View Controllers(Chapter 7 of iOS Programming: The Big Nerd Ranch Guide)

UIViewController has several methods that get called at certain times:

viewWillAppear: when its view is about to be added to the window

viewDidAppear: when its view has been added to the window

viewWillDisappear: when its view is about to be dismissed, covered, or otherwise hidden from view

viewDidDisappear: when its view has been dismissed, covered, or otherwise hidden from view