CODE EXAMPLE FOR DART

using email signin or phone number in flutter firebase

class PhoneAuthForm extends StatefulWidget {
  PhoneAuthForm({Key? key}) : super(key: key);

  @override
  _PhoneAuthFormState createState() => _PhoneAuthFormState();
}

class _PhoneAuthFormState extends State<PhoneAuthForm> {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  TextEditingController phoneNumber = TextEditingController();
  TextEditingController otpCode = TextEditingController();

  OutlineInputBorder border = OutlineInputBorder(
      borderSide: BorderSide(color: Constants.kBorderColor, width: 3.0));

  bool isLoading = false;

  String? verificationId;
  @override
  Widget build(BuildContext context) {
    Size size = MediaQuery.of(context).size;
    return Scaffold(
      appBar: AppBar(title: Text("Verify OTP"),
                backwardsCompatibility: false,
          systemOverlayStyle: SystemUiOverlayStyle(statusBarColor: Colors.blue),),
      backgroundColor: Constants.kPrimaryColor,
      body:  Center(
        child: Form(
              key: _formKey,
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  SizedBox(
                    width: size.width * 0.8,
                    child: TextFormField(
                        keyboardType: TextInputType.phone,
                        controller: phoneNumber,
                        decoration: InputDecoration(
                          labelText: "Enter Phone",
                          contentPadding: EdgeInsets.symmetric(
                              vertical: 15.0, horizontal: 10.0),
                          border: border,
                        )),
                  ),
                  SizedBox(
                    height: size.height * 0.01,
                  ),
                  SizedBox(
                    width: size.width * 0.8,
                    child: TextFormField(
                      keyboardType: TextInputType.number,
                      controller: otpCode,
                      obscureText: true,
                      decoration: InputDecoration(
                        labelText: "Enter Otp",
                        contentPadding: EdgeInsets.symmetric(
                            vertical: 15.0, horizontal: 10.0),
                        border: border,
                        suffixIcon: Padding(
                          child: FaIcon(
                            FontAwesomeIcons.eye,
                            size: 15,
                          ),
                          padding: EdgeInsets.only(top: 15, left: 15),
                        ),
                      ),
                    ),
                  ),
                  Padding(padding: EdgeInsets.only(bottom: size.height * 0.05)),
                  !isLoading
                      ? SizedBox(
                          width: size.width * 0.8,
                          child: OutlinedButton(
                            onPressed: () async {
                              FirebaseService service = new FirebaseService();
                              if (_formKey.currentState!.validate()) {
                                setState(() {
                                  isLoading = true;
                                });
                                await phoneSignIn(phoneNumber: phoneNumber.text);
                              }
                            },
                            child: Text(Constants.textSignIn),
                            style: ButtonStyle(
                                foregroundColor: MaterialStateProperty.all<Color>(
                                    Constants.kPrimaryColor),
                                backgroundColor: MaterialStateProperty.all<Color>(
                                    Constants.kBlackColor),
                                side: MaterialStateProperty.all<BorderSide>(
                                    BorderSide.none)),
                          ),
                        )
                      : CircularProgressIndicator(),
                ],
              ),
            ),
      ));
  }