Is it possible to add objects or data members to category in objective C ??

Question

we know that using category we can add functions to existing class. But i have a doubt that, Is it possible to add objects or data members to category in objective C ??

Answer 1

You're right, you cannot add instance variables / members / properties via a category. You can only work with what the class you're expanding already offers, but of course your new methods may receive arguments upon being called.

Answer 2

You can use Objective-C associated references. You can read the Apple documentation about them here: Associative References

Basically this lets you attach objects to any instance using the objc_setAssociatedObject function as follows:

#import <objc/runtime.h>

/* The key for the associated object must be void* so you can use a static variable to get a
   unique pointer. Alternatively you can use _cmd since selectors are constant and unique. */
static char key;
objc_setAssociatedObject(theInstanceToAddYourObjectTo,
                         &key,
                         @"The object you want to add",
                         OBJC_ASSOCIATION_RETAIN);

To get the associated object again:

id value = objc_getAssociatedObject(theInstanceToAddYourObjectTo, &key);

And to clear the associated object, pass nil as the value:

objc_setAssociatedObject(theInstanceToAddYourObjectTo,
                         &key,
                         nil,
                         OBJC_ASSOCIATION_ASSIGN);

So using these functions you can add your own instance variables for use in category methods etc.