Flutter firebase OTP Service

OTPService.dart

///
///   OTPService
///   cloud_firestore: ^0.12.9
///   firebase_storage: ^3.0.6
///   firebase_auth: ^0.14.0
///

import 'package:firebase_auth/firebase_auth.dart';

typedef void PhoneSubmitCallBack(String status);

class OTPService {
  late FirebaseUser _firebaseUser;

  late AuthCredential _phoneAuthCredential;
  late String _verificationId;

  static final OTPService _singleton = OTPService._internal();

  factory OTPService() {
    return OTPService._singleton;
  }

  OTPService._internal() {
    // _getFirebaseUser();
  }

  Future<void> _getFirebaseUser() async {
    this._firebaseUser = await FirebaseAuth.instance.currentUser();
    String status =
        (_firebaseUser == null) ? 'Not Logged In\n' : 'Already LoggedIn\n';
    print(status);
  }

  void _handleError(e) {
    print(e.message);
  }

  Future<String> submitPhoneNumber(phone, PhoneSubmitCallBack callBack) async {
    String phoneNumber = phone;
    print(phoneNumber);
    String _status = "";

    void verificationCompleted(AuthCredential phoneAuthCredential) {
      print('verificationCompleted');
      _status = 'verificationCompleted\n';
      this._phoneAuthCredential = phoneAuthCredential;
      print(phoneAuthCredential);
      callBack(_status);
    }

    void verificationFailed(AuthException error) {
      _status = 'verificationFailed';
      _handleError(error);
      callBack(_status);
    }

    void codeSent(String verificationId, [int? code]) {
      _status = 'codeSent';
      this._verificationId = verificationId;
      print(verificationId);
      print(code.toString());
      print('Code Sent\n');
      callBack(_status);
    }

    void codeAutoRetrievalTimeout(String verificationId) {
      _status = 'codeAutoRetrievalTimeout';
      print(verificationId);
      callBack(_status);
    }

    await FirebaseAuth.instance.verifyPhoneNumber(
      phoneNumber: phoneNumber,
      timeout: Duration(milliseconds: 10000),
      verificationCompleted: verificationCompleted,
      verificationFailed: verificationFailed,
      codeSent: codeSent,
      codeAutoRetrievalTimeout: codeAutoRetrievalTimeout,
    );
    return _status;
  }

  Future<String> submitOTP(otp) async {
    String smsCode = otp;
    this._phoneAuthCredential = PhoneAuthProvider.getCredential(
        verificationId: this._verificationId, smsCode: smsCode);

    return _login();
  }

  Future<String> _login() async {
    try {
      return FirebaseAuth.instance
          .signInWithCredential(this._phoneAuthCredential)
          .then((AuthResult authRes) {
        _firebaseUser = authRes.user;
        return _firebaseUser.toString();
      }).catchError((e) {
        return e.toString();
      });
    } catch (e) {
      return e.toString();
    }
  }
}