I’ve played around with making turn-based games using GameCenter. I understand that by default, GameCenter assumes that out of a number of participants, at any given time, one player holds the “play baton”, and that this player is the only one who can affect the current game state. Gameplay is asynchronous, i.e. whoever’s turn it is can take their time, and the other players will be notified once it’s their turn.
So far, so good.
Now I want to use GameCenter to implement a similar, but slightly different kind of turn-based game: an asynchronous game where, instead of a serial player succession, players make their turns in parallel, which are then consolidated into a new game state once all players have “turned in” their moves.
A good model game for this would be Rock, Paper, Scissors: both players secretly decide on their move (“rock“, or “paper“, or “scissors”). The order in which those are then submitted to the server is arbitrary; i.e. no player should ever get a “not your turn”-type error when they try to submit a move in an ongoing round. Once they both turned in their moves, all player choices are revealed, and the winner of the current round is determined/declared.
The question is: is it possible at all to use the GameCenter infrastructure for this kind of game, either by design or by work-around? And if so, what would be considered a good approach?
It is not possible to implement this with Game Center the way you suggested, but you can take an approach that will look as if you did manage to do this.
When you start a turn-based match, it’s always the local player’s turn. Either Game Center provides you with a blank match, or you will receive a match in which someone else already took their turn. There is no way to control this, so you need to be prepared for both.
The approach you can take is to have a player always take their turn before you show them anyone else’s move. Only then do you check if in your local case, everyone has now taken their turn and you show the result. This will provide the illusion of what you are asking for. In the case of Rock-Paper-Scissors you can now decide the match outcome. The other player will be notified.
However, if not everyone has taken their turn in this round, don’t show anything, update the game state as well, but tell the user you’re now waiting for others to take their turn. You will be able to show the result when you are notified that it’s your turn again, with a game state already indicating the outcome.