iOS Game Center GKTapper is twisting my melon, HELP!

[thedollarhunter]

Please please could someone explain what is going on in the GKTapper sample code!

Specifically - I call authenticateLocalUser and it works just fine BUT what is callDelegateOnMainThread actually doing and why does it not seem to pass any argument (@selector???) and thus outputs "Missed Method" to the console?

If someone understands this please take a moment to explain it to me!

Code is unchanged from the GKTapper sample from Apple:

- (void) authenticateLocalUser
{
	if([GKLocalPlayer localPlayer].authenticated == NO)
	{
		[[GKLocalPlayer localPlayer] authenticateWithCompletionHandler:^(NSError *error) 
		{
			[self callDelegateOnMainThread: @selector(processGameCenterAuth:) withArg: NULL error: error];
		}];
	}
}


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


- (void) callDelegate: (SEL) selector withArg: (id) arg error: (NSError*) err
{
	assert([NSThread isMainThread]);
	if([delegate respondsToSelector: selector])
	{
		if(arg != NULL)
		{
			[delegate performSelector: selector withObject: arg withObject: err];
			NSLog(@"delegate with arg");
		}
		else
		{
			[delegate performSelector: selector withObject: err];
			NSLog(@"delegate without arg");
		}
	}
	else
	{
		NSLog(@"delegate Missed Method");
	}
}