[Not for review] Discardable memory emulation

content/renderer/render_process_visibility_manager.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 CONTENT_RENDERER_RENDER_PROCESS_VISIBILITY_MANAGER_H_
+#define CONTENT_RENDERER_RENDER_PROCESS_VISIBILITY_MANAGER_H_
+
+#include "base/memory/singleton.h"
+#include "base/observer_list.h"
+#include "content/common/content_export.h"
+
+namespace content {
+
+class CONTENT_EXPORT RenderProcessVisibilityObserver {
+ public:
+  virtual void RenderProcessVisibilityChanged(bool visible) = 0;
+};
+
+class CONTENT_EXPORT RenderProcessVisibilityManager {
+ public:
+  static RenderProcessVisibilityManager* GetInstance();
+
+  void WidgetVisibilityChanged(bool visible);
+
+  void AddObserver(RenderProcessVisibilityObserver* observer);
+  void RemoveObserver(RenderProcessVisibilityObserver* observer);
+
+ private:
+  friend struct DefaultSingletonTraits<RenderProcessVisibilityManager>;
+
+  RenderProcessVisibilityManager();
+  ~RenderProcessVisibilityManager();
+
+  int num_visible_render_widgets_;
+  ObserverList<RenderProcessVisibilityObserver> observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(RenderProcessVisibilityManager);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_RENDERER_RENDER_PROCESS_VISIBILITY_MANAGER_H_