Notify launcher page with onTransitionChanged event

ui/app_list/views/app_list_main_view.h

Index: ui/app_list/views/app_list_main_view.h
diff --git a/ui/app_list/views/app_list_main_view.h b/ui/app_list/views/app_list_main_view.h
index a87ae16f213f374064dc64f5e3a245b5d7d25c15..4dc534cb2002f3128592484ca8d74a4707dbf95c 100644
--- a/ui/app_list/views/app_list_main_view.h
+++ b/ui/app_list/views/app_list_main_view.h
@@ -9,6 +9,7 @@
 
 #include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
+#include "base/observer_list.h"
 #include "base/timer/timer.h"
 #include "ui/app_list/app_list_export.h"
 #include "ui/app_list/views/apps_grid_view_delegate.h"
@@ -31,6 +32,18 @@ class ContentsView;
 class PaginationModel;
 class SearchBoxView;
 
+class AppListMainViewObserver {
+ public:
+  // Invoked after a new contents view is created.
+  virtual void OnContentsViewCreated() {}
+
+  // Invoked before the contens view is destroyed.
+  virtual void OnContentsViewDestroying() {}
+
+ protected:
+  virtual ~AppListMainViewObserver() {}
+};
+
 // AppListMainView contains the normal view of the app list, which is shown
 // when the user is signed in.
 class APP_LIST_EXPORT AppListMainView : public views::View,
@@ -79,6 +92,9 @@ class APP_LIST_EXPORT AppListMainView : public views::View,
   // Called when the search box's visibility is changed.
   void NotifySearchBoxVisibilityChanged();
 
+  void AddObserver(AppListMainViewObserver* observer);
+  void RemoveObserver(AppListMainViewObserver* observer);
+
  private:
   class IconLoader;
 
@@ -130,6 +146,8 @@ class APP_LIST_EXPORT AppListMainView : public views::View,
 
   base::WeakPtrFactory<AppListMainView> weak_ptr_factory_;
 
+  ObserverList<AppListMainViewObserver, true> observers_;
+
   DISALLOW_COPY_AND_ASSIGN(AppListMainView);
 };