You can allow users to sign in to your app using multiple authentication providers by linking auth provider credentials to an existing user account. Users are identifiable by the same Firebase user ID regardless of the authentication provider they used to sign in. For example, a user who signed in with a password can link a Google account and sign in with either method in the future. Or, an anonymous user can link a Facebook account and then, later, sign in with Facebook to continue using your app.
Before you begin
Add support for two or more authentication providers (possibly including anonymous authentication) to your app.
Link auth provider credentials to a user account
To link auth provider credentials to an existing user account:
- Sign in the user using any authentication provider or method.
- Complete the sign-in flow for the new authentication provider up to, but not
including, calling one of the
FIRAuth.signInWithmethods. For example, get the user's Google ID token, Facebook access token, or email and password. -
Get a
FIRAuthCredentialfor the new authentication provider:Google Sign-In
Swift
guard let authentication = user ?. authentication , let idToken = authentication . idToken else { return } let credential = GoogleAuthProvider . credential ( withIDToken : idToken , accessToken : authentication . accessToken )
Objective-C
FIRAuthCredential * credential = [ FIRGoogleAuthProvider credentialWithIDToken : result . user . idToken . tokenString accessToken : result . user . accessToken . tokenString ];
Facebook Login
Swift
let credential = FacebookAuthProvider . credential ( withAccessToken : AccessToken . current !. tokenString )
Objective-C
FIRAuthCredential * credential = [ FIRFacebookAuthProvider credentialWithAccessToken :[ FBSDKAccessToken currentAccessToken ]. tokenString ];
Email-password sign-in
Swift
let credential = EmailAuthProvider . credential ( withEmail : email , password : password )
Objective-C
FIRAuthCredential * credential = [ FIREmailAuthProvider credentialWithEmail : email password : password ];
-
Pass the
FIRAuthCredentialobject to the signed-in user'slinkWithCredential:completion:method:Swift
user . link ( with : credential ) { authResult , error in // ... } }
Objective-C
[[ FIRAuth auth ]. currentUser linkWithCredential : credential completion : ^ ( FIRAuthDataResult * result , NSError * _Nullable error ) { // ... }];
The call to
linkWithCredential:completion:will fail if the credentials are already linked to another user account. In this situation, you must handle merging the accounts and associated data as appropriate for your app:Swift
let prevUser = Auth . auth (). currentUser Auth . auth (). signIn ( with : credential ) { authResult , error in if let error = error { let authError = error as NSError if isMFAEnabled , authError . code == AuthErrorCode . secondFactorRequired . rawValue { // The user is a multi-factor user. Second factor challenge is required. let resolver = authError . userInfo [ AuthErrorUserInfoMultiFactorResolverKey ] as ! MultiFactorResolver var displayNameString = "" for tmpFactorInfo in resolver . hints { displayNameString += tmpFactorInfo . displayName ?? "" displayNameString += " " } self . showTextInputPrompt ( withMessage : "Select factor to sign in \n \( displayNameString ) " , completionBlock : { userPressedOK , displayName in var selectedHint : PhoneMultiFactorInfo ? for tmpFactorInfo in resolver . hints { if displayName == tmpFactorInfo . displayName { selectedHint = tmpFactorInfo as ? PhoneMultiFactorInfo } } PhoneAuthProvider . provider () . verifyPhoneNumber ( with : selectedHint !, uiDelegate : nil , multiFactorSession : resolver . session ) { verificationID , error in if error != nil { print ( "Multi factor start sign in failed. Error: \( error . debugDescription ) " ) } else { self . showTextInputPrompt ( withMessage : "Verification code for \( selectedHint ?. displayName ?? "" ) " , completionBlock : { userPressedOK , verificationCode in let credential : PhoneAuthCredential ? = PhoneAuthProvider . provider () . credential ( withVerificationID : verificationID !, verificationCode : verificationCode !) let assertion : MultiFactorAssertion ? = PhoneMultiFactorGenerator . assertion ( with : credential !) resolver . resolveSignIn ( with : assertion !) { authResult , error in if error != nil { print ( "Multi factor finanlize sign in failed. Error: \( error . debugDescription ) " ) } else { self . navigationController ?. popViewController ( animated : true ) } } } ) } } } ) } else { self . showMessagePrompt ( error . localizedDescription ) return } // ... return } // User is signed in // ... } // Merge prevUser and currentUser accounts and data // ... }
Objective-C
FIRUser * prevUser = [ FIRAuth auth ]. currentUser ; [[ FIRAuth auth ] signInWithCredential : credential completion : ^ ( FIRAuthDataResult * _Nullable authResult , NSError * _Nullable error ) { if ( isMFAEnabled && error && error . code == FIRAuthErrorCodeSecondFactorRequired ) { FIRMultiFactorResolver * resolver = error . userInfo [ FIRAuthErrorUserInfoMultiFactorResolverKey ]; NSMutableString * displayNameString = [ NSMutableString string ]; for ( FIRMultiFactorInfo * tmpFactorInfo in resolver . hints ) { [ displayNameString appendString : tmpFactorInfo . displayName ]; [ displayNameString appendString : @" " ]; } [ self showTextInputPromptWithMessage : [ NSString stringWithFormat : @"Select factor to sign in \n %@" , displayNameString ] completionBlock : ^ ( BOOL userPressedOK , NSString * _Nullable displayName ) { FIRPhoneMultiFactorInfo * selectedHint ; for ( FIRMultiFactorInfo * tmpFactorInfo in resolver . hints ) { if ([ displayName isEqualToString : tmpFactorInfo . displayName ]) { selectedHint = ( FIRPhoneMultiFactorInfo * ) tmpFactorInfo ; } } [ FIRPhoneAuthProvider . provider verifyPhoneNumberWithMultiFactorInfo : selectedHint UIDelegate : nil multiFactorSession : resolver . session completion : ^ ( NSString * _Nullable verificationID , NSError * _Nullable error ) { if ( error ) { [ self showMessagePrompt : error . localizedDescription ]; } else { [ self showTextInputPromptWithMessage : [ NSString stringWithFormat : @"Verification code for %@" , selectedHint . displayName ] completionBlock : ^ ( BOOL userPressedOK , NSString * _Nullable verificationCode ) { FIRPhoneAuthCredential * credential = [[ FIRPhoneAuthProvider provider ] credentialWithVerificationID : verificationID verificationCode : verificationCode ]; FIRMultiFactorAssertion * assertion = [ FIRPhoneMultiFactorGenerator assertionWithCredential : credential ]; [ resolver resolveSignInWithAssertion : assertion completion :^ ( FIRAuthDataResult * _Nullable authResult , NSError * _Nullable error ) { if ( error ) { [ self showMessagePrompt : error . localizedDescription ]; } else { NSLog ( @"Multi factor finanlize sign in succeeded." ); } }]; }]; } }]; }]; } else if ( error ) { // ... return ; } // User successfully signed in. Get user data from the FIRUser object if ( authResult == nil ) { return ; } FIRUser * user = authResult . user ; // ... }]; // Merge prevUser and currentUser accounts and data // ... }];
If the call to linkWithCredential:completion:
succeeds, the user can now sign in using
any linked authentication provider and access the same Firebase data.
Unlink an auth provider from a user account
You can unlink an auth provider from an account, so that the user can no longer sign in with that provider.
To unlink an auth provider from a user account, pass the provider ID to the unlink
method. You can get the provider IDs
of the auth providers linked to a user from the providerData
property.
Swift
Auth . auth (). currentUser ?. unlink ( fromProvider : providerID !) { user , error in // ... }
Objective-C
[[ FIRAuth auth ]. currentUser unlinkFromProvider : providerID completion : ^ ( FIRUser * _Nullable user , NSError * _Nullable error ) { // ... }];
Troubleshooting
If you encounter errors when trying to link multiple accounts, see the documentation on verified email addresses .
