Add ViewObserver to View for view updates

ui/views/view.h

Index: ui/views/view.h
diff --git a/ui/views/view.h b/ui/views/view.h
index bb5ebdf78cb1213e16c4dd0cd41c4ad73e510466..616938e77319e47aa59b81f9f0adac840bf81956 100644
--- a/ui/views/view.h
+++ b/ui/views/view.h
@@ -74,6 +74,7 @@ class FocusTraversable;
 class LayoutManager;
 class NativeViewAccessibility;
 class ScrollView;
+class ViewObserver;
 class Widget;
 class WordLookupClient;
 
@@ -1000,6 +1001,10 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   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.
@@ -1435,6 +1440,13 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
               const gfx::Point& press_pt,
               ui::DragDropTypes::DragEventSource source);
 
+  // Called after the |child| has been added to its parent. Called on observers
+  // listening to |child|'s parent.
+  void NotifyChildViewAdded(View* child);

[sky, 2016/11/22 04:40:48]

You only call these functions from one place, inline the implementation there.

[Sarmad Hashmi, 2016/11/22 04:47:00]

Moved the implementations to where they were called from instead.

+  // Called after the |child| has been removed from its |parent|. Called on
+  // observers listening to |parent|.
+  void NotifyChildViewRemoved(View* child);
+
   //////////////////////////////////////////////////////////////////////////////
 
   // Creation and lifetime -----------------------------------------------------
@@ -1575,6 +1587,10 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // 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);
 };