Core Data & iCloud: exception when NSPersistentStoreCoordinatorStoresWillChangeNotification received

Question

I'm following the Apple's iCloud Programming Guide for Core Data, the Using the SQLite Store with iCloud section, and you are told there to listen for the NSPersistentStoreCoordinatorStoresWillChangeNotification iCloud event this way:

[[NSNotificationCenter defaultCenter] addObserverForName:NSPersistentStoreCoordinatorStoresWillChangeNotification
                                                  object:self.managedObjectContext.persistentStoreCoordinator
                                                   queue:[NSOperationQueue mainQueue]
                                              usingBlock:^(NSNotification *note) {
                                                  [self.managedObjectContext performBlock:^{
                                                      // disable user interface with setEnabled: or an overlay

                                                      if ([self.managedObjectContext hasChanges]) {
                                                          NSError *saveError;
                                                          if (![self.managedObjectContext save:&saveError]) {
                                                              NSLog(@"Save error: %@", saveError);
                                                          }
                                                      }
                                                      else {
                                                          // drop any managed object references
                                                          [self.managedObjectContext reset];
                                                      }
                                                  }];
                                              }];

I've put such code within my AppDelegate's application:didFinishLaunchingWithOptions: method and, when the notification is received, I get this exception:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Can only use -performBlock: on an NSManagedObjectContext that was created with a queue.'

According to the document, this should work... what could am I doing wrong?

Thanks

Answer 1

The error:

Can only use -performBlock: on an NSManagedObjectContext that was created with a queue.

...is quite clear. You cannot call performBlock: on a managed object context unless that context was created using one of the queue-type concurrency options. Meaning that when you created the context, you must have used initWithConcurrencyType: and used either NSPrivateQueueConcurrencyType or NSMainQueueConcurrencyType as the argument.

This has nothing at all to do with the notification, only with the fact that you're calling performBlock: when you didn't create the context correctly.