wezm.net/content/technical/2011/12/fun-with-objc-setassociatedobject-and-uialertview.html

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][github-search]
shows just about every iOS developer has had the same thought and had a crack at it.

[UIAlertView]: http://developer.apple.com/library/ios/#documentation/uikit/reference/UIAlertView_Class/UIAlertView/UIAlertView.html
[github-search]: https://github.com/search?type=Repositories&language=&q=uialertview&repo=&langOverride=&x=14&y=17&start_value=1

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`][set-object]. 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.

[set-object]: http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocAssociativeReferences.html

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][bbum].

[bbum]: http://twitter.com/bbum/status/3609098005

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][gist] is available as a [gist on GitHub][gist].

[gist]: https://gist.github.com/1392611