How to check If user is new or existing when login with google using firebase in flutter?

Question

I am working on a project in which user data will be saved in Fire store in this i am making new document for user when the user sign In with google. here is my code below

import 'package:firebase_auth/firebase_auth.dart';
import 'package:gfd_official/User/User.dart';
import 'package:gfd_official/services/database.dart';
import 'package:google_sign_in/google_sign_in.dart';

class GSignInhelp {
 final FirebaseAuth _auth = FirebaseAuth.instance;

 //Firebase User
 Userdat _userFromFirebase(User user) {
 return user != null ? Userdat(uid: user.uid) : null;
 }

 //auth change user stream
 Stream<Userdat> get user {
 return _auth.authStateChanges().map(_userFromFirebase);
}

Future signInWithGoogle() async {
GoogleSignIn googleSignIn = GoogleSignIn();
final account = await googleSignIn.signIn();
final auth = await account.authentication;
final credential = GoogleAuthProvider.credential(
accessToken: auth.accessToken,
idToken: auth.idToken,
);
try {
final res = await _auth.signInWithCredential(credential);
User user = res.user;
await DatabaseService(uid: user.uid)
    .updateUserRole(user.displayName, user.email, 'basic');
return _userFromFirebase(user);
} catch (e) {
  print(e.toString());
  return null;
  }
}

Future logOut() async {
try {
  GoogleSignIn().signOut();
  return await _auth.signOut();
} catch (e) {
  print(e.toString());
  return null;
}
}
}

I am successfully able to change user role in fire store but as you can see when user signs in the database values will be set to default but I don't want this. So, my question is how can I check that if user is signed in for first time or returning user.

If user is returning then I don't want to reset user data in Fire store and If user is new then create document in Fire store with default values.

Conclusion, How can I check that When user signs in with google that is user a existing user or new a user?

Answer 1

You can check the user whether he is a new or previously registered user via User Credential and through you can retrieve the authResult.additionalUserInfo.isNewUser if the user is not already registered before.

You can use this code:

 Future<UserCredential> signInWithGoogle(BuildContext context) async {

    FirebaseAuth _auth = FirebaseAuth.instance;
    // Trigger the authentication flow
    final GoogleSignInAccount googleUser = await GoogleSignIn().signIn();

    // Obtain the auth details from the request
    final GoogleSignInAuthentication googleAuth =
        await googleUser.authentication;

    // Create a new credential
    final GoogleAuthCredential credential = GoogleAuthProvider.credential(
      accessToken: googleAuth.accessToken,
      idToken: googleAuth.idToken,
    );

    final UserCredential authResult =
        await _auth.signInWithCredential(credential);

    final User user = authResult.user;

    ///Her to check isNewUser OR Not
    if (authResult.additionalUserInfo.isNewUser) {
      if (user != null) {
      //You can her set data user in Fire store
      //Ex: Go to RegisterPage()
        }

      }
    } else {
     
      //Ex: Go to HomePage()
    }

    return await FirebaseAuth.instance.signInWithCredential(credential);
  }

Answer 2

Here is the full code for Google sign in and sign out :-

class GSignInhelp {  
final FirebaseAuth _auth = FirebaseAuth.instance;   //Firebase User   
Userdat _userFromFirebaseUser(User user) {    
          return user != null ? Userdat(uid: user.uid) : null;   }   //auth 
         change user stream   
         Stream get user {    
             return _auth.authStateChanges().map(_userFromFirebaseUser);   
        }   
   Future signInWithGoogle() async {     
         GoogleSignIn googleSignIn = 
    GoogleSignIn();