ozone: Initialize a subsystem only if necessary.

ui/events/ozone/evdev/event_factory_evdev.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 "ui/events/ozone/evdev/event_factory_evdev.h"
 
 #include <fcntl.h>
 #include <linux/input.h>
 
 #include "base/debug/trace_event.h"
@@ skipping 178 matching lines @@
     }
       break;
     case DeviceEvent::REMOVE: {
       TRACE_EVENT1("ozone", "OnDeviceRemoved", "path", event.path().value());
       DetachInputDevice(event.path());
     }
       break;
   }
 }
 
+void EventFactoryEvdev::OnDispatcherListChanged() {
+  if (ui_task_runner_)
+    return;
+  ui_task_runner_ = base::MessageLoopProxy::current();
+  StartProcessingEvents();
+}
+
 void EventFactoryEvdev::DetachInputDevice(const base::FilePath& path) {
   TRACE_EVENT1("ozone", "DetachInputDevice", "path", path.value());
   CHECK(ui_task_runner_->RunsTasksOnCurrentThread());
 
   // Remove device from map.
   scoped_ptr<EventConverterEvdev> converter(converters_[path]);
   converters_.erase(path);
 
   if (converter) {
     // Cancel libevent notifications from this converter. This part must be
     // on UI since the polling happens on UI.
     converter->Stop();
 
     // Dispatch task to close from the worker pool, since close may block.
     base::WorkerPool::PostTask(
         FROM_HERE,
         base::Bind(&CloseInputDevice, path, base::Passed(&converter)),
         true);
   }
 }
 
 void EventFactoryEvdev::StartProcessingEvents() {
+  if (!ui_task_runner_)
+    return;
   CHECK(ui_task_runner_->RunsTasksOnCurrentThread());
 
   if (device_manager_ && !has_started_processing_events_) {
     has_started_processing_events_ = true;
     // Scan & monitor devices.
     device_manager_->AddObserver(this);
     device_manager_->ScanDevices(this);
   }
 }
 
 void EventFactoryEvdev::WarpCursorTo(gfx::AcceleratedWidget widget,
                                      const gfx::PointF& location) {
   if (cursor_) {
     cursor_->MoveCursorTo(widget, location);
     MouseEvent mouse_event(ET_MOUSE_MOVED,
                            cursor_->location(),
                            cursor_->location(),
                            modifiers_.GetModifierFlags(),
                            /* changed_button_flags */ 0);
     DispatchEvent(&mouse_event);
   }
 }
 
 }  // namespace ui