Add the actual location coordinates to the touch cancel event

ui/aura/window_event_dispatcher.cc

 // Copyright 2014 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.
 
 #include "ui/aura/window_event_dispatcher.h"
 
 #include "base/bind.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
@@ skipping 122 matching lines @@
   DispatchDetails details =
       DispatchEvent(focused_window ? focused_window : window(), &event);
   if (details.dispatcher_destroyed)
     return;
 }
 
 void WindowEventDispatcher::DispatchGestureEvent(ui::GestureEvent* event) {
   DispatchDetails details = DispatchHeldEvents();
   if (details.dispatcher_destroyed)
     return;
-
   Window* target = GetGestureTarget(event);
   if (target) {
     event->ConvertLocationToTarget(window(), target);
     DispatchDetails details = DispatchEvent(target, event);
     if (details.dispatcher_destroyed)
       return;
   }
 }
 
 DispatchDetails WindowEventDispatcher::DispatchMouseExitAtPoint(
@@ skipping 369 matching lines @@
 ////////////////////////////////////////////////////////////////////////////////
 // WindowEventDispatcher, ui::GestureEventHelper implementation:
 
 bool WindowEventDispatcher::CanDispatchToConsumer(
     ui::GestureConsumer* consumer) {
   Window* consumer_window = ConsumerToWindow(consumer);
   return (consumer_window && consumer_window->GetRootWindow() == window());
 }
 
 void WindowEventDispatcher::DispatchCancelTouchEvent(ui::TouchEvent* event) {
+  // We observe that the cancel touch event's location got scale twice,
+  // because the location we use to create the touch cancel event
+  // is already scaled, and we scale it again in the PrepareEventForDispatch
+  // function. Thus, we revert the location back in the below function.

[tdresser, 2014/08/15 14:22:04]

I think this could be a bit clearer. The key thing to mention is what units we
have now, and what units we need to convert to.

Perhaps:
The touchcancel event's location is based on the last known location of the
pointer, in dips. OnEventFromSource expects events with co-ordinates in raw
pixels, so we convert back to raw pixels here.

[sadrul, 2014/08/15 16:10:43]

What if you we don't transform the event in
WindowEventDispatcher::PrepareEventForDispatch() if the event was not created
from a native event (i.e. check event->HasNativeEvent() before doing the
transform)? Would that work?

[tdresser, 2014/08/15 16:48:44]

Yeah, that sounds reasonable.

[lanwei, 2014/08/15 17:09:28]

This is a very good suggestion, I will give it a try.

[lanwei, 2014/08/27 03:54:03]

Done.

+  event->UpdateForRootTransform(host_->GetRootTransform());
   DispatchDetails details = OnEventFromSource(event);
   if (details.dispatcher_destroyed)
     return;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // WindowEventDispatcher, WindowObserver implementation:
 
 void WindowEventDispatcher::OnWindowDestroying(Window* window) {
   if (!host_->window()->Contains(window))
@@ skipping 355 matching lines @@
     if (!ui::GestureRecognizer::Get()->ProcessTouchEventPreDispatch(orig_event,
                                                                     target)) {
       event->SetHandled();
     }
   }
 
   PreDispatchLocatedEvent(target, event);
 }
 
 }  // namespace aura