Cocoa touch dimming background darker

Hey! I am making an app with a slider which slides out from the bottom and I would like to make other parts of开发者_如何学C the app a bit darker to show that the picker is the active component, like when alertbox comes, the background darkens. How do archieve this?

Overlay the elements you want to darken with a view that has a semi-transparent dark background (e.g. a black background with an alpha value of 0.5).