Category vs Extension

What is a category and when is it used?

A category is a way of adding additional methods to a class without extending it. It is often used to add a collection of related methods. A common use case is to add additional methods to built in classes in the Cocoa frameworks. For example adding async download methods to the UIImage class

The main difference is that with an extension, the compiler will expect you to implement the methods within your main @implementation, whereas with a category you have a separate @implementation block. So you should pretty much only use an extension at the top of your main .m file (the only place you should care about ivars, incidentally) -- it's meant to be just that, an extension.

Use class extensions to Hide Private Information

Class extensions are often used to extend the public interface with additional private methods or properties for use within the implementation of the class itself. It’s common, for example, to define a property as readonly in the interface, but as readwrite in a class extension declared above the implementation, in order that the internal methods of the class can change the property value directly.

As an example, the XYZPerson class might add a property called uniqueIdentifier, designed to keep track of information like a Social Security Number in the US.

It usually requires a large amount of paperwork to have a unique identifier assigned to an individual in the real world, so the XYZPerson class interface might declare this property as readonly, and provide some method that requests an identifier be assigned, like this:

@interface XYZPerson : NSObject
    ...
    @property (readonly) NSString *uniqueIdentifier;
    - (void)assignUniqueIdentifier;
@end

In order for the XYZPerson class to be able to change the property internally, it makes sense to redeclare the property in a class extension that’s defined at the top of the implementation file for the class:

@property (readwrite) NSString *uniqueIdentifier;

Note: The readwrite attribute is optional, because it’s the default. You may like to use it when redeclaring a property, for clarity.