Add a UIView above all, even the navigation bar

Question

You can do that by adding your view directly to the keyWindow:

UIView *myView = /* <- Your custom view */;
UIWindow *currentWindow = [UIApplication sharedApplication].keyWindow;
[currentWindow addSubview:myView];

UPDATE — For Swift 4.1 and above

let currentWindow: UIWindow? = UIApplication.shared.keyWindow
currentWindow?.addSubview(myView)

UPDATE for iOS13 and above

keyWindow is deprecated. You should use the following:

UIApplication.shared.windows.first(where: { $0.isKeyWindow })?.addSubview(myView)