It is useful to know that you can reference the top level of your application from anywhere since it is a Singleton. This enables access to the application delegate as follows:
[[UIApplication sharedApplication] delegate];
This can be useful if you want declare your model at the top level of your app and need to reference it from other places. In this case you would need to cast the delegate reference:
StemStopwatchAppDelegate* ref =
(StemStopwatchAppDelegate*) [[UIApplication sharedApplication] delegate];
I have also found it necessary to reference UIApplicationDelegate in a view with a UITabBarController when I wanted to use a UIActionSheet (a bit like a popup dialog). This ensured that the actionsheet had focus and not the tabbar underneath when it was invoked:
[actionSheet showInView:ref.window];
