How to do Phone Authentication in Flutter using Firebase?

Currently _signInPhoneNumber is deprecated, so use this:

try {
    AuthCredentialauthCredential = PhoneAuthProvider.getCredential(verificationId: verificationId, verificationsCode: smsCode);

    await _firebaseAuth
      .signInWithCredential(authCredential)
      .then((FirebaseUser user) async {
         final FirebaseUser currentUser = await _firebaseAuth.currentUser();
         assert(user.uid == currentUser.uid);
         print('signed in with phone number successful: user -> $user');
    }
}

Below are the steps:-

1. Get OTP based on Phone Number:-

   void sendOTP(String phoneNumber, PhoneCodeSent codeSent,
         PhoneVerificationFailed verificationFailed) {
       if (!phoneNumber.contains('+')) phoneNumber = '+91' + phoneNumber;
       _firebaseAuth.verifyPhoneNumber(
           phoneNumber: phoneNumber,
           timeout: Duration(seconds: 30),
           verificationCompleted: null,
           verificationFailed: verificationFailed,
           codeSent: codeSent,
           codeAutoRetrievalTimeout: null);   }
   

2. Use the codeSent function to retrieve verification_id and pass it to OTP screen

   void codeSent(String verificationId, [int forceResendingToken]) {
   Navigator.push(
       context,
       MaterialPageRoute(
           builder: (context) => Otp(
                 phoneNumber: _phoneNumber,
                 verificationId: verificationId,
               )));
   }
   

3. Get user based on verification_id and OTP

   Future<bool> verifyOTP(String verificationId, String otp) async {
   AuthCredential credential = PhoneAuthProvider.getCredential(
     verificationId: verificationId,
     smsCode: otp,
   );
   AuthResult result;
   try {
     result = await _firebaseAuth.signInWithCredential(credential);
   } catch (e) {
     // throw e;
     return false;
   }
   print(result);
   (await result.user.getIdToken()).claims.forEach((k, v) {
     print('k= $k and v= $v');
   });
   if (result.user.uid != null) return true;
   return false;
   }
   

For more details watch the below video

https://youtu.be/e5M3EwJJYS4

I had the same issue but I was building an Ionic app.

you can do firebase phone auth on the web. the idea is:

• take the user's phone number and send to a webpage.

• create a recaptchaVerifier

  var recaptchaVerifier = new firebase.auth.RecaptchaVerifier('recaptcha-container');
  

• sign in the user with phone number:

  firebase.auth().signInWithPhoneNumber(phoneNumber, recaptchaVerifier)
          .then(confirmationResult => {
              myVerificationId = confirmationResult.verificationId;
          })
          .catch(error => {
              console.log('error', error);
          })
  

• send the confirmation id back to the flutter app through a deeplink

• Firebase will send the sms code.

• sign in the user with any method that takes the confirmation Id and the code as parameters. for the web it goes like this:

    let signinCredintial = firebase.auth.PhoneAuthProvider.credential(this.verificationId, this.code);
    firebase.auth().signInWithCredential(signinCredintial)
     .then(response => {
       // user is signed in
  
  })
  

I have implemented the phone auth singnIn with firebase in flutter its quite easy just import the firebase_auth library and validate the phone number is in proper format i.e it has a "+" sign in the beginning followed by country code then the phone number then the code goes like this

if (phoneExp.hasMatch(phon))
   {
      final PhoneVerificationCompleted verificationCompleted=(FirebaseUser user){
        setState(() {
                      _message=Future<String>.value("auto sign in succedded $user");
                      debugPrint("Sign up succedded");
                      _pref.setString("phonkey",user.phoneNumber.toString());
                      MyNavigator.goToDetail(context);
//called when the otp is variefied automatically
                    });
      };
  final PhoneVerificationFailed verificationFailed=(AuthException authException){
    setState(() {
                  _message=Future<String>.value("verification failed code: ${authException.code}. Message: ${authException.message}");
                });
  };

  final PhoneCodeSent codeSent=(String verificationId,[int forceResendingToken]) async {
    this.verificationId=verificationId;


  };

  final PhoneCodeAutoRetrievalTimeout codeAutoRetrievalTimeout = (String verificationId){
    this.verificationId=verificationId;

  };

  await _auth.verifyPhoneNumber(
    phoneNumber: phon,
    timeout: Duration(seconds: 60),
    verificationCompleted: verificationCompleted,
    verificationFailed: verificationFailed,
    codeSent: codeSent,
    codeAutoRetrievalTimeout: codeAutoRetrievalTimeout
    );



}

and if the phone wasn't able to detect the otp sent automatically, get the otp in a string and implement this function

void _signInWithOtp() async{
  final FirebaseUser user = await _auth.signInWithPhoneNumber(
  verificationId: verificationId,
  smsCode: _otpController.text,
);

Use the flutter_otp package to send SMS with OTP easily. The advantages are:

1. You can send custom OTP messages.
2. OTP can be generated in any range.

Example of using it :

 import 'package:flutter_otp/flutter_otp.dart';

 ...

 FlutterOtp myOtpObj = FlutterOtp();
 myOtpObj.sendOtp('7975235555');

 ...

 // you can check as follows
 bool correctOrNot = myOtpObj.resultChecker(<OTP entered by user>);

Check out the flutter_otp repository here