Gamekit

GameKit

Use GameKit for Game Center authentication, competition, matchmaking, social surfaces, and saved-game handoffs; keep rendering, board logic, and full SharePlay group-activity design in their owning framework skills.

Contents

• Authentication
• Access Point
• Dashboard
• Leaderboards
• Achievements
• Real-Time Multiplayer
• Turn-Based Multiplayer
• Common Mistakes
• Review Checklist
• References

Authentication

All GameKit features require the local player to authenticate first. Set the authenticateHandler on GKLocalPlayer.local early in the app lifecycle. GameKit calls the handler multiple times during initialization.

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import GameKit

func authenticatePlayer() {
    GKLocalPlayer.local.authenticateHandler = { viewController, error in
        if let viewController {
            // Present so the player can sign in or create an account.
            present(viewController, animated: true)
            return
        }
        if let error {
            // Player could not sign in. Disable Game Center features.
            disableGameCenter()
            return
        }

        // Player authenticated. Check restrictions before starting.
        let player = GKLocalPlayer.local

        if player.isUnderage {
            hideExplicitContent()
        }
        if player.isMultiplayerGamingRestricted {
            disableMultiplayer()
        }
        if player.isPersonalizedCommunicationRestricted {
            disableInGameChat()
        }

        configureAccessPoint()
    }
}

Guard on GKLocalPlayer.local.isAuthenticated before calling any GameKit API. For server-side identity verification, see references/gamekit-patterns.md.

Access Point

GKAccessPoint displays a Game Center control in a corner of the screen. When tapped, it opens the Game Center dashboard. Configure it after authentication.

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func configureAccessPoint() {
    GKAccessPoint.shared.location = .topLeading
    GKAccessPoint.shared.showHighlights = true
    GKAccessPoint.shared.isActive = true
}

Hide the access point during gameplay and show it on menu screens:

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GKAccessPoint.shared.isActive = false  // Hide during active gameplay
GKAccessPoint.shared.isActive = true   // Show on pause or menu

Open the dashboard to a specific state programmatically. Specific leaderboard access-point triggers require iOS 18+.

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// Open directly to a leaderboard
GKAccessPoint.shared.trigger(
    leaderboardID: "com.mygame.highscores",
    playerScope: .global,
    timeScope: .allTime
) { }

// Open directly to achievements
GKAccessPoint.shared.trigger(state: .achievements) { }

Dashboard

Present the Game Center dashboard using GKGameCenterViewController. The presenting object must conform to GKGameCenterControllerDelegate.

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final class GameViewController: UIViewController, GKGameCenterControllerDelegate {

    func showDashboard() {
        let vc = GKGameCenterViewController(state: .dashboard)
        vc.gameCenterDelegate = self
        present(vc, animated: true)
    }

    func showLeaderboard(_ leaderboardID: String) {
        let vc = GKGameCenterViewController(
            leaderboardID: leaderboardID,
            playerScope: .global,
            timeScope: .allTime
        )
        vc.gameCenterDelegate = self
        present(vc, animated: true)
    }

    func gameCenterViewControllerDidFinish(
        _ gameCenterViewController: GKGameCenterViewController
    ) {
        gameCenterViewController.dismiss(animated: true)
    }
}

Dashboard states include .dashboard, .leaderboards, .achievements, .challenges, .localPlayerProfile, and .localPlayerFriendsList.

Leaderboards

Configure leaderboards in App Store Connect before submitting scores. Supports classic (persistent) and recurring (time-limited, auto-resetting) types.

Submitting Scores

Submit to one or more leaderboards using the class method:

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func submitScore(_ score: Int, leaderboardIDs: [String]) async throws {
    try await GKLeaderboard.submitScore(
        score,
        context: 0,
        player: GKLocalPlayer.local,
        leaderboardIDs: leaderboardIDs
    )
}

Loading Entries

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func loadTopScores(
    leaderboardID: String,
    count: Int = 10
) async throws -> (GKLeaderboard.Entry?, [GKLeaderboard.Entry]) {
    let leaderboards = try await GKLeaderboard.loadLeaderboards(
        IDs: [leaderboardID]
    )
    guard let leaderboard = leaderboards.first else { return (nil, []) }

    let (localEntry, entries, _) = try await leaderboard.loadEntries(
        for: .global,
        timeScope: .allTime,
        range: 1...count
    )
    return (localEntry, entries)
}

GKLeaderboard.Entry provides player, rank, score, formattedScore, context, and date. For recurring leaderboard timing, leaderboard images, and leaderboard sets, see references/gamekit-patterns.md.

