Objective-C: Instance Variable in Category

Question

I am just asking whether it was possible to add an instance variable via a category. My special problem is, that I need to add an NSIndexPath property to an ASIHTTPRequest object but I don't wanna subclass the ASIHTTPRequest as a matter of principle.

Do I have any kind of alternative?

Thanks for your answers, Christian

Unipersonal

possible duplicate of Objective-C: Property in Category – Hammad

Answer 1

A category can not declare additional instance variables but since OS X 10.6 and iOS 3.1 you can work around this with associative references.

You can use associative references to simulate the addition of object instance variables to an existing class. Using associative references, you can add storage to an object without modifying the class declaration. This is done via objc_setAssociatedObject and objc_getAssociatedObject. If you wrap these calls in a property’s custom getter and setter, you can fake an ivar.

Have a look at this post by Ole Begemann.

Nunatak

Thanks, but just found an easier solution: userInfo-Dictionary in ASIHTTPRequest! But anyway, thanks! – Unipersonal

I'm happy you solved it! You may want to answer the question yourself with your solution and accept it or accept one of the other answers so the question does not remain unaccepted. – Nunatak

It is in fact now possible to add ivars in a class extension. See: #5826845 – Lux

That would require to synthesize/implement accessors of the property in the main @implementation block for the corresponding class – Nunatak

anyone have a correct url for the associative references link? – Cloraclorinda