Question

trying to implement a multiplayer. Using the sample from Game Center – Sending and receiving data.

Everything seems okay, but in apple documentation there is also said about invitation handler.

[GKMatchmaker sharedMatchmaker].inviteHandler = ^(GKInvite *acceptedInvite, NSArray *playersToInvite) {
   // Insert application-specific code here to clean up any games in progress.
   if (acceptedInvite) {
        GKMatchmakerViewController *mmvc = [[[GKMatchmakerViewController alloc] initWithInvite:acceptedInvite] autorelease];
        mmvc.matchmakerDelegate = self;
        [self presentModalViewController:mmvc animated:YES];
    } else if (playersToInvite) {
        GKMatchRequest *request = [[[GKMatchRequest alloc] init] autorelease];
        request.minPlayers = 2;
        request.maxPlayers = 4;
        request.playersToInvite = playersToInvite;

        GKMatchmakerViewController *mmvc = [[[GKMatchmakerViewController alloc] initWithMatchRequest:request] autorelease];
        mmvc.matchmakerDelegate = self;
        [self presentModalViewController:mmvc animated:YES];
    }
};

The problem is quite simple: I do not know where to add this code.

Answer 1

As stated in the docs

Your application should set the
invitation handler as early as
possible after your application is
launched; an appropriate place to set
the handler is in the completion block
you provided that executes after the
local player is authenticated.

Somewhere in your code, you should have authenticated the local player with something like this

[[GKLocalPlayer localPlayer] authenticateWithCompletionHandler:^(NSError *error) {
    if (error == nil) {
        // Insert your piece of code here
    } else {
        // Handle the error
    }
}];

Hope that helps