Get Application's main window
The UIApplicationDelegate usually has a reference to the "main window":
[[[UIApplication sharedApplication] delegate] window];Furthermore, UIApplication has an array of windows [[UIApplication sharedApplication] windows].
See the UIApplication Class Reference.
I'm not 100% sure this works in every case but this should work:
UIWindow *mainWindow = [UIApplication sharedApplication].windows[0];The windows are ordered back to front so the main window should always be at index 0.
Swift 3.0 version of rmaddy's answer:
let window = UIApplication.shared.windows.firstI also should add that since iOS 8.0 UIAlertController has replaced UIAlertView and being a view controller you may no longer face the issue of new windows being created.