Authenticate Using Game Center

You can use Game Center to sign players in to an Apple platforms game built on Firebase. To use Game Center Sign-in with Firebase, first make sure the local player is signed in with Game Center, and then use the GameCenterAuthProvider object to generate a Firebase credential, which you can use to authenticate with Firebase.

Before you begin

Use Swift Package Manager to install and manage Firebase dependencies.

  1. In Xcode, with your app project open, navigate to File > Add Packages.
  2. When prompted, add the Firebase Apple platforms SDK repository:
    3.   https://github.com/firebase/firebase-ios-sdk.git
  3. Choose the Firebase Authentication library.
  4. Add the -ObjC flag to the Other Linker Flags section of your target's build settings.
  5. When finished, Xcode will automatically begin resolving and downloading your dependencies in the background.

Next, perform some configuration steps:

  1. Make sure you register your Apple app with Firebase. This means entering your app's bundle ID in the registration section along with additional optional information such as App Store ID and Team ID, etc. This will be required for securely verifying the audience of the user's Game Center credential before completing sign-in.
  2. Enable Game Center as a sign-in provider for your Firebase project:
    1. In the Firebase console, open the Authentication section.
    2. On the Sign in method tab, enable the Game Center sign-in provider.

Integrate Game Center Sign-in into your game

First, if your game doesn't already use Game Center, follow the instructions in Incorporating Game Center into Your Game and Authenticating a Local Player on the Device on the Apple developer site.

Be sure the bundle ID you provide to iTunes Connect matches the bundle ID you used when you connected your app to your Firebase project.

As part of your Game Center integration, you define an authentication handler that is called at multiple points in the Game Center authentication process. In this handler, check if the player is signed in with Game Center. If so, you can continue to sign in to Firebase.

Swift

let localPlayer = GKLocalPlayer.localPlayer()
localPlayer.authenticateHandler = { (gcAuthViewController?, error) in
  if let gcAuthViewController = gcAuthViewController {
    // Pause any activities that require user interaction, then present the
    // gcAuthViewController to the player.
  } else if localPlayer.isAuthenticated {
    // Player is signed in to Game Center. Get Firebase credentials from the
    // player's Game Center credentials (see below).
  } else {
    // Error
  }
}

Objective-C

__weak GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
localPlayer.authenticateHandler = ^(UIViewController *gcAuthViewController,
                                    NSError *error) {
  if (gcAuthViewController != nil) {
    // Pause any activities that require user interaction, then present the
    // gcAuthViewController to the player.
  } else if (localPlayer.isAuthenticated) {
    // Player is signed in to Game Center. Get Firebase credentials from the
    // player's Game Center credentials (see below).
  } else {
    // Error
  }
};

Authenticate with Firebase

After you determine that the local player has signed in with Game Center, sign the player in to your game by creating an AuthCredential object with GameCenterAuthProvider.getCredential() and passing that object to signIn(with:):

Swift

// Get Firebase credentials from the player's Game Center credentials
GameCenterAuthProvider.getCredential() { (credential, error) in
  if let error = error {
    return
  }
  // The credential can be used to sign in, or re-auth, or link or unlink.
  Auth.auth().signIn(with:credential) { (user, error) in
    if let error = error {
      return
    }
    // Player is signed in!
  }

Objective-C

// Get Firebase credentials from the player's Game Center credentials
[FIRGameCenterAuthProvider getCredentialWithCompletion:^(FIRAuthCredential *credential,
                                                         NSError *error) {
  // The credential can be used to sign in, or re-auth, or link or unlink.
  if (error == nil) {
    [[FIRAuth auth] signInWithCredential:credential
                              completion:^(FIRUser *user, NSError *error) {
      // If error is nil, player is signed in.
    }];
  }
}];

Next steps

After a user signs in for the first time, a new user account is created and linked to their Game Center ID. This new account is stored as part of your Firebase project, and can be used to identify a user across every app in your project.

In your game, you can get the user's Firebase UID from the User object:

Swift

let user = Auth.auth().currentUser
if let user = user {
  let playerName = user.displayName

  // The user's ID, unique to the Firebase project.
  // Do NOT use this value to authenticate with your backend server,
  // if you have one. Use getToken(with:) instead.
  let uid = user.uid
}

Objective-C

FIRUser *user = [FIRAuth auth].currentUser;
if (user) {
  NSString *playerName = user.displayName;

  // The user's ID, unique to the Firebase project.
  // Do NOT use this value to authenticate with your backend server,
  // if you have one. Use getTokenWithCompletion:completion: instead.
  NSString *uid = user.uid;
}

In your Firebase Realtime Database and Cloud Storage Security Rules, you can get the signed-in user's unique user ID from the auth variable, and use it to control what data a user can access.

To get a user's Game Center player information or to access Game Center services, use the APIs provided by Game Kit.

To sign a user out of Firebase, call Auth.signOut():

Swift

let firebaseAuth = Auth.auth()
do {
  try firebaseAuth.signOut()
} catch let signOutError as NSError {
  print ("Error signing out: %@", signOutError)
}

Objective-C

NSError *signOutError;
BOOL status = [[FIRAuth auth] signOut:&signOutError];
if (!status) {
  NSLog(@"Error signing out: %@", signOutError);
  return;
}