Firebase Phone Auth along with account linking - ios

Question

I am having a problem with phone authentication credentials persistence/expiry.
My scenario is like this:
I have a guest user, that I want to link with a phone number. The flow works perfectly if an account is not registered with that phone number. But if it does exist, then I have to:
SignIn and Unlink account.
Link account.
Sign In.
This requires 3 different credentials. But credentials expire for phone authentication after it gets used once - as per my understanding from the error message :
The SMS code has expired. Please re-send the verification code to try
again.
I do not want to ask user 3 times in a row for verification code on his mobile so new credentials can be generated. Any way to make credentials stick or a way around this problem ?
I can share the code if needed but I do not think it would be of any help.

Answer 1

Here is what you should do:
Initialize the phone auth credential first. Try to link that credential to the guest account always. If it fails with an error "credential already in user", the error userinfo will contain a new credential. This credential can then be used to sign in to the existing phone number account. Here is an example in objective-c.
[[FIRAuth auth].currentUser linkWithCredential:credential
completion:^(FIRUser *_Nullable user,
NSError *_Nullable error) {
if (user) {
// Successfully linked credential.
return;
}
if (error.code == FIRAuthErrorCodePhoneAlreadyInUse) {
// Save guest user data.
// Sign in the user instead if applicable.
FIRAuthCredential *credential = error.userInfo[FIRAuthUpdatedCredentialKey];
[[FIRAuth auth] signInWithCredential:credential
completion:^(FIRUser *_Nullable user,
NSError *_Nullable error) {
// copy guest user data to existing phone number user.
}];
return;
}
// Other errors.
}];
You can then programmatically copy the data of the guest user to the existing user and delete the guest user.
All this can be done efficiently with one SMS sent.