Purpose of "Simulated User Interface Elements" for Nav Controllers?

What's the purpose of having the Simulated User Interface elements, i.e. the top and bottom toolbars? Is this to allow you to design with the space accounted f开发者_如何学JAVAor but the actual toolbars would be provided by the Navigation Controller?

Is this to allow you to design with the space accounted for but the actual toolbars would be provided by the Navigation Controller?

Yes. This makes it easier for you (and IB) to position and scale everything when these bars are actually used.

Edit:

As the name "Simulated User Interface Elements" tells you: This elements have no effect on the view they're simulated in. If your app happens to never use a navigation controller (or a toolbar etc.), the view will be scaled up and its content, too. For this the autoresizingMasks become handy.