mus: Add watcher for all touch events.

services/ui/public/interfaces/window_tree.mojom

Index: services/ui/public/interfaces/window_tree.mojom
diff --git a/services/ui/public/interfaces/window_tree.mojom b/services/ui/public/interfaces/window_tree.mojom
index a15f7bf5c7117e6b21cc6037a3bb34d943b92fab..9d3bd14f209ef30fd33cd2651dca4548035d5163 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -72,8 +72,11 @@ interface WindowTree {
   // it to the WindowTreeClient via OnWindowInputEvent (if the event target is
   // this window tree) or OnEventObserved (if the target is another tree). The
   // client must supply a non-zero |observer_id|, which is reported back with
-  // observed events.  Set the matcher to null to clear the observer.
-  SetEventObserver(EventMatcher? matcher, uint32 observer_id);
+  // observed events.
+  AddEventObserver(EventMatcher? matcher, uint32 observer_id);

[sky, 2016/07/08 15:41:11]

In thinking about this a bit could we we implement > 1 matchers in the client
library? The client library would maintain the set of matchers and build a
matcher that matches all the supplied matchers. This way we minimize interaction
with the window server and keep the complexity in the client. Is that possible?

[sadrul, 2016/07/08 15:51:30]

I think this would be possible (and how I was thinking initially as well). But
it's not clear if we are going to need this. The touch-hud is going to be a
separate app, and it would need just the TouchEventWatcher. ash currently needs
just PointerWatcher. For some accessibility features (which I think should be a
separate app), we are going to need some event-watcher that match all touch and
mouse events. So with the use-cases we have right now, every client needs just
one type of watcher. So it would make sense to not introduce this yet, either on
the client or the server side, until we have a use-case where the client needs
more than one type of watcher. Maybe just leave a TODO for now?

[riajiang, 2016/07/11 20:13:22]

Changed back to support only one event matcher with TODOs and DCHECKs.

+
+  // Remove the event observer of the window tree with the observer_id.
+  RemoveEventObserver(uint32 observer_id);
 
   // Sets the specified bounds of the specified window.
   SetWindowBounds(uint32 change_id, uint32 window_id, gfx.mojom.Rect bounds);
@@ -353,12 +356,12 @@ interface WindowTreeClient {
   OnWindowInputEvent(uint32 event_id,
                      uint32 window,
                      ui.mojom.Event event,
-                     uint32 event_observer_id);
+                     array<uint32> event_observer_id);
 
   // Invoked when an |event| is sent via the EventObserver and not targeted at a
   // specific window. The |event_observer_id| is the one supplied to
   // SetEventObserver. The client should not acknowledge these events.
-  OnEventObserved(ui.mojom.Event event, uint32 event_observer_id);
+  OnEventObserved(ui.mojom.Event event, array<uint32> event_observer_id);
 
   // 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