ibrews/apple-platform-skills/gamekit-multiplayer

import GameKit
 
func authenticateLocalPlayer() {
    GKLocalPlayer.local.authenticateHandler = { [weak self] viewController, error in
        if let vc = viewController {
            // Present Game Center login UI
            self?.present(vc, animated: true)
        } else if GKLocalPlayer.local.isAuthenticated {
            self?.enableMultiplayerUI()
        } else {
            // Player declined or unavailable — disable multiplayer
            print("GameKit auth failed: \(error?.localizedDescription ?? "unknown")")
        }
    }
}

// Programmatic (custom UI)
let request = GKMatchRequest()
request.minPlayers = 2
request.maxPlayers = 4
 
GKMatchmaker.shared().findMatch(for: request) { match, error in
    guard let match, error == nil else { return }
    DispatchQueue.main.async { self.startGame(with: match) }
}
 
// Built-in Game Center UI
let vc = GKMatchmakerViewController(matchRequest: request)
vc?.matchmakerDelegate = self
present(vc!, animated: true)

// Encode messages with Codable
struct GameMessage: Codable {
    enum Kind: String, Codable { case move, shoot, stateSync }
    let kind: Kind
    let payload: Data
}
 
func send(_ message: GameMessage, reliable: Bool = true) throws {
    let data = try JSONEncoder().encode(message)
    try match.sendData(toAllPlayers: data, with: reliable ? .reliable : .unreliable)
}
// .reliable   → guaranteed, ordered (use for game state changes)
// .unreliable → fast, may drop (use for position streaming)
 
// GKMatchDelegate — always dispatch UI work to main thread
func match(_ match: GKMatch, didReceive data: Data, fromRemotePlayer player: GKPlayer) {
    guard let message = try? JSONDecoder().decode(GameMessage.self, from: data) else { return }
    DispatchQueue.main.async { self.apply(message, from: player) }
}

func match(_ match: GKMatch, player: GKPlayer, didChange state: GKPlayerConnectionState) {
    switch state {
    case .connected:
        // Wait until ALL expected players are connected
        if match.expectedPlayerCount == 0 { startGame() }
    case .disconnected:
        pauseGame()
        // Optionally: GKMatchmaker.shared().addPlayers(to: match, ...) to fill slot
    @unknown default: break
    }
}