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Fun With objc_setAssociatedObject and UIAlertView

Published on Mon, 12 December 2011

When it comes time to present errors or other messages in iOS with UIAlertView it is immediately obvious that a more convenient interface would involve the use of blocks. A search on GitHub shows just about every iOS developer has had the same thought and had a crack at it.

After reviewing the better options (I.e. those that actually had a README with more than a few lines of content) on GitHub it appeared that one of the challenges was how to handle memory. This stems from the wrapper becoming the delegate for the UIAlertView but not having a strong reference from the caller. I saw various solutions to this, most involving adding a extra retain call to keep everything around until the delegate methods were called.

Since my project is using ARC I looked for a solution that didn’t involve marking the wrapper file as non-ARC and invoking retain. The solution I came up with was to use objc_setAssociatedObject. This Objective-C runtime function allows one object to be associated with another using various memory management strategies. I used this to associate the UIAlertView blocks based wrapper with the UIAlertView.

In the init method the wrapper instance is associated with its UIAlertView:

objc_setAssociatedObject(alertView, _cmd, self, OBJC_ASSOCIATION_RETAIN);

Note that I’m using the implicit second argument to the method, its selector, _cmd as the key for the associated object. This was suggested in a tweet by Bill Bumgarner.

Then in alertView:didDismissWithButtonIndex:, the association is removed, dealloc of the wrapper called as a result and the UIAlertView also released.

SEL key = @selector(initWithTitle:message:);
objc_setAssociatedObject(self.alertView, key, nil, OBJC_ASSOCIATION_RETAIN);

The full MIT licensed code is available as a gist on GitHub.

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