Timer slack for slacker developers

If you have a lot of programs running that subscribe to these events, then you have the problem that half the world wakes up to handle them and your performance goes south. So there needs to be also some mechanism that delivers these events only to apps where it's still relevant. When such state events are relevant to an app, varies between states.

As an example from phone UIs: non-visible applications shouldn't be interested about screen blanking event. Apps themselves could unsubscribe from events when they become non-visible, and subscribe back when they again become visible, but with composited windows there's AFAIK no standard message/property to tell about that (N900 Hildon widgets & N9 MeegoTouch have their own window properties for this).