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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.
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You use categories to define additional methods of an existing class—even one whose source code is unavailable to you—without subclassing. You typically use a category to add methods to an existing class, such as one defined in the Cocoa frameworks.
In Objective C they were called categories, but in Swift they are called extensions. The purpose of both of them are to give additional functionality to existing classes without having to create subclasses.
A category allows you to add methods to an existing class—even to one for which you do not have the source. Categories are a powerful feature that allows you to extend the functionality of existing classes without subclassing. Check the apple doc for the Category in Objective-C.
The methods declared by a class extension are implemented in the implementation block for the original class, so you can't, for example, declare a class extension on a framework class, such as a Cocoa or Cocoa Touch class like NSString. Extensions are actually categories without the category name.
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:
The category has no name. It is declared like this:
@interface SomeClass ()@end- (void) anAdditionalMethod;
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).
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