Introduce EventProcessor::OnEventProcessingStarted()

ui/events/event_processor.cc

Index: ui/events/event_processor.cc
diff --git a/ui/events/event_processor.cc b/ui/events/event_processor.cc
index 25c8c3fae92ca2b74e723fce69ee2fe6792b6c49..ddd18d84cf3c012ccec8cfeea1fdf0fa08372c36 100644
--- a/ui/events/event_processor.cc
+++ b/ui/events/event_processor.cc
@@ -14,7 +14,6 @@ EventDispatchDetails EventProcessor::OnEventFromSource(Event* event) {
   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.
@@ -26,32 +25,35 @@ EventDispatchDetails EventProcessor::OnEventFromSource(Event* 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 (!dispatch_original_event) {
-      if (event_to_dispatch->stopped_propagation())
-        event->StopPropagation();
-      else if (event_to_dispatch->handled())
-        event->SetHandled();
-    }
+  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 (details.dispatcher_destroyed)
-      return details;
+      if (details.dispatcher_destroyed)
+        return details;
 
-    if (details.target_destroyed || event->handled())
-      break;
+      if (details.target_destroyed || event->handled())
+        break;
 
-    target = targeter->FindNextBestTarget(target, event_to_dispatch);
+      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) {