gistfile1.m

- (void)authenticateLocalPlayer
{
    [[GKLocalPlayer localPlayer] authenticateWithCompletionHandler:^(NSError *error) {
		[self callSelfOnMainThread:@selector(didAuthenticateLocalPlayer:) withArg:error error:nil];
	}];
}

- (void)callSelf:(SEL)selector withArg:(id)arg error:(NSError*)err
{
	assert([NSThread isMainThread]);
	if([self respondsToSelector: selector])
	{
		if(arg != NULL)
		{
			[self performSelector: selector withObject: arg withObject: err];
		}
		else
		{
			[self performSelector: selector withObject: err];
		}
	}
	else
	{
		NSLog(@"Missed Method");
	}
}

- (void)callSelfOnMainThread:(SEL)selector withArg:(id)arg error:(NSError*)err
{
	dispatch_async(dispatch_get_main_queue(), ^(void) {
		[self callSelf:selector withArg:arg error:err];
	});
}