How to do Phone Authentication in Flutter using Firebase?

Well Documented Working Demo project here

Below is the detailed procedure

Steps

1. Ask for user's phoneNumber
2. Get OTP from Firebase
3. SignIn to Firebase

Rules

• SignIn/Login is done in the same way.
• The OTP is only used to get AuthCrendential object
• AuthCredential object is the only thing that is used to signIn the user. It is obtained either from verificationCompleted callback function in verifyPhoneNumber or from the PhoneAuthProvider.

(Don't worry if it's confusing, keep reading, you'll get it)

Workflow

1. User gives the phoneNumber
2. Firebase sends OTP
3. SignIn the user
   • If the SIM card with the phoneNumber is not in the device that is currently running the app,
     • We have to first ask the OTP and get AuthCredential object
     • Next we can use that AuthCredential to signIn This method works even if the phoneNumber is in the device
   • Else if user provided SIM phoneNumber is in the device running the app,
     • We can signIn without the OTP.
     • because the verificationCompleted callback from submitPhoneNumber function gives the AuthCredential object which is needed to signIn the user
     • But in the previous case, it ain't called because the SIM is not in the phone.

Functions

• SubmitPhoneNumber

Future<void> _submitPhoneNumber() async {
    /// NOTE: Either append your phone number country code or add in the code itself
    /// Since I'm in India we use "+91 " as prefix `phoneNumber`
    String phoneNumber = "+91 " + _phoneNumberController.text.toString().trim();
    print(phoneNumber);

    /// The below functions are the callbacks, separated so as to make code more readable
    void verificationCompleted(AuthCredential phoneAuthCredential) {
      print('verificationCompleted');
      ...
      this._phoneAuthCredential = phoneAuthCredential;
      print(phoneAuthCredential);
    }

    void verificationFailed(AuthException error) {
      ...
      print(error);
    }

    void codeSent(String verificationId, [int code]) {
      ...
      print('codeSent');
    }

    void codeAutoRetrievalTimeout(String verificationId) {
      ...
      print('codeAutoRetrievalTimeout');
    }

    await FirebaseAuth.instance.verifyPhoneNumber(
      /// Make sure to prefix with your country code
      phoneNumber: phoneNumber,

      /// `seconds` didn't work. The underlying implementation code only reads in `milliseconds`
      timeout: Duration(milliseconds: 10000),

      /// If the SIM (with phoneNumber) is in the current device this function is called.
      /// This function gives `AuthCredential`. Moreover `login` function can be called from this callback
      verificationCompleted: verificationCompleted,

      /// Called when the verification is failed
      verificationFailed: verificationFailed,

      /// This is called after the OTP is sent. Gives a `verificationId` and `code`
      codeSent: codeSent,

      /// After automatic code retrival `tmeout` this function is called
      codeAutoRetrievalTimeout: codeAutoRetrievalTimeout,
    ); // All the callbacks are above
  }

• SubmitOTP

void _submitOTP() {
    /// get the `smsCode` from the user
    String smsCode = _otpController.text.toString().trim();

    /// when used different phoneNumber other than the current (running) device
    /// we need to use OTP to get `phoneAuthCredential` which is inturn used to signIn/login
    this._phoneAuthCredential = PhoneAuthProvider.getCredential(
        verificationId: this._verificationId, smsCode: smsCode);

    _login();
  }

• SignIn/LogIn

Future<void> _login() async {
    /// This method is used to login the user
    /// `AuthCredential`(`_phoneAuthCredential`) is needed for the signIn method
    /// After the signIn method from `AuthResult` we can get `FirebaserUser`(`_firebaseUser`)
    try {
      await FirebaseAuth.instance
          .signInWithCredential(this._phoneAuthCredential)
          .then((AuthResult authRes) {
        _firebaseUser = authRes.user;
        print(_firebaseUser.toString());
      });
      ...
    } catch (e) {
      ...
      print(e.toString());
    }
  }

• Logout

  Future<void> _logout() async {
    /// Method to Logout the `FirebaseUser` (`_firebaseUser`)
    try {
      // signout code
      await FirebaseAuth.instance.signOut();
      _firebaseUser = null;
      ...
    } catch (e) {
      ...
      print(e.toString());
    }
  }

For more details on implementation please refer to the lib/main.dart file here.

If you found issues, edits are welcome to this answer and to this repo README

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');
    }
}