Programming iOS 6, 3rd Edition

Bars

As you saw in Chapter 19, the three bar types — UINavigationBar, UIToolbar, and UITabBar — are often used in conjunction with a built-in view controller:

• A UINavigationController has a UINavigationBar.
• A UINavigationController has a UIToolbar.
• A UITabBarController has a UITabBar.

You can also use these bar types independently. You are most likely to do that with a UIToolbar, which is often used as an independent bottom bar. On the iPad, it can also be used as a top bar, adopting a role analogous to a menu bar on the desktop. That’s such a common interface, in fact, that certain special automatic behaviors are associated with it; for example, a UISearchBar in a UIToolbar and managed by a UISearchDisplayController will automatically display its search results table in a popover (Chapter 22), which is different from what happens if the UISearchBar is not in a UIToolbar.

This section summarizes the facts about the three bar types and the items that populate them.

UINavigationBar

A UINavigationBar is populated by UINavigationItems. The UINavigationBar maintains a stack; UINavigationItems are pushed onto and popped off of this stack. Whatever UINavigationItem is currently topmost in the stack (the UINavigationBar’s topItem), in combination with the UINavigationItem just beneath it in the stack (the UINavigationBar’s backItem), determines what appears in the navigation bar:

• The title (string) or titleView (UIView) of the topItem appears in the center of the navigation bar.
• The prompt ...