Add View::AddedToWidget and RemovedFromWidget.

ui/views/view.cc

Index: ui/views/view.cc
diff --git a/ui/views/view.cc b/ui/views/view.cc
index 04c5f2fae25957c3ca3b706654e263fc8b3c6638..1a173b8760e1448c638325907cd0983f28aebee1 100644
--- a/ui/views/view.cc
+++ b/ui/views/view.cc
@@ -196,8 +196,10 @@ void View::AddChildViewAt(View* view, int index) {
   // If |view| has a parent, remove it from its parent.
   View* parent = view->parent_;
   ui::NativeTheme* old_theme = NULL;
+  Widget* old_widget = NULL;
   if (parent) {
     old_theme = view->GetNativeTheme();
+    old_widget = view->GetWidget();
     if (parent == this) {
       ReorderChildView(view, index);
       return;
@@ -248,6 +250,9 @@ void View::AddChildViewAt(View* view, int index) {
   if (widget) {
     RegisterChildrenForVisibleBoundsNotification(view);
 
+    if (widget != old_widget)
+      view->PropagateAddedToWidget();

[sky, 2017/02/27 16:46:54]

I would like to avoid more tree walking. Can't you incorporate the additional
notification in ViewHierarchyChangedImpl?

[msimonides, 2017/02/28 10:07:27]

Ok.
I will merge PropagateAddedToWidget into PropagateAddNotifications and
PropagateRemovedFromWidget into PropagateRemoveNotifications.
Calling the notifications from ViewHierarchyChangedImpl would require more
complex argument passing (widget added, widget removed, widget not changed).

Alternatively, ViewHierarchyChangedDetails could be extended with the
information about widget change. I think this would be overly complex but I can
do this if you wish.

[sky, 2017/02/28 17:35:33]

That's fine. These are implementation details of Views.
I agree. I prefer separate functions for subclasses.

[msimonides, 2017/03/01 10:31:52]

Done.

+
     if (view->visible())
       view->SchedulePaint();
   }
@@ -1516,6 +1521,10 @@ void View::NativeViewHierarchyChanged() {
   }
 }
 
+void View::AddedToWidget() {}
+
+void View::RemovedFromWidget() {}
+
 // Painting --------------------------------------------------------------------
 
 void View::PaintChildren(const ui::PaintContext& context) {
@@ -1934,8 +1943,10 @@ void View::DoRemoveChildView(View* view,
     if (view->visible())
       view->SchedulePaint();
 
-    if (!new_parent || new_parent->GetWidget() != widget)
+    if (!new_parent || new_parent->GetWidget() != widget) {
       widget->NotifyWillRemoveView(view);
+      view->PropagateRemovedFromWidget();
+    }
   }
 
   // Make sure the layers belonging to the subtree rooted at |view| get
@@ -2027,6 +2038,24 @@ void View::PropagateNativeThemeChanged(const ui::NativeTheme* theme) {
   OnNativeThemeChanged(theme);
 }
 
+void View::PropagateAddedToWidget() {
+  {
+    internal::ScopedChildrenLock lock(this);
+    for (auto* child : children_)
+      child->PropagateAddedToWidget();
+  }
+  AddedToWidget();
+}
+
+void View::PropagateRemovedFromWidget() {
+  {
+    internal::ScopedChildrenLock lock(this);
+    for (auto* child : children_)
+      child->PropagateRemovedFromWidget();
+  }
+  RemovedFromWidget();
+}
+
 // Size and disposition --------------------------------------------------------
 
 void View::PropagateVisibilityNotifications(View* start, bool is_visible) {