iOS 13 Programming Fundamentals with Swift

Chapter 10. Cocoa Classes

When you program iOS through Foundation and UIKit, you’re programming Cocoa. The Cocoa API is written mostly in Objective-C, and Cocoa itself consists mostly of Objective-C classes, derived from the root class, NSObject.

This chapter introduces Cocoa’s class structure and explains how Cocoa is conceptually organized, in terms of its underlying Objective-C features, along with a survey of some of the most commonly encountered Cocoa utility classes. The chapter then discusses Objective-C instance properties and Cocoa key–value coding, and concludes with a description of the Cocoa root class and its features, which are inherited by all Cocoa classes.

Subclassing

Cocoa supplies a large repertory of objects that already know how to behave in certain desirable ways. A UIButton 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, and how to accept keyboard input. When the default behavior or appearance of an object supplied by Cocoa isn’t quite what you’re after, you’ll want to customize it.

But that does not necessarily mean you need to subclass! In fact, subclassing is one of the rarer ways in which your code will relate to Cocoa. Most built-in Cocoa Touch classes will never need subclassing (and some, in their documentation, downright forbid it).

Instead, Cocoa classes are often heavily endowed with methods that you can call and properties that you can set precisely ...