Introduce EventProcessor::OnEventProcessingStarted()

ui/events/event_processor.cc

 // Copyright 2013 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.
 
 #include "ui/events/event_processor.h"
 
 #include "ui/events/event_target.h"
 #include "ui/events/event_targeter.h"
 
 namespace ui {
 
 EventDispatchDetails EventProcessor::OnEventFromSource(Event* event) {
   EventTarget* root = GetRootTarget();
   CHECK(root);
   EventTargeter* targeter = root->GetEventTargeter();
   CHECK(targeter);
-  PrepareEventForDispatch(event);
 
   // If |event| is in the process of being dispatched or has already been
   // dispatched, then dispatch a copy of the event instead.
   bool dispatch_original_event = event->phase() == EP_PREDISPATCH;
   Event* event_to_dispatch = event;
   scoped_ptr<Event> event_copy;
   if (!dispatch_original_event) {
     event_copy = Event::Clone(*event);
     event_to_dispatch = event_copy.get();
   }
 
-  EventTarget* target = targeter->FindTargetForEvent(root, event_to_dispatch);
   EventDispatchDetails details;
-  while (target) {
-    details = DispatchEvent(target, event_to_dispatch);
+  if (OnEventProcessingStarted(event)) {
+    EventTarget* target = targeter->FindTargetForEvent(root, event_to_dispatch);
+    while (target) {
+      details = DispatchEvent(target, event_to_dispatch);
 
-    if (!dispatch_original_event) {
-      if (event_to_dispatch->stopped_propagation())
-        event->StopPropagation();
-      else if (event_to_dispatch->handled())
-        event->SetHandled();
+      if (!dispatch_original_event) {
+        if (event_to_dispatch->stopped_propagation())
+          event->StopPropagation();
+        else if (event_to_dispatch->handled())
+          event->SetHandled();
+      }
+
+      if (details.dispatcher_destroyed)
+        return details;
+
+      if (details.target_destroyed || event->handled())
+        break;
+
+      target = targeter->FindNextBestTarget(target, event_to_dispatch);
     }
-
-    if (details.dispatcher_destroyed)
-      return details;
-
-    if (details.target_destroyed || event->handled())
-      break;
-
-    target = targeter->FindNextBestTarget(target, event_to_dispatch);
   }
 
   OnEventProcessingFinished(event);
   return details;
 }
 
-void EventProcessor::PrepareEventForDispatch(Event* event) {
+bool EventProcessor::OnEventProcessingStarted(Event* event) {
+  return true;
 }
 
 void EventProcessor::OnEventProcessingFinished(Event* event) {
 }
 
 }  // namespace ui