mus: Add EventObserver to allow passively listening to UI events

components/mus/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 "components/mus/ws/window_manager_state.h"
 
 #include "base/memory/weak_ptr.h"
 #include "components/mus/ws/accelerator.h"
 #include "components/mus/ws/display_manager.h"
 #include "components/mus/ws/platform_display.h"
@@ skipping 311 matching lines @@
                                         ? base::TimeDelta::FromDays(1)
                                         : GetDefaultAckTimerDelay();
   event_ack_timer_.Start(FROM_HERE, max_delay, this,
                          &WindowManagerState::OnEventAckTimeout);
 
   tree_awaiting_input_ack_ = tree;
   if (accelerator) {
     event_awaiting_input_ack_ = ui::Event::Clone(event);
     post_target_accelerator_ = accelerator;
   }
+
+  // Ignore |tree| because it will receive the event via normal dispatch.
+  window_server()->SendToEventObservers(event, user_id_, tree);
+
   tree->DispatchInputEvent(target, event);
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// EventDispatcherDelegate:
+
 void WindowManagerState::OnAccelerator(uint32_t accelerator_id,
                                        const ui::Event& event) {
   DCHECK(IsActive());
   tree_->OnAccelerator(accelerator_id, event);
 }
 
 void WindowManagerState::SetFocusedWindowFromEventDispatcher(
     ServerWindow* new_focused_window) {
   DCHECK(IsActive());
   display_->SetFocusedWindow(new_focused_window);
@@ skipping 31 matching lines @@
     return;
   }
 
   base::WeakPtr<Accelerator> weak_accelerator;
   if (accelerator)
     weak_accelerator = accelerator->GetWeakPtr();
   DispatchInputEventToWindowImpl(target, in_nonclient_area, event,
                                  weak_accelerator);
 }
 
+void WindowManagerState::OnEventTargetNotFound(const ui::Event& event) {
+  window_server()->SendToEventObservers(event, user_id_,
+                                        nullptr /* ignore_tree */);
+}
+
 }  // namespace ws
 }  // namespace mus