Enumerate the return value of dispatchEvent so it is clear.

third_party/WebKit/Source/core/events/EventTarget.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  *           (C) 2007, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ skipping 231 matching lines @@
 }
 
 bool EventTarget::clearAttributeEventListener(const AtomicString& eventType)
 {
     EventListener* listener = getAttributeEventListener(eventType);
     if (!listener)
         return false;
     return removeEventListener(eventType, listener, false);
 }
 
-bool EventTarget::dispatchEventForBindings(PassRefPtrWillBeRawPtr<Event> event, ExceptionState& exceptionState)
+bool EventTarget::dispatchEventForBindings(PassRefPtrWillBeRawPtr<Event> dispatchEvent, ExceptionState& exceptionState)
 {
+    RefPtrWillBeRawPtr<Event> event = dispatchEvent;

[philipj_slow, 2016/02/25 15:23:46]

If this is to protect the event for its use in the return clause, it can be
reverted if that is changed.

[dtapuska, 2016/02/25 22:06:10]

Done.

     if (event->type().isEmpty()) {
         exceptionState.throwDOMException(InvalidStateError, "The event provided is uninitialized.");
         return false;
     }
     if (event->isBeingDispatched()) {
         exceptionState.throwDOMException(InvalidStateError, "The event is already being dispatched.");
         return false;
     }
 
     if (!executionContext())
         return false;
 
     event->setTrusted(false);
-    return dispatchEventInternal(event);
+
+    // Return whether the event was cancelled or not to JS not that it
+    // might have actually been handled; so throw away the result of
+    // dispatchEventInternal.
+    dispatchEventInternal(event);
+    return !event->defaultPrevented();

[philipj_slow, 2016/02/25 15:23:46]

Will this not be equivalent to dispatchEventInternal(event) !=
DispatchEventResult::CanceledByEventHandler? It would be worrying if there were
any difference.

[dtapuska, 2016/02/25 22:06:10]

Done.

 }
 
-bool EventTarget::dispatchEvent(PassRefPtrWillBeRawPtr<Event> event)
+DispatchEventResult EventTarget::dispatchEvent(PassRefPtrWillBeRawPtr<Event> event)
 {
     event->setTrusted(true);
     return dispatchEventInternal(event);
 }
 
-bool EventTarget::dispatchEventInternal(PassRefPtrWillBeRawPtr<Event> event)
+DispatchEventResult EventTarget::dispatchEventInternal(PassRefPtrWillBeRawPtr<Event> event)
 {
     event->setTarget(this);
     event->setCurrentTarget(this);
     event->setEventPhase(Event::AT_TARGET);
-    bool defaultWasNotPrevented = fireEventListeners(event.get());
+    DispatchEventResult result = fireEventListeners(event.get());
     event->setEventPhase(0);
-    return defaultWasNotPrevented;
+    return result;
 }
 
 void EventTarget::uncaughtExceptionInEventHandler()
 {
 }
 
 static const AtomicString& legacyType(const Event* event)
 {
     if (event->type() == EventTypeNames::transitionend)
         return EventTypeNames::webkitTransitionEnd;
@@ skipping 47 matching lines @@
             if (listenersVector)
                 UseCounter::count(executingWindow->document(), prefixedAndUnprefixedFeature);
             else
                 UseCounter::count(executingWindow->document(), prefixedFeature);
         } else if (listenersVector) {
             UseCounter::count(executingWindow->document(), unprefixedFeature);
         }
     }
 }
 
-bool EventTarget::fireEventListeners(Event* event)
+DispatchEventResult EventTarget::fireEventListeners(Event* event)
 {
     ASSERT(!EventDispatchForbiddenScope::isEventDispatchForbidden());
     ASSERT(event && !event->type().isEmpty());
 
     EventTargetData* d = eventTargetData();
     if (!d)
-        return true;
+        return DispatchEventResult::NotCanceled;
 
     EventListenerVector* legacyListenersVector = nullptr;
     AtomicString legacyTypeName = legacyType(event);
     if (!legacyTypeName.isEmpty())
         legacyListenersVector = d->eventListenerMap.find(legacyTypeName);
 
     EventListenerVector* listenersVector = d->eventListenerMap.find(event->type());
 
     if (listenersVector) {
         fireEventListeners(event, d, *listenersVector);
     } else if (legacyListenersVector) {
         AtomicString unprefixedTypeName = event->type();
         event->setType(legacyTypeName);
         fireEventListeners(event, d, *legacyListenersVector);
         event->setType(unprefixedTypeName);
     }
 
     Editor::countEvent(executionContext(), event);
     countLegacyEvents(legacyTypeName, listenersVector, legacyListenersVector);
-    return !event->defaultPrevented();
+    return event->eventResult();
 }
 
 void EventTarget::fireEventListeners(Event* event, EventTargetData* d, EventListenerVector& entry)
 {
     RefPtrWillBeRawPtr<EventTarget> protect(this);
 
     // Fire all listeners registered for this event. Don't fire listeners removed
     // during event dispatch. Also, don't fire event listeners added during event
     // dispatch. Conveniently, all new event listeners will be added after or at
     // index |size|, so iterating up to (but not including) |size| naturally excludes
@@ skipping 87 matching lines @@
     // they have one less listener to invoke.
     if (d->firingEventIterators) {
         for (size_t i = 0; i < d->firingEventIterators->size(); ++i) {
             d->firingEventIterators->at(i).iterator = 0;
             d->firingEventIterators->at(i).end = 0;
         }
     }
 }
 
 } // namespace blink