How to Get Custom Stateful Widget value in another class widget?

Question

In My Applicatios's first Page user will select the No from 1 to 10, after selecting the number my second page will display the 5 GradeScreen widget if user select 5, like wise i can create widget using below code, but i can't get the values which user selects from dropdownbutton widget in my GradeScreen widget,

Please help me to get value from this GradeScreen widget. I'm New to Flutter. Thanks in Advance.

import 'package:flutter/material.dart';

class CalculateCGPAScreen extends StatelessWidget {
  final int noOfSubjectsCount;

  CalculateCGPAScreen(this.noOfSubjectsCount);

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      appBar: AppBar(),
      body: Column(
        children: noOfSubjects(noOfSubjectsCount),
      ),
    );
  }

  List<Widget> noOfSubjects(int subjectCount) {
    List<Widget> list = [];
    for (int i = 1; i <= subjectCount; i++) {
      list.add(GradeScreen("Subject $i", 1));
    }
    list.add(RaisedButton(
      onPressed: () {},
      color: Colors.amber,
      splashColor: Colors.amberAccent,
      child: Text("Calculate CGPA"),
    ));
    return list;
  }
}

class GradeScreen extends StatefulWidget {
  final String subjectName;
  final int gradeSystem;

  GradeScreen(this.subjectName, this.gradeSystem);

  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return _GradeScreenState();
  }
}

class _GradeScreenState extends State<GradeScreen> {
  String selectedGrade = "S";
  List<int> creditList = [1, 2, 3, 4, 5, 6];
  int selectedCredit = 1;

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: <Widget>[
        Text(widget.subjectName),
        DropdownButton(
          items: getGradeList(widget.gradeSystem).map((String loopValue) {
            return DropdownMenuItem(
              child: Text(loopValue),
              value: loopValue,
            );
          }).toList(),
          onChanged: (String value) {
            setState(() {
              selectedGrade = value;
            });
          },
          value: selectedGrade,
        ),
        DropdownButton(
          items: creditList.map((int credit) {
            return DropdownMenuItem(
              child: Text(credit.toString()),
              value: credit,
            );
          }).toList(),
          onChanged: (int value) {
            setState(() {
              selectedCredit = value;
            });
          },
          value: selectedCredit,
        )
      ],
    );
  }

  List<String> getGradeList(int option) {
    List<String> gradeSystem;
    if (option == 1) {
      gradeSystem = ['S', 'A', 'B', 'C', 'D', 'E'];
    } else {
      gradeSystem = ['O', 'A+', 'A', 'B+', 'B'];
    }
    return gradeSystem;
  }
}

Answer 1

import 'package:flutter/material.dart';

class CalculateCGPAScreen extends StatelessWidget {
  final int noOfSubjectsCount;

  CalculateCGPAScreen(this.noOfSubjectsCount);

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      appBar: AppBar(),
      body: Column(
        children: noOfSubjects(noOfSubjectsCount),
      ),
    );
  }

  List<Widget> noOfSubjects(int subjectCount) {
    List<Widget> list = [];
    for (int i = 1; i <= subjectCount; i++) {
      list.add(GradeScreen("Subject $i", 1));
    }
    list.add(RaisedButton(
      onPressed: () {
        for (var item in list) {
          if (item is GradeScreen) {
            GradeScreen gs = item;
            print(gs.grade);
            print(gs.credit);
          }
        }
      },
      color: Colors.amber,
      splashColor: Colors.amberAccent,
      child: Text("Calculate CGPA"),
    ));
    return list;
  }
}

class GradeScreen extends StatefulWidget {
  final String subjectName;
  final int gradeSystem;
  int _credit;
  String _grade;
  GradeScreen(this.subjectName, this.gradeSystem);

  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return _GradeScreenState();
  }

  set credit(int value) {
    _credit = value;
  }

  get credit => _credit;

  set grade(String value) {
    _grade = value;
  }

  get grade => _grade;
}

class _GradeScreenState extends State<GradeScreen> {
  String selectedGrade = "S";
  List<int> creditList = [1, 2, 3, 4, 5, 6];
  int selectedCredit = 1;

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: <Widget>[
        Text(widget.subjectName),
        DropdownButton(
          items: getGradeList(widget.gradeSystem).map((String loopValue) {
            return DropdownMenuItem(
              child: Text(loopValue),
              value: loopValue,
            );
          }).toList(),
          onChanged: (String value) {
            setState(() {
              selectedGrade = value;
              widget.grade = value;
            });
          },
          value: selectedGrade,
        ),
        DropdownButton(
          items: creditList.map((int credit) {
            return DropdownMenuItem(
              child: Text(credit.toString()),
              value: credit,
            );
          }).toList(),
          onChanged: (int value) {
            setState(() {
              selectedCredit = value;
              widget.credit = value;
            });
          },
          value: selectedCredit,
        )
      ],
    );
  }

  List<String> getGradeList(int option) {
    List<String> gradeSystem;
    if (option == 1) {
      gradeSystem = ['S', 'A', 'B', 'C', 'D', 'E'];
    } else {
      gradeSystem = ['O', 'A+', 'A', 'B+', 'B'];
    }
    return gradeSystem;
  }
}

Change the dropdown value and get the result into debug console.

Answer 2

import 'package:flutter/material.dart';

class CalculateCGPAScreen extends StatelessWidget {
  final int noOfSubjectsCount;

  CalculateCGPAScreen(this.noOfSubjectsCount);

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      appBar: AppBar(),
      body: Column(
        children: noOfSubjects(noOfSubjectsCount),
      ),
    );
  }

  List<Widget> noOfSubjects(int subjectCount) {
    List<Widget> list = [];
    for (int i = 1; i <= subjectCount; i++) {
      list.add(GradeScreen("Subject $i", 1));
    }
    list.add(RaisedButton(
      onPressed: () {},
      color: Colors.amber,
      splashColor: Colors.amberAccent,
      child: Text("Calculate CGPA"),
    ));
    return list;
  }
}

class GradeScreen extends StatefulWidget {
  final String subjectName;
  final int gradeSystem;
  int _data;
  GradeScreen(this.subjectName, this.gradeSystem);

  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return _GradeScreenState();
  }

  set data(int value) {
    _data = value;
  }

  get data => _data;
}

class _GradeScreenState extends State<GradeScreen> {
  String selectedGrade = "S";
  List<int> creditList = [1, 2, 3, 4, 5, 6];
  int selectedCredit = 1;

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: <Widget>[
        Text(widget.subjectName),
        DropdownButton(
          items: getGradeList(widget.gradeSystem).map((String loopValue) {
            return DropdownMenuItem(
              child: Text(loopValue),
              value: loopValue,
            );
          }).toList(),
          onChanged: (String value) {
            setState(() {
              selectedGrade = value;
            });
          },
          value: selectedGrade,
        ),
        DropdownButton(
          items: creditList.map((int credit) {
            return DropdownMenuItem(
              child: Text(credit.toString()),
              value: credit,
            );
          }).toList(),
          onChanged: (int value) {
            setState(() {
              selectedCredit = value;
              widget.data = selectedCredit;
            });
          },
          value: selectedCredit,
        )
      ],
    );
  }

  List<String> getGradeList(int option) {
    List<String> gradeSystem;
    if (option == 1) {
      gradeSystem = ['S', 'A', 'B', 'C', 'D', 'E'];
    } else {
      gradeSystem = ['O', 'A+', 'A', 'B+', 'B'];
    }
    return gradeSystem;
  }
}