iOS Programming: The Big Nerd Ranch Guide

For the More Curious: Overriding Autorotation

In most cases, autorotation does the right thing if the autoresizing masks are properly set. However, you might want to take additional action on an autorotation or override the autorotation process altogether to change the way the view looks when it rotates. You can do this by overriding willAnimateRotation​ToInterfaceOrientation:​duration: in a view controller subclass.

When a view controller is about to autorotate its view, it checks to see if you have implemented this method. If you have, then this method is invoked during the animation block of the rotation code. Therefore, all changes to subviews in this method will be animated as well. You can also perform some custom code within this method. ...