mus: Add EventObserver to allow passively listening to UI events

components/mus/public/cpp/lib/window_tree_client_impl.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 "components/mus/public/cpp/lib/window_tree_client_impl.h"
 
 #include <stddef.h>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 847 matching lines @@
 
   InFlightPropertyChange new_change(window, name, new_data);
   if (ApplyServerChangeToExistingInFlightChange(new_change))
     return;
 
   WindowPrivate(window).LocalSetSharedProperty(name, std::move(new_data));
 }
 
 void WindowTreeClientImpl::OnWindowInputEvent(uint32_t event_id,
                                               Id window_id,
-                                              mojom::EventPtr event) {
+                                              mojom::EventPtr event,
+                                              uint32_t event_observer_id) {
+  // TODO(jamescook): If event_observer_id is non-zero, also route the event to
+  // PointerWatchers.
+
   Window* window = GetWindowByServerId(window_id);
   if (!window || !window->input_event_handler_) {
     tree_->OnWindowInputEventAck(event_id, mojom::EventResult::UNHANDLED);
     return;
   }
 
   std::unique_ptr<base::Callback<void(mojom::EventResult)>> ack_callback(
       new base::Callback<void(mojom::EventResult)>(
           base::Bind(&mojom::WindowTree::OnWindowInputEventAck,
                      base::Unretained(tree_), event_id)));
   window->input_event_handler_->OnWindowInputEvent(
       window, *event.To<std::unique_ptr<ui::Event>>().get(), &ack_callback);
 
   // The handler did not take ownership of the callback, so we send the ack,
   // marking the event as not consumed.
   if (ack_callback)
     ack_callback->Run(mojom::EventResult::UNHANDLED);
 }
 
+void WindowTreeClientImpl::OnEventObserved(mojom::EventPtr event_in,
+                                           uint32_t event_observer_id) {
+  // TODO(jamescook): Route the observed event to PointerWatchers.
+}
+
 void WindowTreeClientImpl::OnWindowFocused(Id focused_window_id) {
   Window* focused_window = GetWindowByServerId(focused_window_id);
   InFlightFocusChange new_change(this, focused_window);
   if (ApplyServerChangeToExistingInFlightChange(new_change))
     return;
 
   LocalSetFocus(focused_window);
 }
 
 void WindowTreeClientImpl::OnWindowPredefinedCursorChanged(
@@ skipping 134 matching lines @@
 
 void WindowTreeClientImpl::SetUnderlaySurfaceOffsetAndExtendedHitArea(
     Window* window,
     const gfx::Vector2d& offset,
     const gfx::Insets& hit_area) {
   window_manager_internal_client_->SetUnderlaySurfaceOffsetAndExtendedHitArea(
       server_id(window), offset.x(), offset.y(), mojo::Insets::From(hit_area));
 }
 
 }  // namespace mus