Programming iOS 6, 3rd Edition

Appearance Proxy

Instead of sending messages that customize the look of an interface object to the object itself, you can send them to an appearance proxy for that object’s class. The appearance proxy then passes that same message along to the actual future instances of that class. You’ll usually configure your appearance proxies very early in the lifetime of the app, and never again. The app delegate’s application:didFinishLaunchingWithOptions:, before the app’s window has been displayed, is the most obvious and common location.

Thus, for example, instead of sending setTitleTextAttributes:forState: to a particular UIBarButtonItem, you could send it to a UIBarButtonItem appearance proxy. All actual UIBarButtonItems from then on would have the text attributes you specified.

This architecture has two chief uses:

• It simplifies the task of giving your app a consistent overall appearance. Suppose you want all UIBarButtonItems to have a certain title font. Instead of having to remember to send setTitleTextAttributes:forState: to each UIBarButtonItem your app ever instantiates, you send it once to the appearance proxy and it is sent to those UIBarButtonItems for you.
• It provides access to interface objects that might otherwise be difficult to refer to. For example, you don’t get direct access to a search bar’s external Cancel button, but it is a UIBarButtonItem and you can customize it through the UIBarButtonItem appearance proxy.

There are two class methods for obtaining an appearance ...