===================================================== main.dart =====================================================================
import 'package:flutter/material.dart';
import 'HomePage.dart';
import 'PhoneAuth.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:firebase_auth/firebase_auth.dart';

bool sdg_msk=false;
String no_hp;
void Savemasuk() async {
  SharedPreferences pref = await SharedPreferences.getInstance();
  pref.setBool("lg_msk", sdg_msk);
  pref.setString("nomorhp", no_hp);
}

Future<bool> get_login() async {
  SharedPreferences pref = await SharedPreferences.getInstance();
pref.getBool("lg_msk") == null ? false : true;}

Future<String> get_akun() async { // = 'tidak ada nama'
  SharedPreferences pref = await SharedPreferences.getInstance();
  return pref.getString("nomorhp") ?? "tidak ada nomor hp";
}

void main() {
  runApp(new MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}
FirebaseUser user;
class _MyAppState extends State<MyApp> {
  bool loggedIn = false;
  @override
  Widget build(BuildContext context) {
    return FutureBuilder<bool>(
      future: get_login(),
      builder: (BuildContext context, AsyncSnapshot<bool> snapshot) {
        if (snapshot.data== false){
          return MaterialApp(home: PhoneAuth(),);
        }else{
          return MaterialApp(home: HomePage(user),);
        }
      },
    );
  }
}













======================================== PhoneAuth.dart ======================================
import 'package:country_pickers/country.dart';
import 'package:flutter/material.dart';
import 'package:country_pickers/country_pickers.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'HomePage.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'main.dart';


class PhoneAuth extends StatefulWidget {
  @override
  _PhoneAuthState createState() => _PhoneAuthState();
}






class _PhoneAuthState extends State<PhoneAuth> {

String name;

  Country _selectedDialogCountry =
      CountryPickerUtils.getCountryByPhoneCode('62');

  final TextEditingController _phoneController = new TextEditingController();
  final TextEditingController _smsController = new TextEditingController();
  String _phoneCode = '62';

  String verificationId;
  FirebaseUser user;
  var _keyField = GlobalKey<FormFieldState>();

  Widget _buildDialogItem(Country country) => Row(
        children: <Widget>[
          CountryPickerUtils.getDefaultFlagImage(country),
          SizedBox(width: 8.0),
          Text("+${country.phoneCode}"),
          SizedBox(width: 8.0),
          Flexible(child: Text(country.name))
        ],
      );

  void _openCountryPickerDialog() => showDialog(
        context: context,
        builder: (context) => Theme(
            data: Theme.of(context).copyWith(primaryColor: Colors.orange),
            child: CountryPickerDialog(
              titlePadding: EdgeInsets.all(8.0),
              searchCursorColor: Colors.orange,
              searchInputDecoration: InputDecoration(hintText: 'Search...'),
              isSearchable: true,
              title: Text('Select your phone code'),
              onValuePicked: (Country country) {
                print("${country.name}");
                print("${country.phoneCode}");
                print(_selectedDialogCountry);
                setState(() {
                  _selectedDialogCountry = country;
                  this._phoneCode = country.phoneCode;
                  itemBuilder:
                  _buildDialogItem;
                });
              },
            )),
      );

  Future<void> verifyPhone(BuildContext context) async {
    try {
      final PhoneCodeAutoRetrievalTimeout autoRetrieve = (String verId) {
        this.verificationId = verId;
      };

      final PhoneCodeSent smsCodeSent = (String verId, [int forceCodeResend]) {
        this.verificationId = verId;
        smsCodeDialog(context).then((value) {
          print('Signed in');
        });
      };

      final PhoneVerificationCompleted verifiedSuccess = (FirebaseUser user) {

        if (user != null) {
                        sdg_msk=true;
          print("Successful verification user is: +${_phoneCode + _phoneController.text.trim()}");
          no_hp= "+${_phoneCode + _phoneController.text.trim()}";
          Navigator.push(
              context, MaterialPageRoute(builder: (context) => HomePage(user)));
        } else {
          print("user is null");
        }
      };

      final PhoneVerificationFailed veriFailed = (AuthException exception) {
        print('Failed verification: ${exception.message}');
      };
String temp = "${_phoneCode + _phoneController.text.trim()}";

      await FirebaseAuth.instance.verifyPhoneNumber(
         phoneNumber : (temp == null) ? "" : temp,
          codeAutoRetrievalTimeout: autoRetrieve,
          codeSent: smsCodeSent,
          timeout: const Duration(seconds: 5),
          verificationCompleted: verifiedSuccess,
          verificationFailed: veriFailed);
    } catch (e) {
      print("error: $e");
    }
  }

  Future<bool> smsCodeDialog(BuildContext context) {
    return showDialog(
        context: context,
        barrierDismissible: false,
        builder: (BuildContext context) {
          return new AlertDialog(
            title: Text('Enter sms Code'),
            content: TextField(
              keyboardType: TextInputType.number,
              controller: _smsController,
            ),
            contentPadding: EdgeInsets.all(10.0),
            actions: <Widget>[
              new FlatButton(
                child: Text('Done'),
                onPressed: () async {
                  AuthCredential credential = PhoneAuthProvider.getCredential(
                    verificationId: this.verificationId,
                    smsCode: _smsController.text.trim(),
                  );

                  user = await FirebaseAuth.instance
                      .signInWithCredential(credential)
                      .then((user) {
                    if (user != null) {
                      Navigator.of(context).pop();
                      Navigator.push(
                          context,
                          MaterialPageRoute(
                              builder: (context) => HomePage(user)));
                    } else {
                      print("Failed verification");
                    }
                  }).catchError((e) {
                    print("error: $e");
                  });
                },
              )
            ],
          );
        });
  }

  // login shared prefs
  bool alreadyLogin = false;
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
home: Scaffold(
      body: SafeArea(
        child: Center(
            child: SingleChildScrollView(
                child: Center(
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              FractionallySizedBox(
                widthFactor: 0.7,
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.center,
                  children: <Widget>[
                    ListTile(
                        onTap: _openCountryPickerDialog,
                        title: _buildDialogItem(_selectedDialogCountry)),
                    Container(
                      width: 170,
                      child: TextFormField(
                        key: _keyField,
                        controller: _phoneController,
                        keyboardType: TextInputType.phone,
                        decoration: InputDecoration(
                          labelText: "Nomor Telepon",
                          prefixText: "+${_phoneCode} ",
                          counterText: 'masukkan tanpa angka 0',
                        ),
                        validator: (val) {
                          if (val.isEmpty) {
                            return "this field cannot be empty";
                          }
                        },
                      ),
                    )
                  ],
                ),
              ),
              new RaisedButton(
                child: new Text("Verifikasi"),
                color: Colors.orange.shade800,
                textColor: Colors.white,
                onPressed: () async {
                  if (_phoneCode != null) {
                    if (_keyField.currentState.validate()) {
                      name="+${_phoneCode + _phoneController.text.trim()}";
                      print(name);
                      verifyPhone(context);


                                           }
                                         }
                                       },
                                     ),
                                   ],
                                 ),
                               ))),

                             ),
                       ),
                           ); }


}