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? Examplif

Answer 1

@interface SomeClass ()


- (void) anAdditionalMethod;


@end

I think it is not the way to declare Category. Category must have a name

@interface SomeClass (XYZ)


- (void) anAdditionalMethod;


@end

for example

@interface NSMutableArray (NSMutableArrayCreation)

+ (id)arrayWithCapacity:(NSUInteger)numItems;
- (id)initWithCapacity:(NSUInteger)numItems;

@end

Declared for NSMutableArray by Apple

Answer 2

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).

Answer 3

1. Category

=> In Objective C, when you want to add some more functionality to a class without inheritance, you simply use category for it.

=> it comes with its own .h and .m file

=> Category uses to add new method not properties.

2. Class Extension

-> In Objective C, when you want to make behaviour of some property private you use class extension.

-> it comes with **.h** file only.

-> mainly for properties.

Note: when we add a new file and select a option of objective c category shows category and "category on" not "subclass of" so it shows like

@interface className (categoryName)
@end

-You will get two file .h and .m with file name as (className+categoryName.h and className+categoryName.m)

and in extension case you will get

@interface className()
@end

-You will get only one file with name as className_extensionName.h

• In category you don't own the class but in extension you are.