Make event-targeting asynchronous in window server.

services/ui/ws/window_manager_state.cc

 // Copyright 2016 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 "services/ui/ws/window_manager_state.h"
 
 #include <utility>
 
 #include "base/logging.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 272 matching lines @@
   // The tree is no longer active, so no point in dispatching any further
   // events.
   std::queue<std::unique_ptr<QueuedEvent>> event_queue;
   event_queue.swap(event_queue_);
 }
 
 void WindowManagerState::ProcessEvent(const ui::Event& event,
                                       int64_t display_id) {
   // If this is still waiting for an ack from a previously sent event, then
   // queue up the event to be dispatched once the ack is received.
-  if (in_flight_event_details_) {
+  if (in_flight_event_details_ || event_dispatcher_.HittestInFlight()) {

[sky, 2017/05/15 21:20:23]

The code you have here will only work if EventDispatchDelegate calls to its
delegate when the queue is empty so that WMS knows to start processing the next
event.

[riajiang, 2017/05/17 02:01:59]

Actually I was looking at |in_flight_event_details_|
(https://cs.chromium.org/chromium/src/services/ui/ws/window_manager_state.h?ty...)
again and was wondering if it's true that it would be non-null until after we've
successfully dispatched the event to the client? (cuz it seems like we only call
ProcessNextEventFromQueue() when we are done processing+dispatching the event)
If that is true, then it means if there's a hit-test in flight in
EventDispatcher, the |in_flight_event_details_| would be non-null so it would
already go inside this if-block without checking IsHitTestInFlight()? 

[sky, 2017/05/17 18:14:43]

in_flight_event_details_ is only created from callbacks from EventDispatcher
(DispatchInputEventToWindow for one). So, if calling to EventDispatcher doesn't
immediately call to DispatchInputEventToWindow() in_flight_event_details_ won't
be set to a non-null value.

[riajiang, 2017/05/19 16:49:30]

I ended up keeping event_dispatcher_.IsHitTestInFlight() and notifying
WindowManagerState when hit-tests are done in EventDispatcher. Now the queue for
event targeting and event dispatching is in WindowManagerState and the queue for
other hit-test queries is in EventDispatcher. PTAL, thanks!

     if (!event_queue_.empty() && !event_queue_.back()->processed_target &&
         EventsCanBeCoalesced(*event_queue_.back()->event, event)) {
       event_queue_.back()->event = CoalesceEvents(
           std::move(event_queue_.back()->event), ui::Event::Clone(event));
       event_queue_.back()->display_id = display_id;
       return;
     }
     QueueEvent(event, nullptr, display_id);
     return;
   }
@@ skipping 408 matching lines @@
       window->RemoveObserver(this);
       orphaned_window_manager_display_roots_.erase(iter);
       return;
     }
   }
   NOTREACHED();
 }
 
 }  // namespace ws
 }  // namespace ui