Adding Properties to an Objective-C Category

Posted by David Hamrick on

Let’s say you have a category that needs to store some information. Unfortunately you can’t add an instance variable, but you can add something called an associated reference. From the documentation:

Associative references, available starting in Mac OS X v10.6, simulate the addition of object instance variables to an existing class

To create an association you use the objc_setAssociatedObject function and to retrieve an association use the objc_getAssociatedObject method.

Using an associated reference as storage for a property

We can use this technique to make a built in class have a property that we want. In this example, I am storing the name of a style that I want to assign to a UIView.

When instances of UIView are released they will also release their associated references.

This technique let’s use set this custom property on plain old UIViews. If you only need to do something simple like add a property, this provides a nice alternative to subclassing.


David Hamrick is a partner at Hamrick Software, the makers of VueScan, the worlds most popular 3rd party scanner software.