Learning the iOS 4 SDK for JavaScript Programmers

Welcome to Class Files

Most, if not all, of the code you write for an iOS app is contained in what are called class files. A class is an object-oriented programming term and is vital to understanding most of what goes on in iOS SDK programming.

A class is code that defines the characteristics of an object. You’ve worked with tons of objects in JavaScript—whether they be pure language objects, such as the Date object, or DOM objects that represent HTML elements on the page—so you should already be familiar with the concept of an object having properties (which can hold data) and methods (which perform actions). In fact, when you write the code for a JavaScript object constructor function, you are doing essentially the same job that you will be doing when you write the code for an Objective-C class definition. It’s just that the format and environment for the Objective-C variety is radically different from the JavaScript constructor. To help you visualize the differences between the JavaScript object constructor and Objective-C class definitions, I’ll show you a variation on a venerable JavaScript object constructor example, which I will then rewrite in Objective-C (you can even test it in Workbench by the end of the chapter). Both examples focus on using the constructor or class as a way to obtain object instances.

The JavaScript Way

I’ll begin with a variation of an “old chestnut” example, one that defines an object that represents an automobile. I’ll add a function to the old standard ...