11. Categories, Posing, and Protocols
In this chapter you'll learn about how to add methods to a class in a modular fashion through the use of categories, how one class can substitute for another one, and how to create a standardized list of methods for others to implement.
Categories
Sometimes you might be working with a class definition and want to add some new methods to it. For example, you might decide for your Fraction class that, in addition to the add: method for adding two fractions, you'd like to have methods to subtract, multiply, and divide two fractions.
As another example, say you are working on a large programming project and as part of that project a new class is being defined containing many different methods. You have been assigned ...
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