Make ui::InputDeviceEventObserver provide a default implementation

ui/events/devices/input_device_event_observer.h

Index: ui/events/devices/input_device_event_observer.h
diff --git a/ui/events/devices/input_device_event_observer.h b/ui/events/devices/input_device_event_observer.h
index 707fea2095d6cf2d40675aba9cfcd4d5500716dc..a4b5ac99488ae0fcfdbd70cdc09438acc3a694ed 100644
--- a/ui/events/devices/input_device_event_observer.h
+++ b/ui/events/devices/input_device_event_observer.h
@@ -12,12 +12,13 @@ namespace ui {
 // DeviceDataManager observer used to announce input hotplug events.
 class EVENTS_DEVICES_EXPORT InputDeviceEventObserver {
  public:
-  virtual ~InputDeviceEventObserver() {}
+  virtual void OnKeyboardDeviceConfigurationChanged() {}
+  virtual void OnTouchscreenDeviceConfigurationChanged() {}
+  virtual void OnMouseDeviceConfigurationChanged() {}
+  virtual void OnTouchpadDeviceConfigurationChanged() {}
 
-  virtual void OnKeyboardDeviceConfigurationChanged() = 0;
-  virtual void OnTouchscreenDeviceConfigurationChanged() = 0;
-  virtual void OnMouseDeviceConfigurationChanged() = 0;
-  virtual void OnTouchpadDeviceConfigurationChanged() = 0;
+ protected:
+  virtual ~InputDeviceEventObserver() {}

[sadrul, 2015/02/20 20:12:27]

I think this should remain public. Maybe make the default ctor protected
instead?

[pkotwicz, 2015/02/20 21:22:50]

Ok, I've made the destructor public!

We seem to make the destructor protected in some cases. WidgetObserver,
ShellObserver and EnvObserver do this. Is there a reason for the preference in
whether the destructor should be public/protected

[sadrul, 2015/02/23 17:02:52]

I think the point of making the dtor protected is to disallow creating this
observer (because it doesn't do anything by itself). But this can also prevent
someone from doing scoped_ptr<Observer> obs(new
SomeImplementationOfObserver(...));. So instead, the better way would be to make
the ctor protected, so that it can't be instantiated directly.

The default impl of some observers manages their own lifetime (e.g. deletes
themselves from some override). It makes sense to make the dtor protected in
those cases. But that's not the case here (or for the other observers you list).

 };
 
 }  // namespace ui