objective c - Difference between Category and Class Extension?

Question

What is the difference between a Category and a Class Extension. I believe both are used to add custom methods in existing classes. Can someone throw light on this? Examplification with code will be really appreciated.

Answer 1

A category is a way to add methods to existing classes. They usually reside in files called "Class+CategoryName.h", like "NSView+CustomAdditions.h" (and .m, of course).

A class extension is a category, except for 2 main differences:

1. The category has no name. It is declared like this:

   @interface SomeClass ()
   
   - (void) anAdditionalMethod;
   

   @end

2. The implementation of the extension must be in the main @implementation block of the file.

It's quite common to see a class extension at the top of a .m file declaring more methods on the class, that are then implemented below in the main @implementation section of the class. This is a way to declare "pseudo-private" methods (pseudo-private in that they're not really private, just not externally exposed).