[Firebase Auth] Possibility to use AuthCredential with Apple provider more than once.

[yamauchieduardo]

Explanation why I need this feature:

In Firebase Projects we have some diffent types of AuthCredential, some can be used once time, and another more than once.

AuthCredential that can be once time: PhoneAuth.
AuthCredential that can be used more once time: GoogleAuthCredential & FacebookAuthCredential.

• Use the credential is when I call a method that will confirm if the credential is valid like signInWithCredential, linkWithCredential.

For Example:

• Using a credential that can be used MORE THAN ONCE, I can use the credential to linkWithCredential and after that I CAN REUSE the same credential to signInWithCredential (GoogleAuthCredential or FacebookAuthCredential).
• Using a credential that can be used JUST ONCE, if I use the credential to linkWithCredential and CAN'T REUSE the same credential to signInWithCredential (PhoneAuthCredential).

Actually Apple Login doesn't have any specific class to login, usually we use OAuthCredential to login with Apple.

 /// Get the credential from Apple.	
final appleCredential = await SignInWithApple.getAppleIDCredential(
 scopes: [
    AppleIDAuthorizationScopes.email,
    AppleIDAuthorizationScopes.fullName,
 ];
);

final oAuthProvider = OAuthProvider('apple.com');

final oAuthCredential = oAuthProvider.credential(
   idToken: appleCredential.identityToken,
   accessToken: appleCredential.authorizationCode,
);

/// Use the apple credential to login.
await FirebaseAuth.instance.signInWithCredential(oAuthCredential);

Observation:

Providers like Google, Facebook, Twitter and Github have their own classes. (GoogleAuthCredential, FacebookAuthCredential, TwitterAuthCredential, GithubAuthCredential, etc).

In my project have an important requirement: The users NEED to have a custom UID.

To solve this problem, I build two paths:

The first path is when the user give me an AuthCredentials than can be used MORE THAN ONCE (GoogleAuthCredential, FacebookAuthCredential, etc).

1 - I login the user using signInWithCredential method.
1.a - if the FirebaseCurrentUser have a custom Id finishes the login process.
1.b - if the FirebaseCurrentUser don't have a custom id:
1.b.1 - I loggout the user.
1.b.2 - I delete the user account.
1.b.3 - I create a new account using custom-token (that have a custom-id).
1.b.4 - Link the new account (custom-id) with the AuthCredential (that the users give me) and finish the login process.

The second path is when the user give me an AuthCredential than can be used JUST ONCE (PhoneCredential).

1 - I use a Firebase-Function (admin.auth) to check if the user already have an account (using some identifier, like phone for PhoneCredential).
1.a - If the user already have an account, login the user using signInWithCredential method and finish the login process.
1.b - If the user don't have account:
1.b.1 - I create a new account using custom-token (that have a custom-id).
1.b.2 - Link the new account (custom-id) with the AuthCredential (that the users give me) and finish the login process.

This worked fine, until I notice that AppleProvider could don't have ANY IDENTIFIER (because the user can choose to anonymize all data).

If the Apple Credential can be REUSED, I can use the first path to build accounts with custom-id.

Without the possibility to reuse the Apple AuthCredential this type of workaround can't be possible.

Thanks for the attention.