We use categories to define additional methods of an existing class—even one whose source code is unavailable to you—without subclassing. You typically use a category to add methods to an existing class, such as one defined in the Cocoa frameworks. The added methods are inherited by subclasses and are indistinguishable at runtime from the original methods of the class. You can also use categories of your own classes to:
Distribute the implementation of your own classes into separate source files—for example, you could group the methods of a large class into several categories and put each category in a different file.
Declare private methods.
You add methods to a class by declaring them in an interface file under a category name and defining them in an implementation file under the same name. The category name indicates that the methods are an extension to a class declared elsewhere, not a new class.
Distribute the implementation of your own classes into separate source files—for example, you could group the methods of a large class into several categories and put each category in a different file.
Declare private methods.
You add methods to a class by declaring them in an interface file under a category name and defining them in an implementation file under the same name. The category name indicates that the methods are an extension to a class declared elsewhere, not a new class.