How can I learn if a key of any of the objects stored in an array change?

I know there is an addObserver: for the array, but this needs an NSIndexSet – so would I need to remove/add the observer, each time items are removed/added and change the NSIndexSet?

On a related note, is it possible to add an observer to an NSDictionary? cause in fact, this would be a better way to store my objects.