Add ViewObserver to View for view updates

ui/views/view_observer.h

Index: ui/views/view_observer.h
diff --git a/ui/views/view_observer.h b/ui/views/view_observer.h
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+// 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/view.h"
+
+namespace gfx {
+class Rect;
+}
+
+namespace views {
+
+// Observer can listen to various events on the Views.
+class VIEWS_EXPORT ViewObserver {
+ public:
+  // Called whenever target |view| is being moved. If |old_parent| is null,
+  // the target is being added, otherwise it is being removed/reparented.
+  virtual void OnViewParentChanged(View* target, View* old_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_