When running the app from Xcode, make sure that -com.apple.CoreData.ConcurrencyDebug 1 is included in your launch arguments.
This code is supposed to test the Core Data concurrency debugging flag in iOS 8.
It fails in line 67 of the AppDelegate class (let didSave = backgroundContext!.save(&potentialSaveError)) with a multithreading violation exception and I have no idea why. I believe it should work because I wrapped the access to backgroundContext in a performBlock { } call.
When I remove the performBlock { } wrapper, the code fails on a previous line with a multithreading violation (NSEntityDescription.insertNewObjectForEntityForName("Person", inManagedObjectContext: backgroundContext!), which is expected behavior because we are accessing the background context from the wrong queue.
I also tested essentially the same code with Objective-C so it is probably not a Swift issue.