A category is an Objective-C language feature that allows you to reach right into an existing class and define additional methods. You can do this even if you don’t have the code for the class, as with Cocoa’s classes. Your instance methods can refer to self, and this will mean the instance to which the message was originally sent, as usual. A category, unlike a subclass, cannot define additional instance variables; it can override methods, but you should probably not take advantage of this ability.

Defining a category is just like defining the class on which the category is being defined: you need an interface section and an implementation section, and you’ll typically distribute them into the standard .h and .m class file pair. At the start of both the interface section and the implementation section, where you give the class’s name, you add a category name in parentheses. The .h file will probably need to import the header for the original class (or the header of the framework that defines it), and the .m file will, as usual, import the corresponding header file.

For example, in one of my apps I found myself performing a bunch of string transformations in order to derive the path to various resource files inside the app bundle based on the resource’s name and purpose. I ended up with half a dozen utility methods. Given that these methods all operated on an NSString, it was appropriate to implement them as a category of NSString, thus allowing any NSString, anywhere ...

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