How to create UIView that fills or dim the background as alertview does?

I have an app with tab bar and navigation bar.I want to show some loading kind of view full screen semi transparent so that user cannot use the tab bar and navigation bar untill the processing finishes. For eg:I want a look of that view when a alert view pops up and the backg开发者_开发知识库round goes dim and user cant use anything

You create a semi transparent view, add an opaque progress indicating subview in middle of it and add it as a subview over your tabbar controller's view. The semi transparent view should block all touches.