Adding a gray semi transparent backdrop behind the topmost window within the default container

ash/wm/maximize_mode/maximize_mode_window_manager.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 ASH_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_WINDOW_MANAGER_H_
 #define ASH_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_WINDOW_MANAGER_H_
 
 #include <map>
 #include <set>
 
 #include "ash/ash_export.h"
+#include "ash/shell_observer.h"
 #include "ash/wm/window_state.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "ui/aura/window_observer.h"
 #include "ui/gfx/display_observer.h"
 
 namespace ash {
 class Shell;
 
 namespace internal{
 
 // A window manager which - when created - will force all windows into maximized
 // mode. Exception are panels and windows which cannot be maximized.
 // Windows which cannot be maximized / resized are centered with a layer placed
 // behind the window so that no other windows are visible and/or obscured.
 // With the destruction of the manager all windows will be restored to their
 // original state.
 class ASH_EXPORT MaximizeModeWindowManager : public aura::WindowObserver,
-                                             public gfx::DisplayObserver {
+                                             public gfx::DisplayObserver,
+                                             public ShellObserver {
  public:
   // This should only be deleted by the creator (ash::Shell).
   virtual ~MaximizeModeWindowManager();
 
- // Returns the number of maximized & tracked windows by this manager.
+  // Returns the number of maximized & tracked windows by this manager.
   int GetNumberOfManagedWindows();
 
   // Overridden from WindowObserver:
   virtual void OnWindowDestroying(aura::Window* window) OVERRIDE;
   virtual void OnWindowAdded(aura::Window* window) OVERRIDE;
   virtual void OnWindowBoundsChanged(aura::Window* window,
                                      const gfx::Rect& old_bounds,
                                      const gfx::Rect& new_bounds) OVERRIDE;
 
   // aura::DisplayObserver overrides:
   virtual void OnDisplayBoundsChanged(
       const gfx::Display& display) OVERRIDE;
   virtual void OnDisplayAdded(const gfx::Display& display) OVERRIDE;
   virtual void OnDisplayRemoved(const gfx::Display& display) OVERRIDE;
 
+  // ShellObserver overrides:
+  void OnOverviewModeStarted();
+  void OnOverviewModeEnded();
+
  protected:
   friend class ash::Shell;
 
   // The object should only be created by the ash::Shell.
   MaximizeModeWindowManager();
 
  private:
   typedef std::map<aura::Window*, wm::WindowStateType> WindowToStateType;
 
   // Maximize all windows and restore their current state.
@@ skipping 30 matching lines @@
   // Remove Window creation observers.
   void RemoveWindowCreationObservers();
 
   // Change the internal state (e.g. observers) when the display configuration
   // changes.
   void DisplayConfigurationChanged();
 
   // Returns true when the |window| is a container window.
   bool IsContainerWindow(aura::Window* window);
 
+  // Add a backdrop behind the currently active window on each desktop.
+  void EnableBackdropBehindTopWindowOnEachDisplay(bool enable);
+
   // Every window which got touched by our window manager gets added here.
   WindowToStateType initial_state_type_;
 
   // All container windows which have to be tracked.
   std::set<aura::Window*> observed_container_windows_;
 
+  // True if all backdrops are hidden.
+  bool backdrops_hidden_;
+
   DISALLOW_COPY_AND_ASSIGN(MaximizeModeWindowManager);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_WM_MAXIMIZE_MODE_MAXIMIZE_MODE_WINDOW_MANAGER_H_