When to use associated objects in objective-c

Question

I have recently learned about associated objects in objective-c and how to implement them. From my understanding, they are helpful if you have a property that you want only a single instance of an object to have.

I can't think of any specific use cases for associated objects in objective-c (meaning use cases that I can't do using some other means).

Does anyone have specific examples of when to use associated objects?

Answer 1

It is useful when Apple has designed a class incorrectly, such that you can't subclass it.

A good example is NSURLSessionDownloadTask. Let's suppose that when you create the task you want to attach a piece of data to it, to be retrieved when the final task delegate message arrives.

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    // retrieve data from downloadTask here
}

But you can't subclass NSURLSessionDownloadTask; you have to accept the task that NSURLSession gives you. So you can't attach any data to it by normal means. One solution in this situation is an associated object.

Personally I wish every object behaved like CALayer and CAAnimation, where you can attach as much data to it as you like in the form of key-value pairs, as if the object were secondarily a kind of NSDictionary.