how to add google authentication in flutter firebase code example

Example 1: signIn google firebase flutter

import 'package:firebase_auth/firebase_auth.dart';
import 'package:google_sign_in/google_sign_in.dart';

final FirebaseAuth _auth = FirebaseAuth.instance;
final GoogleSignIn googleSignIn = GoogleSignIn();

Future<String> signInWithGoogle() async {
  final GoogleSignInAccount googleSignInAccount = await googleSignIn.signIn();
  final GoogleSignInAuthentication googleSignInAuthentication =
      await googleSignInAccount.authentication;

  final AuthCredential credential = GoogleAuthProvider.getCredential(
    accessToken: googleSignInAuthentication.accessToken,
    idToken: googleSignInAuthentication.idToken,
  );

  final AuthResult authResult = await _auth.signInWithCredential(credential);
  final FirebaseUser user = authResult.user;

  assert(!user.isAnonymous);
  assert(await user.getIdToken() != null);

  final FirebaseUser currentUser = await _auth.currentUser();
  assert(user.uid == currentUser.uid);

  return 'signInWithGoogle succeeded: $user';
}

void signOutGoogle() async{
  await googleSignIn.signOut();

  print("User Sign Out");
}

Example 2: flutter firebase google auth

import 'package:firebase_auth/firebase_auth.dart';
import 'package:google_sign_in/google_sign_in.dart';

class GoogleAuth {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  final GoogleSignIn googleSignIn = GoogleSignIn();

  User getUser() {
    return _auth.currentUser;
  }

  Future<String> getUserId() async {
    final User user = _auth.currentUser;
    final uid = user.uid;
    return uid;
  }

  Future<User> signInWithGoogle() async {
    final GoogleSignInAccount googleSignInAccount = await googleSignIn.signIn();
    final GoogleSignInAuthentication googleSignInAuthentication =
        await googleSignInAccount.authentication;

    final AuthCredential credential = GoogleAuthProvider.credential(
      accessToken: googleSignInAuthentication.accessToken,
      idToken: googleSignInAuthentication.idToken,
    );

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

    assert(!user.isAnonymous);
    assert(await user.getIdToken() != null);

    final User currentUser = _auth.currentUser;
    assert(user.uid == currentUser.uid);

    print('signInWithGoogle succeeded: $user');

    return currentUser;
  }

  void signOutGoogle() async {
    await googleSignIn.signOut();
    print("User Sign Out");
  }
}