Achievements

Configure achievements in App Store Connect. Each achievement has a unique identifier, point value, and localized title/description.

Reporting Progress

Set percentComplete from 0...100. The property type is Double, but Apple requires an integer value. GameKit only accepts increases.

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func reportAchievement(identifier: String, percentComplete: Int) async throws {
    let achievement = GKAchievement(identifier: identifier)
    achievement.percentComplete = Double(min(max(percentComplete, 0), 100))
    achievement.showsCompletionBanner = true
    try await GKAchievement.report([achievement])
}

// Unlock an achievement completely
func unlockAchievement(_ identifier: String) async throws {
    try await reportAchievement(identifier: identifier, percentComplete: 100)
}

Loading Player Achievements

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func loadPlayerAchievements() async throws -> [GKAchievement] {
    try await GKAchievement.loadAchievements()
}

If an achievement is not returned, the player has no progress on it yet. Create a new GKAchievement(identifier:) to begin reporting. Use GKAchievement.resetAchievements() to reset all progress during testing.

Real-Time Multiplayer

Real-time multiplayer connects players in a peer-to-peer network for simultaneous gameplay. Players exchange data directly through GKMatch.

Matchmaking with GameKit UI

Use GKMatchmakerViewController for the standard matchmaking interface:

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func presentMatchmaker() {
    let request = GKMatchRequest()
    request.minPlayers = 2
    request.maxPlayers = 4
    request.inviteMessage = "Join my game!"

    guard let matchmakerVC = GKMatchmakerViewController(matchRequest: request) else {
        return
    }
    matchmakerVC.matchmakerDelegate = self
    present(matchmakerVC, animated: true)
}

Implement GKMatchmakerViewControllerDelegate:

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extension GameViewController: GKMatchmakerViewControllerDelegate {
    func matchmakerViewController(
        _ viewController: GKMatchmakerViewController,
        didFind match: GKMatch
    ) {
        match.delegate = self
        viewController.dismiss(animated: true)
        startGame(with: match)
    }

    func matchmakerViewControllerWasCancelled(
        _ viewController: GKMatchmakerViewController
    ) {
        viewController.dismiss(animated: true)
    }

    func matchmakerViewController(
        _ viewController: GKMatchmakerViewController,
        didFailWithError error: Error
    ) {
        viewController.dismiss(animated: true)
    }
}

Exchanging Data

Send and receive game state through GKMatch and GKMatchDelegate:

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extension GameViewController: GKMatchDelegate {
    func sendAction(_ action: GameAction, to match: GKMatch) throws {
        let data = try JSONEncoder().encode(action)
        try match.sendData(toAllPlayers: data, with: .reliable)
    }

    func match(_ match: GKMatch, didReceive data: Data, fromRemotePlayer player: GKPlayer) {
        guard let action = try? JSONDecoder().decode(GameAction.self, from: data) else {
            return
        }
        handleRemoteAction(action, from: player)
    }

    func match(_ match: GKMatch, player: GKPlayer, didChange state: GKPlayerConnectionState) {
        switch state {
        case .connected:
            checkIfReadyToStart(match)
        case .disconnected:
            handlePlayerDisconnected(player)
        default:
            break
        }
    }
}

Data modes: .reliable sends until delivery succeeds or the connection times out; .unreliable sends once and may arrive out of order. Use .reliable for critical state and .unreliable for small, time-sensitive updates. Treat received match data as untrusted input. Register the local player as a listener (GKLocalPlayer.local.register(self)) to receive invitations. For programmatic matchmaking and custom match UI, see references/gamekit-patterns.md.

Turn-Based Multiplayer

Turn-based games store match state on Game Center servers. Players take turns asynchronously and do not need to be online simultaneously.

Starting a Match

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let request = GKMatchRequest()
request.minPlayers = 2
request.maxPlayers = 4

let matchmakerVC = GKTurnBasedMatchmakerViewController(matchRequest: request)
matchmakerVC.turnBasedMatchmakerDelegate = self
present(matchmakerVC, animated: true)

Taking Turns

Encode game state into Data, end the turn, and specify the next participants:

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func endTurn(match: GKTurnBasedMatch, gameState: GameState) async throws {
    let data = try JSONEncoder().encode(gameState)

    // Build next participants list: remaining active players
    let nextParticipants = match.participants.filter {
        $0.status != .done && $0 != match.currentParticipant
    }

    try await match.endTurn(
        withNextParticipants: nextParticipants,
        turnTimeout: GKTurnTimeoutDefault,
        match: data
    )
}

Ending the Match

Set outcomes for all participants, then end the match:

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func endMatch(_ match: GKTurnBasedMatch, winnerIndex: Int, data: Data) async throws {
    for (index, participant) in match.participants.enumerated() {
        participant.matchOutcome = (index == winnerIndex) ? .won : .lost
    }
    try await match.endMatchInTurn(withMatch: data)
}

Listening for Turn Events

Register as a listener. Prefer GKLocalPlayerListener when one object handles multiple Game Center event categories.

