Convert ui::GestureEvent coordinates during targeting

ui/events/event_targeter.h

 // Copyright (c) 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.
 
 #ifndef UI_EVENTS_EVENT_TARGETER_H_
 #define UI_EVENTS_EVENT_TARGETER_H_
 
 #include "base/compiler_specific.h"
 #include "ui/events/event.h"
 #include "ui/events/events_export.h"
 
 namespace ui {
 
 class Event;
 class EventTarget;
 class LocatedEvent;
 
 class EVENTS_EXPORT EventTargeter {
  public:
   virtual ~EventTargeter();
 
   // Returns the target |event| should be dispatched to. If there is no such
-  // target, this should return NULL.
+  // target, return NULL. If |event| is a located event, the location of |event|
+  // is in the coordinate space of |root|. Furthermore, the targeter can mutate
+  // the event (e.g., by changing the location of the event to be in the
+  // returned target's coordinate space) so that it can be dispatched to the
+  // target without any further modification.
   virtual EventTarget* FindTargetForEvent(EventTarget* root,
                                           Event* event);
 
   // Same as FindTargetForEvent(), but used for positional events. The location
   // etc. of |event| are in |root|'s coordinate system. When finding the target
   // for the event, the targeter can mutate the |event| (e.g. change the
   // coordinate to be in the returned target's coordinate system) so that it can
-  // be dispatched to the target without any farther modification.
+  // be dispatched to the target without any further modification.
+  // TODO(tdanderson|sadrul): This should not be in the public API of
+  //                          EventTargeter.
   virtual EventTarget* FindTargetForLocatedEvent(EventTarget* root,
                                                  LocatedEvent* event);
 
   // Returns true if |target| or one of its descendants can be a target of
   // |event|. This requires that |target| and its descendants are not
   // prohibited from accepting the event, and that the event is within an
   // actionable region of the target's bounds. Note that the location etc. of
   // |event| is in |target|'s parent's coordinate system.
   // TODO(tdanderson|sadrul): This function should be made non-virtual and
   //                          non-public.
   virtual bool SubtreeShouldBeExploredForEvent(EventTarget* target,
                                                const LocatedEvent& event);
 
   // Returns the next best target for |event| as compared to |previous_target|.
+  // |event| is in the local coordinate space of |previous_target|.
   // Also mutates |event| so that it can be dispatched to the returned target
   // (e.g., by changing |event|'s location to be in the returned target's
   // coordinate space).
   virtual EventTarget* FindNextBestTarget(EventTarget* previous_target,
                                           Event* event);
 
  protected:
   // Returns false if neither |target| nor any of its descendants are allowed
   // to accept |event| for reasons unrelated to the event's location or the
   // target's bounds. For example, overrides of this function may consider
   // attributes such as the visibility or enabledness of |target|. Note that
   // the location etc. of |event| is in |target|'s parent's coordinate system.
   virtual bool SubtreeCanAcceptEvent(EventTarget* target,
                                      const LocatedEvent& event) const;
 
   // Returns whether the location of the event is in an actionable region of the
   // target. Note that the location etc. of |event| is in the |target|'s
   // parent's coordinate system.
   virtual bool EventLocationInsideBounds(EventTarget* target,
                                          const LocatedEvent& event) const;
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_EVENT_TARGETER_H_