Darken screen, the same way an alert would?

Is there a way to darken the whole screen, the same wa开发者_开发知识库y that it would if an alert was shown?

If i'd have to go for a custom method of putting a black image over it with a low alpha setting, how would i manage to cover the navigationbar too?

Lower the alpha of the App Delegate's window:

AppDelegate *appDel = (AppDelegate *)[[UIApplication sharedApplication] delegate];
[appDel.window setAlpha:0.5];