lemonkey

[[managedObject managedObjectContext] performBlock:^{
	NSString *value = [managedObject someValue];
	[NSOperationQueue mainQueue] addOperationWithBlock:^{
		[[self label] setText:value];
	}];
}];

lemonkey

[managedObjectContext performBlock:^{
	NSError	*error	= nil;

	if (![managedObjectContext save:&error]){
		[self handleError:error];
	}
}];

lemonkey

- (void) allPeopleInManagedObjectContext:(NSManagedObjectContext *)managedObjectContext withCompletion:(void (^)(NSArray*, NSError*))completion {
	[managedObjectContext performBlock:^{
		NSError				*error			= nil;
		NSArray				*results		= nil;
		NSFetchRequest		*fetchRequest	= nil;
		NSPredicate			*predicate		= nil;
		NSEntityDescription	*entity			= nil;

		fetchRequest = [[NSFetchRequest alloc] init];
		entity = [NSEntityDescription entityForName:@"Person" inManagedObjectContext:context];

lemonkey

- (void) saveManagedObjectContext:(NSManagedObjectContext *)managedObjectContext withCompletion::(void (^)(BOOL, NSError *))completio{
	managedObjectContext performBlock:^{
		NSError	*error	= nil;

		if (![managedObjectContext save:&error]){
			completion(NO, error);
		} else {
			if ([managedObjectContext parentContext] != nil){
				[self saveManagedObjectContext:[managedObjectContext parentContext] withCompletion:completion];
			} else {

lemonkey

group = dispatch_group_create();
dispatch_group_enter(group);
[context performBlock:^{
    result = [context executeFetch:...
    dispatch_group_leave(group);
}];
groupResult = dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

lemonkey

- (void) fetchThingsWithCompletion:(void (^)(NSArray *, NSError *))completion{
    [context performBlock:^{
         result = [context executeFetch:...
          if (completion != nil){
              completion(result, error);
          }
    }];
}