How do you create an invisible button that covers the whole screen including navigation controller?

I have a simple tableview that navigates t开发者_运维知识库o a detailed view with a navigation controller. In the detailed view I need to create a button which when pressed isolates all other buttons displayed on the screen including the navigation controller back button. I know how to get the invisible button to cover most of buttons the screen but I can't get it to cover the navigation controller back button as this has been created in the parent view. How do I pop it over the top of the navigation controller from the detail view?

You can insert the button as a direct subview of the key window.

[[UIApplication sharedApplication].keyWindow addSubview:theButton];

(why not use an UIAlertView or UIActionSheet?)