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Difference between Objective-C Category and Extension

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1.Category is a way to add methods to a class whether or not source code is available implies you can add category to foundation classes like NSString and also to your own custom classes.

2.We can add extra instance variable and property in class extension but not in Category.

3.Any variable and method inside the extension is not even accessible to inherited class.

4.Category and Extension both are basically made to handle large code base but category is a way to extend class API in multiple source file while extension is a way to add required methods out side the main interface file.

5.Use category when you have to break your same class code into different source file according to different functionality and Extension when you just need to add some required methods to existing class outside the main interface file. also when you need to modify a publicly declared instance variable in a class. for ex: readonly to readwrite you can re declare it in extension.

Read : What is Category?

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