mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/window_manager_state.cc

Index: components/mus/ws/window_manager_state.cc
diff --git a/components/mus/ws/window_manager_state.cc b/components/mus/ws/window_manager_state.cc
index c81bb8cb86b78b4d1ccfc2e6e0649a9e30cc96f3..a864cdea4013c5fe009578513acf89c10966e693 100644
--- a/components/mus/ws/window_manager_state.cc
+++ b/components/mus/ws/window_manager_state.cc
@@ -226,6 +226,15 @@ void WindowManagerState::ProcessEvent(const ui::Event& event) {
     QueueEvent(event, nullptr);
     return;
   }
+
+  // Only send events to any event observers after the previous event was acked,
+  // to maintain ordering with events sent via normal dispatch.
+  // TODO(jamescook): If the event dispatcher finds a target window, and that
+  // window's window tree has an event observer that matches the event, just
+  // send the event once via normal dispatch and let the client library trigger
+  // its event observer mechanism. http://crbug.com/605580
+  window_server()->SendToEventObservers(event);

[sky, 2016/04/21 17:23:03]

This comment doesn't match with the code. This function is called when Display
gets an Event, before EventDispatcher has processed the event.

If you want to send after the event has been acked I would expect this code to
be in OnEventAck (and you'll have to deal with the client we're waiting on going
away).

[James Cook, 2016/04/22 18:22:54]

This code is gone.

+
   event_dispatcher_.ProcessEvent(event);
 }
 
@@ -274,6 +283,11 @@ void WindowManagerState::ProcessNextEventFromQueue() {
       event_dispatcher_.ProcessEvent(*queued_event->event);
       return;
     }
+
+    // If the event has a target it was already sent to observers before its
+    // initial processing.
+    window_server()->SendToEventObservers(*queued_event->event);
+
     if (queued_event->processed_target->IsValid()) {
       DispatchInputEventToWindowImpl(
           queued_event->processed_target->window(),