Add ViewObserver to View for view updates

ui/views/view_observer.h

+// Copyright 2016 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_OBSERVER_H_
+#define UI_VIEWS_VIEW_OBSERVER_H_
+
+#include "ui/views/views_export.h"
+
+namespace views {
+
+class View;
+
+// Observer can listen to various events on the Views.
+class VIEWS_EXPORT ViewObserver {
+ public:
+  // Called when |child| is added to its parent. Invoked on the parent which has
+  // the |child| added to it.
+  virtual void OnChildViewAdded(View* child) {}
+
+  // Called when |child| is removed from its |parent|.
+  virtual void OnChildViewRemoved(View* child, View* parent) {}
+
+  // Called when View::SetVisible() is called with a new value.
+  // The |view| may still be hidden at this point.
+  virtual void OnViewVisibilityChanged(View* view) {}
+
+  virtual void OnViewEnabledChanged(View* view) {}
+
+  virtual void OnViewBoundsChanged(View* view) {}
+
+  // Invoked whenever a child is moved to another index. This is called on the
+  // parent view. |view| is the child view being moved.
+  virtual void OnChildViewReordered(View* view) {}
+
+ protected:
+  virtual ~ViewObserver() {}
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_VIEW_OBSERVER_H_