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

ui/base/events/event.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_H_
 #define UI_BASE_EVENTS_EVENT_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/event_types.h"
@@ skipping 143 matching lines @@
   }
 
   bool IsFlingScrollEvent() const {
     return type_ == ET_SCROLL_FLING_CANCEL ||
            type_ == ET_SCROLL_FLING_START;
   }
 
   // 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);
   Event(const Event& copy);
   void set_type(EventType type) { type_ = type; }
   void set_delete_native_event(bool delete_native_event) {
     delete_native_event_ = delete_native_event;
   }
   void set_time_stamp(base::TimeDelta time_stamp) { time_stamp_ = time_stamp; }
 
@@ skipping 473 matching lines @@
   // This value is stored as a bitfield because the number of touch ids varies,
   // but we currently don't need more than 32 touches at a time.
   const unsigned int touch_ids_bitfield_;
 
   DISALLOW_COPY_AND_ASSIGN(GestureEvent);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_EVENTS_EVENT_H_