Monday, December 5, 2011

Extending Classes with Categories

Much of the day to day work in object-oriented programming involves using classes that are already written. Sometimes we want our classes to do just a little bit more. That means they need more properties and methods and adding these entities changes the nature of the class. This is called sub-classing and we already covered how to do that.

There are times, however, when we want to extend the abilities of class but we don't want to define a completely new class. You will encounter these situations more and more as you plunge into Objective-C programming. Perhaps more importantly, you will find examples of category use throughout the Mac and iOS programming examples written by Apple and other developers.

So, to extend a class without sub-classing you have the option of using categories. Let's talk about how to do this.