- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    [center addObserver:self
               selector:@selector(contextChanged:)
                   name:NSManagedObjectContextObjectsDidChangeNotification
                 object:self.document.managedObjectContext];
}
- (void)viewWillDisappear:(BOOL)animated
{
    [center removeObserver:self
                      name:NSManagedObjectContextObjectsDidChangeNotification
                    object:self.document.managedObjectContext];
    [super viewWillDisappear:animated];
}