Add View::AddedToWidget and RemovedFromWidget.

ui/views/view.h

Index: ui/views/view.h
diff --git a/ui/views/view.h b/ui/views/view.h
index fcbee85c1bf2155eef7639c5b96e75c6980c1b4f..d4868e6092bedee91fd9913049deb1ca7f88cce5 100644
--- a/ui/views/view.h
+++ b/ui/views/view.h
@@ -1116,6 +1116,14 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // FocusManager manages this view.
   virtual void NativeViewHierarchyChanged();
 
+  // This method is invoked for a view when it is attached to a hierarchy with
+  // a widget, i.e. GetWidget() starts returning a non-null result.
+  virtual void AddedToWidget();
+
+  // This method is invoked for a view when it is removed from a hierarchy with
+  // a widget.
+  virtual void RemovedFromWidget();
+
   // Painting ------------------------------------------------------------------
 
   // Responsible for calling Paint() on child Views. Override to control the
@@ -1308,7 +1316,8 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   void PropagateNativeViewHierarchyChanged();
 
   // Takes care of registering/unregistering accelerators if
-  // |register_accelerators| true and calls ViewHierarchyChanged().
+  // |register_accelerators| true, calls ViewHierarchyChanged() and
+  // AddedToWidget() or RemovedFromWidget() if necessary.
   void ViewHierarchyChangedImpl(bool register_accelerators,
                                 const ViewHierarchyChangedDetails& details);
 
@@ -1491,6 +1500,9 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // This view's children.
   Views children_;
 
+  // The widget this view is (possibily indirectly) attached to.
+  Widget* attached_widget_;

[sky, 2017/02/24 18:26:14]

We shouldn't need to cache the widget. This likely requires changing the
signature of ViewHierarchyChangedImpl() to indicate one of added to widget,
removed from widget or no change in widget status.

[msimonides, 2017/02/27 09:52:28]

Done.

+
 #if DCHECK_IS_ON()
   // True while iterating over |children_|. Used to detect and DCHECK when
   // |children_| is mutated during iteration.