Programming iOS 4

Chapter 10. Cocoa Classes

Using the Cocoa frameworks requires an understanding of how those frameworks organize their classes. Cocoa class organization depends upon certain Objective-C language features that are introduced in this chapter. The chapter also surveys some commonly used Cocoa utility classes, along with a discussion of the Cocoa root class.

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 summon the keyboard when the user taps in it, how to accept keyboard input, and how to respond when the user finishes inputting text.

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 and properties 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 ...