<< shiftedbits

Key value observing is quite a useful tool, no doubt about it. But it has a singularly annoying manner of informing the observer of a change. The -[NSObject observeValueForKeyPath:ofObject:change:context:] method is sent to the observer when a change occurs. It’s up to the implementor to parse the `change’ dictionary to figure out what changed.

When I use observers, I usually want a method called on my class when a change occurs, similar to target/action with UI elements. So my -[NSObject observeValueForKeyPath:ofObject:change:context:] is basically a set of if’s for each key I’m observing that calls the appropriate method.

This turns out to be pretty nasty, especially when you have multiple classes in a hierarchy implementing the method. It wasn’t obvious to me that I needed to call super in this method. Thirty minutes of debugging later, and I was convinced there had to be a better way of doing it.

To this effort, I present two new APIs. To NSObject, I add -[NSObject addObserver:forKeyPath:options:selector:] and to NSArray, -[NSArray addObserver:toObjectsAtIndexes:forKeyPath:options:selector]. The code is available here, or keep reading for the details.

Update 1.1: Two critical bug fixes. Please update to the new version if you have the old one. Also, anonymous svn is now available if you want to use an external. http://svn.shiftedbits.org/public/KVOAdditions/trunk. (more…)