Programming iOS 6, 3rd Edition

Subclassing

Cocoa effectively hands you a large repertory of objects that already know how to behave in certain desirable ways. A UIButton, for example, knows how to draw itself and how to respond when the user taps it; a UITextField knows how to display editable text, how to summon the keyboard when the user taps in it, and how to accept keyboard input.

Often, the default behavior or appearance of an object supplied by Cocoa won’t be quite what you’re after, and you’ll want to customize it. Cocoa classes are heavily endowed with methods that you can call and properties that you can set for precisely this purpose, and these will be your first resort. Always study the documentation for a Cocoa class to see whether instances can already be made to do what you want. For example, the class documentation for UILabel (Chapter 27) shows that you can set the font, size, color, line-breaking behavior, and horizontal alignment of its text, among other things.

Nevertheless, sometimes setting properties and calling methods won’t suffice to customize an instance the way you want to. In such cases, Cocoa may provide methods that are called internally as an instance does its thing, and whose behavior you can customize by subclassing and overriding (Chapter 4). You don’t have the code to any of Cocoa’s built-in classes, but you can still subclass them, creating a new class that acts just like a built-in class except for the modifications you provide.

Oddly enough, however (and you might be particularly ...