events: Dispatch all events through EventHandler::OnEvent from the dispatcher.

ui/base/events/event_handler.h

 // Copyright (c) 2012 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_BASE_EVENTS_EVENT_HANDLER_H_
 #define UI_BASE_EVENTS_EVENT_HANDLER_H_
 
 #include <vector>
 
 #include "ui/base/events/event_constants.h"
@@ skipping 10 matching lines @@
 
 class EventTarget;
 
 // Dispatches events to appropriate targets. The default implementations return
 // ER_UNHANDLED for all events.
 class UI_EXPORT EventHandler {
  public:
   EventHandler();
   virtual ~EventHandler();
 
+  // This is called for all events. The default implementation routes the event
+  // to one of the event-specific callbacks (OnKeyEvent, OnMouseEvent etc.). If
+  // this is overridden, then normally, the override should chain into the
+  // default implementation for un-handled events.
   virtual EventResult OnEvent(Event* event);
 
   virtual EventResult OnKeyEvent(KeyEvent* event);
 
   virtual EventResult OnMouseEvent(MouseEvent* event);
 
   virtual EventResult OnScrollEvent(ScrollEvent* event);
 
   virtual EventResult OnTouchEvent(TouchEvent* event);
 
   virtual EventResult OnGestureEvent(GestureEvent* event);
 };
 
 typedef std::vector<EventHandler*> EventHandlerList;
 
 }  // namespace ui
 
 #endif  // UI_BASE_EVENTS_EVENT_HANDLER_H_