One possible improvement (and maybe it's not feasible—you've obviously spent a lot more time thinking about this than I have) would be to accept any object that

One possible improvement (and maybe it's not feasible—you've obviously spent a lot more time thinking about this than I have) would be to accept any object that implements NSCoding, rather than strictly NSDictionary objects. Depending on how you're isolating instances I could see how that might open up a huge can of worms from a security perspective.