+#include "ui/compositor/compositor_export.h"
+
+namespace cc {
+class DrawingDisplayItem;
+}
+
+namespace ui {
+class PaintContext;
+class PaintRecorder;
+
+// A class that holds the output of a PaintRecorder to be reused when the
+// object that created the PaintRecorder has not been changed/invalidated.
+class COMPOSITOR_EXPORT PaintCache {
+ public:
+  PaintCache();
+  ~PaintCache();
+
+  // Returns true if the PaintCache was able to insert a previously-saved
+  // painting output into the PaintContext. If it returns false, the caller
+  // needs to do the work of painting, which can be stored into the PaintCache
+  // to be used next time.
+  bool UseCache(const PaintContext& context);
+
+ private:
+  // Only PaintRecorder can modify these.
+  friend PaintRecorder;
+
+  void SetCache(scoped_ptr<cc::DrawingDisplayItem> item);
+
+  scoped_ptr<cc::DrawingDisplayItem> display_item_;
+
+  DISALLOW_COPY_AND_ASSIGN(PaintCache);
+};
+
+}  // namespace ui
+
+#endif  // UI_COMPOSITOR_PAINT_CACHE_H_