Programming iOS 5, 2nd Edition

Chapter 2. Object-Based Programming

My object all sublime.

—W. S. Gilbert, The Mikado

Objective-C, the native language for programming the Cocoa API, is an object-oriented language; in order to use it, the programmer must have an appreciation of the nature of objects and object-based programming. There’s little point in learning the syntax of Objective-C message sending or instantiation without a clear understanding of what a message or an instance is. That is what this chapter is about.

Objects

An object, in programming, is based on the concept of an object in the real world. It’s an independent, self-contained thing. These objects, unlike purely inert objects in the real world, have abilities. So an object in programming is more like a clock than a rock; it doesn’t just sit there, but actually does something. Perhaps one could compare an object in programming more to the animate objects of the real world, as opposed to the inanimate objects, except that — unlike real-world animate things — a programming object is supposed to be predictable: in particular, it does what you tell it. In the real world, you tell a dog to sit and anything can happen; in the programming world, you tell a dog to sit and it sits. (This is why so many of us prefer programming to dealing with the real world.)

In object-based programming, a program is organized into many discrete objects. This organization can make life much easier for the programmer. Each object has abilities that are specialized for that object. ...