title: What is NSManagedObjectContext's performBlock: used for?
body: <p>In iOS 5, <code>NSManagedObjectContext</code> has a couple of new methods, <code>performBlock:</code> and <code>performBlockAndWait:</code>. What are these methods actually used for? What do they replace in older versions? What kind of blocks are supposed to be passed to them? How do I decide which to use? If anyone has some examples of their use it would be great.</p>