mus: Add EventObserver to allow passively listening to UI events

components/mus/public/interfaces/window_tree.mojom

Index: components/mus/public/interfaces/window_tree.mojom
diff --git a/components/mus/public/interfaces/window_tree.mojom b/components/mus/public/interfaces/window_tree.mojom
index eb2ef580ef31b7b8fb99e0eab0494d16f6976ed4..698047c14ceed65497f18a0bd8d1ad6ce76dbbad 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -7,6 +7,7 @@ module mus.mojom;
 import "components/mus/public/interfaces/compositor_frame.mojom";
 import "components/mus/public/interfaces/cursor.mojom";
 import "components/mus/public/interfaces/input_events.mojom";
+import "components/mus/public/interfaces/input_event_matcher.mojom";
 import "components/mus/public/interfaces/mus_constants.mojom";
 import "components/mus/public/interfaces/surface_id.mojom";
 import "components/mus/public/interfaces/window_manager.mojom";
@@ -109,6 +110,12 @@ interface WindowTree {
   // if |window_id| does not currently have capture.
   ReleaseCapture(uint32 change_id, uint32 window_id);
 
+  // Sets an observer that monitors all events, even if they are not targeted
+  // at a window in this tree. If an event matchs |matcher| the observer reports
+  // it to the WindowTreeClient via OnEventObserved(). Each client may only have
+  // one observer at a time. Set the matcher to null to clear the observer.
+  SetEventObserver(EventMatcher? matcher);
+
   // Sets the specified bounds of the specified window.
   SetWindowBounds(uint32 change_id, uint32 window_id, mojo.Rect bounds);
 
@@ -358,9 +365,17 @@ interface WindowTreeClient {
   // Invoked when an event is targeted at the specified window. The client must
   // call WindowTree::OnWindowInputEventAck() with the same |event_id| to notify
   // that the event has been processed, and with an EventResult value to notify
-  // if the event was consumed. The client will not receive farther events until
-  // the event is ack'ed.
-  OnWindowInputEvent(uint32 event_id, uint32 window, Event event);
+  // if the event was consumed. |matched_observer| is true if the event also
+  // matched the active event observer for this client. The client will not
+  // receive farther events until the event is ack'ed.
+  OnWindowInputEvent(uint32 event_id,
+                     uint32 window,
+                     Event event,
+                     bool matched_observer);

[sky, 2016/04/22 20:18:48]

Rather than a bool for matched_observer I think you should use an id. If you
don't use an id it means the client doesn't have a way to ensure they know which
eventmatcher matched. For example. Lets say the client has an EventMatcher
installed and wants to replace it with a new one. If there was an in flight
event that only matched the first eventmatcher the client will send it to the
wrong place.

I recommend having SetEventObserver take a client assigned int it that is passed
through OnWindowInputEvent.

[James Cook, 2016/04/22 21:25:08]

Done.

+
+  // Invoked when an event is sent via the EventObserver and not targeted at a
+  // specific window. The client does not need to acknowledge these events.

[sky, 2016/04/22 20:18:48]

'does not need to' -> should not

[James Cook, 2016/04/22 21:25:08]

Done.

+  OnEventObserved(Event event);
 
   // Called in two distinct cases: when a window known to the connection gains
   // focus, or when focus moves from a window known to the connection to a