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GKLocalPlayer.local.register(self)

extension GameViewController: GKLocalPlayerListener {
    func player(_ player: GKPlayer, receivedTurnEventFor match: GKTurnBasedMatch,
                didBecomeActive: Bool) {
        // Load match data and update UI
        loadAndDisplayMatch(match)
    }

    func player(_ player: GKPlayer, matchEnded match: GKTurnBasedMatch) {
        showMatchResults(match)
    }
}

Match Data Size

Check the match object's matchDataMaximumSize before ending a turn. Store larger state externally and keep only compact references in match data.

Common Mistakes

Not authenticating before using GameKit APIs

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// DON'T
func submitScore() {
    GKLeaderboard.submitScore(100, context: 0, player: GKLocalPlayer.local,
                              leaderboardIDs: ["scores"]) { _ in }
}

// DO
func submitScore() async throws {
    guard GKLocalPlayer.local.isAuthenticated else { return }
    try await GKLeaderboard.submitScore(
        100, context: 0, player: GKLocalPlayer.local, leaderboardIDs: ["scores"]
    )
}

Setting authenticateHandler multiple times

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// DON'T: Set handler on every scene transition
override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    GKLocalPlayer.local.authenticateHandler = { vc, error in /* ... */ }
}

// DO: Set the handler once, early in the app lifecycle

Ignoring multiplayer restrictions

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// DON'T
func showMultiplayerMenu() { presentMatchmaker() }

// DO
func showMultiplayerMenu() {
    guard !GKLocalPlayer.local.isMultiplayerGamingRestricted else { return }
    presentMatchmaker()
}

Not setting match delegate immediately

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// DON'T: Set delegate in dismiss completion -- misses early messages
func matchmakerViewController(_ vc: GKMatchmakerViewController, didFind match: GKMatch) {
    vc.dismiss(animated: true) { match.delegate = self }
}

// DO: Set delegate before dismissing
func matchmakerViewController(_ vc: GKMatchmakerViewController, didFind match: GKMatch) {
    match.delegate = self
    vc.dismiss(animated: true)
}

Not calling finishMatchmaking for programmatic matches

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// DON'T
let match = try await GKMatchmaker.shared().findMatch(for: request)
startGame(with: match)

// DO
let match = try await GKMatchmaker.shared().findMatch(for: request)
GKMatchmaker.shared().finishMatchmaking(for: match)
startGame(with: match)

Not disconnecting from match

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// DON'T
func returnToMenu() { showMainMenu() }

// DO
func returnToMenu() {
    currentMatch?.disconnect()
    currentMatch?.delegate = nil
    currentMatch = nil
    showMainMenu()
}

Review Checklist

• GKLocalPlayer.local.authenticateHandler set once at app launch
• isAuthenticated checked before any GameKit API call
• Player restrictions checked (isUnderage, isMultiplayerGamingRestricted, isPersonalizedCommunicationRestricted)
• Game Center capability added in Xcode signing settings
• Leaderboards and achievements configured in App Store Connect
• Access point configured and toggled appropriately during gameplay
• GKGameCenterControllerDelegate dismisses dashboard in gameCenterViewControllerDidFinish
• Match delegate set immediately when match is found
• finishMatchmaking(for:) called for programmatic matches; disconnect() and nil delegate on exit
• Turn-based match data stays under match.matchDataMaximumSize
• Turn-based participants have outcomes set before endMatchInTurn
• Invitation or turn listener registered with GKLocalPlayer.local.register(_:)
• Data mode chosen appropriately: .reliable for state, .unreliable for frequent updates
• Error handling for all async GameKit calls

References

• See references/gamekit-patterns.md for identity verification, legacy voice chat, saved games, custom match UI, leaderboard images, challenge handling, and rule-based matchmaking.
• GameKit documentation
• GKLocalPlayer
• GKAccessPoint
• GKLeaderboard
• GKAchievement
• GKMatch
• GKTurnBasedMatch