Make UserActivityDetector a singleton

ui/wm/core/user_activity_detector.h

 // 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.
 
 #ifndef UI_WM_CORE_USER_ACTIVITY_DETECTOR_H_
 #define UI_WM_CORE_USER_ACTIVITY_DETECTOR_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/observer_list.h"
 #include "base/time/time.h"
 #include "ui/events/event_handler.h"
 #include "ui/wm/wm_export.h"
 
 namespace wm {
 
 class UserActivityObserver;
 
 // Watches for input events and notifies observers that the user is active.
 class WM_EXPORT UserActivityDetector : public ui::EventHandler {
  public:
   // Minimum amount of time between notifications to observers.
   static const int kNotifyIntervalMs;
 
   // Amount of time that mouse events should be ignored after notification
   // is received that displays' power states are being changed.
   static const int kDisplayPowerChangeIgnoreMouseMs;
 
-  UserActivityDetector();
-  ~UserActivityDetector() override;
+  // Creates a UserActivityDetector instance.
+  static void Create();
+
+  // Deletes the UserActivityDetector instance.
+  static void Shutdown();

[oshima, 2014/10/31 16:53:31]

Can you keep ctor/dtor, and creation in shell. Just add static accessor only.

[pkotwicz, 2014/10/31 16:58:29]

For the sake of interest, which other classes have a static getter but not a
static constructor and destructor?

+
+  // Returns the UserActivityDetector instance if one was created.
+  static UserActivityDetector* Get();
 
   base::TimeTicks last_activity_time() const { return last_activity_time_; }
 
   void set_now_for_test(base::TimeTicks now) { now_for_test_ = now; }
 
   bool HasObserver(UserActivityObserver* observer) const;
   void AddObserver(UserActivityObserver* observer);
   void RemoveObserver(UserActivityObserver* observer);
 
   // Called when displays are about to be turned on or off.
   void OnDisplayPowerChanging();
 
   // ui::EventHandler implementation.
   void OnKeyEvent(ui::KeyEvent* event) override;
   void OnMouseEvent(ui::MouseEvent* event) override;
   void OnScrollEvent(ui::ScrollEvent* event) override;
   void OnTouchEvent(ui::TouchEvent* event) override;
   void OnGestureEvent(ui::GestureEvent* event) override;
 
  private:
+  UserActivityDetector();
+  ~UserActivityDetector() override;
+
   // Returns |now_for_test_| if set or base::TimeTicks::Now() otherwise.
   base::TimeTicks GetCurrentTime() const;
 
   // Updates |last_activity_time_|.  Additionally notifies observers and
   // updates |last_observer_notification_time_| if enough time has passed
   // since the last notification.
   void HandleActivity(const ui::Event* event);
 
   ObserverList<UserActivityObserver> observers_;
 
@@ skipping 11 matching lines @@
   // is to avoid reporting mouse events that occur when displays are turned
   // on or off as user activity.
   base::TimeTicks honor_mouse_events_time_;
 
   DISALLOW_COPY_AND_ASSIGN(UserActivityDetector);
 };
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_USER_ACTIVITY_DETECTOR_H_