store data in parent page - flutter

signUp.dart
==================
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:flutter/services.dart';
import 'package:hebo_mobile/screens/signUp/firstScreen.dart';
import 'package:hebo_mobile/screens/signUp/secondScreen.dart';
import 'package:hebo_mobile/screens/signUp/thirdScreen.dart';
import 'package:hebo_mobile/screens/signUp/fourthScreen.dart';
import 'package:hebo_mobile/screens/signUp/fifthScreen.dart';
import 'package:hebo_mobile/screens/signUp/sixthScreen.dart';
import 'package:hebo_mobile/screens/signUp/seventhScreen.dart';

class SignUp extends StatefulWidget {
  @override
  SignUpState createState() => new SignUpState();
}

class SignUpState extends State<SignUp> {
  PageController pageCtr = PageController();
  String firstName;
  String lastName;
  String dateBirth;
  int gender;
  String email;
  String phoneCode;
  String phoneNumber;

  @override
  void dispose() {
    // Clean up the controller when the widget is removed from the
    // widget tree.
    // firstNameCtr.dispose();
    // lastNameCtr.dispose();
    // dateBirthCtr.dispose();
    super.dispose();
  }

  _firstScreenUpdate(val1, val2) {
    setState(() {
      firstName = val1;
      lastName = val2;
    });
  }

  _secondScreenUpdate(val1, val2) {
    setState(() {
      dateBirth = val1;
      gender = val2;
    });
  }

  _thirdScreenUpdateGender(val1, val2, val3) {
    setState(() {
      email = val1;
      phoneCode = val2;
      phoneNumber = val3;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
          child: PageView(
        physics: new NeverScrollableScrollPhysics(),
        controller: pageCtr,
        children: <Widget>[
          Container(
            child: SignUpFirst(
              nextPage: () {
                pageCtr.nextPage(
                    curve: Curves.ease, duration: Duration(milliseconds: 500));
              },
              updateValue: _firstScreenUpdate,
              firstName: firstName,
              lastName: lastName,
            ),
          ),
          Container(
            child: SignUpSecond(
              prevPage: () {
                pageCtr.previousPage(
                    curve: Curves.ease, duration: Duration(milliseconds: 500));
              },
              nextPage: () {
                pageCtr.nextPage(
                    curve: Curves.ease, duration: Duration(milliseconds: 500));
              },
              updateValue: _secondScreenUpdate,
              dateBirth: dateBirth,
              gender: gender,
            ),
          ),
          Container(
            child: SignUpThird(
              prevPage: () {
                pageCtr.previousPage(
                    curve: Curves.ease, duration: Duration(milliseconds: 500));
              },
              nextPage: () {
                pageCtr.nextPage(
                    curve: Curves.ease, duration: Duration(milliseconds: 500));
              },
              updateValue: _thirdScreenUpdateGender,
              email: email,
              phoneCode: phoneCode,
              phoneNumber: phoneNumber,
            ),
          ),
        ],
      )),
    );
  }
}

firstScreen.dart
=============
import 'package:flutter/material.dart';

class SignUpFirst extends StatefulWidget {
  final Function nextPage;
  final Function updateValue;

  final String firstName;
  final String lastName;

  SignUpFirst(
      {@required this.nextPage,
      @required this.updateValue,
      @required this.firstName,
      @required this.lastName});

  @override
  SignUpFirstState createState() => new SignUpFirstState();
}

class SignUpFirstState extends State<SignUpFirst> {
  @override
  Widget build(BuildContext context) {
    TextEditingController firstNameCtr =
        new TextEditingController(text: widget.firstName);
    TextEditingController lastNameCtr =
        new TextEditingController(text: widget.lastName);
    return Scaffold(
      body: SafeArea(
        child: Container(
          width: double.infinity,
          color: Colors.pink,
          child: Padding(
            padding: const EdgeInsets.only(left: 20, right: 20, top: 20),
            child: Column(
              children: <Widget>[
                Row(
                  children: <Widget>[
                    FlatButton(
                      onPressed: () {
                        // pageCtr.previousPage(
                        //     curve: Curves.ease,
                        //     duration: Duration(milliseconds: 500));
                      },
                      color: Colors.transparent,
                      child: Column(
                        children: <Widget>[
                          Icon(
                            Icons.keyboard_arrow_left,
                            color: Colors.white,
                            size: 50,
                          )
                        ],
                      ),
                    ),
                  ],
                ),
                Text(
                  'Halo, Tak Kenal maka Tak Sayang.',
                  style: TextStyle(
                      color: Colors.white,
                      fontSize: 23,
                      fontWeight: FontWeight.bold),
                ),
                TextField(
                  controller: firstNameCtr,
                  decoration: InputDecoration(
                      enabledBorder: UnderlineInputBorder(
                        borderSide: BorderSide(color: Colors.white),
                      ),
                      hintText: 'First Name',
                      hintStyle: TextStyle(
                          color: Colors.white, fontWeight: FontWeight.bold)),
                ),
                TextField(
                  controller: lastNameCtr,
                  decoration: InputDecoration(
                      enabledBorder: UnderlineInputBorder(
                        borderSide: BorderSide(color: Colors.white),
                      ),
                      hintText: 'Last Name',
                      hintStyle: TextStyle(
                          color: Colors.white, fontWeight: FontWeight.bold)),
                ),
                RaisedButton(
                  onPressed: () {
                    widget.nextPage();
                    widget.updateValue(firstNameCtr.text, lastNameCtr.text);
                  },
                  child: Text('Continue'),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}