events: Start changing EventHandler interface to not return a value.

ui/base/events/event.h

Index: ui/base/events/event.h
diff --git a/ui/base/events/event.h b/ui/base/events/event.h
index 06687beb2cf7dfa4e7fc452e80f78fb33cf763ae..ad6b8efde8b3f08b577a29356b108fb58b70ee80 100644
--- a/ui/base/events/event.h
+++ b/ui/base/events/event.h
@@ -161,6 +161,15 @@ class UI_EXPORT Event {
   // Returns true if the event has a valid |native_event_|.
   bool HasNativeEvent() const;
 
+  // Immediately stops the propagation of the event. This must be called only
+  // from an EventHandler during an event-dispatch. Any event handler that may
+  // be in the list will not receive the event after this is called.

[Ben Goodger (Google), 2012/12/03 17:36:54]

One thing to note. Reading the W3C spec for DOMEvents, stop propagation only
takes effect after the current layer, e.g. for pre-target handlers all handlers
at the current level will be fired and then propagation stopped. Not sure we
care here, at least not yet.

[sadrul, 2012/12/03 17:45:20]

One of the use-cases for StopPropagation is when the target is destroyed by an
event-handler. In such cases, continuing to dispatch the event to other handlers
can cause crashes etc.

+  void StopPropagation();
+  bool stopped_propagation() const { return result_ & ui::ER_CONSUMED; }
+
+  void PreventDefault();
+  bool default_prevented() const { return result_ != ui::ER_UNHANDLED; }

[Ben Goodger (Google), 2012/12/03 17:36:54]

This reads a little awkwardly. I'm not sure I have any more precise suggestions
though. DOM's "canceled" is simpler but maybe vaguer?

[sadrul, 2012/12/03 17:45:20]

How about 'handled()' or 'processed()' (and 'MarkHandled'/'MarkProcessed')?

+
  protected:
   Event(EventType type, base::TimeDelta time_stamp, int flags);
   Event(const base::NativeEvent& native_event, EventType type, int flags);