Objective-C category compared to Mixins

Question

Is the concept of the Objective-C categories in anyway similar to the concept of mixins? If so: what are the similarities? In not: what are the differences?

Answer 1

Categories are defined for a particular class, as far as I know, you can't create a category and add the methods it implements to several classes.

Answer 2

With a mixin, you might derive a new class from your base and the mixin, then instantiate this new class to take advantage of it.

With a category, you are effectively adding directly the base class, so that all instances of that base have access to the functionality provided by the category.

Answer 3

To the best of my understanding:

Mixins

• Syntactic sugar for composition
• Added by the developer of the class, not the user
• Can be reused by multiple classes
• Can add instance variables
• Can be implemented using forwarding in Objective-C

Categories

• Similar to extension methods in other languages
• Usually added by the user of the class, not the developer
• Used by exactly one class and its subclasses
• Can't add instance variables