Add ViewObserver to View for view updates

ui/views/view.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_VIEWS_VIEW_H_
 #define UI_VIEWS_VIEW_H_
 
 #include <stddef.h>
 
 #include <algorithm>
@@ skipping 56 matching lines @@
 
 class Background;
 class Border;
 class ContextMenuController;
 class DragController;
 class FocusManager;
 class FocusTraversable;
 class LayoutManager;
 class NativeViewAccessibility;
 class ScrollView;
+class ViewObserver;
 class Widget;
 class WordLookupClient;
 
 namespace internal {
 class PreEventDispatchHandler;
 class PostEventDispatchHandler;
 class RootView;
 }
 
 /////////////////////////////////////////////////////////////////////////////
@@ skipping 906 matching lines @@
   // to scroll. ScrollView interprets a return value of 0 (or negative)
   // to scroll by a default amount.
   //
   // See VariableRowHeightScrollHelper and FixedRowHeightScrollHelper for
   // implementations of common cases.
   virtual int GetPageScrollIncrement(ScrollView* scroll_view,
                                      bool is_horizontal, bool is_positive);
   virtual int GetLineScrollIncrement(ScrollView* scroll_view,
                                      bool is_horizontal, bool is_positive);
 
+  void AddObserver(ViewObserver* observer);
+  void RemoveObserver(ViewObserver* observer);
+  bool HasObserver(const ViewObserver* observer) const;
+
  protected:
   // Used to track a drag. RootView passes this into
   // ProcessMousePressed/Dragged.
   struct DragInfo {
     // Sets possible_drag to false and start_x/y to 0. This is invoked by
     // RootView prior to invoke ProcessMousePressed.
     void Reset();
 
     // Sets possible_drag to true and start_pt to the specified point.
     // This is invoked by the target view if it detects the press may generate
@@ skipping 555 matching lines @@
   // Input  --------------------------------------------------------------------
 
   std::unique_ptr<ViewTargeter> targeter_;
 
   // Accessibility -------------------------------------------------------------
 
   // Belongs to this view, but it's reference-counted on some platforms
   // so we can't use a scoped_ptr. It's dereferenced in the destructor.
   NativeViewAccessibility* native_view_accessibility_;
 
+  // Observers -------------------------------------------------------------
+
+  base::ObserverList<ViewObserver> observers_;
+
   DISALLOW_COPY_AND_ASSIGN(View);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_VIEW_H_