从无状态小部件类调用有状态小部件的问题

Question

从无状态小部件类调用有状态小部件的问题..我创建了无状态小部件类 MyApp，其中绘制了图像滑块（有 4 个图像），单独使用此图像滑块可以正常工作，但是当我尝试连接有状态小部件类 DropDown 时出现问题。 ..如何从 MyApp 类调用 DropDown 类。有人帮忙.pl.

import 'package:flutter/material.dart';
import "dart:convert";
import "package:flutter_linkify/flutter_linkify.dart";
import "package:url_launcher/url_launcher.dart";
import 'package:flutter_image_slideshow/flutter_image_slideshow.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    return MaterialApp(
        home: Scaffold(
            body:

  ImageSlideshow(
  width: double.infinity,
  height: 200,
  initialPage: 0,
  indicatorColor: Colors.blue,
  indicatorBackgroundColor: Colors.grey,
  children: [
  Image.asset(
  'assets/muktagiri.jpg',
  fit: BoxFit.cover,
  ),
  Image.asset(
  'assets/maatapti.jpg',
  fit: BoxFit.cover,
  ),
  Image.asset(
  'assets/kukru.jpg',
  fit: BoxFit.cover,
  ),
  Image.asset(
  'assets/gupteshwar.jpg',
  fit: BoxFit.cover,
  ),
  ],
  onPageChanged: (value) {
  print('Page changed: $value');
  },
  ),

        ),

        );
}
}
class DropDown extends StatefulWidget{
  @override
  DropDownState createState() => DropDownState();
}


class Item {
  int id;
  String name;

  Item(this.id, this.name);

  static List<Item> getDeparment() {
    return <Item>[
      Item(1, 'Social Justice'),
      Item(2, 'NIC'),
      Item(3, 'Education'),
      Item(4, 'Food'),
    ];
  }
}

//class _HomeState extends State<State> {
class DropDownState extends State<DropDown> {
  int _value = 1;
  int index=0;
  List<Item> _dept = Item.getDeparment();
  List<DropdownMenuItem<Item>> _dropdownMenuItems;
  Item _selectedDept;
  // Company _selectedCompany;
  @override
  void initState() {
    _dropdownMenuItems = buildDropdownMenuItems(_dept);
    _selectedDept = _dropdownMenuItems[0].value;
    super.initState();
  }
  List<DropdownMenuItem<Item>> buildDropdownMenuItems(List companies) {
    List<DropdownMenuItem<Item>> items = List();
    for (Item department in _dept) {
      items.add(
        DropdownMenuItem(
          value: department,
          child: Text(department.name),
        ),
      );
    }
    return items;
  }
  onChangeDropdownItem(Item selectedItem) {
    setState(() {
      _selectedDept = selectedItem;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: new Container(
        child: Center(
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text("Select Department"),

              DropdownButton(
                value: _selectedDept,
                items: _dropdownMenuItems,
                onChanged: onChangeDropdownItem,
              ),
              SizedBox(
                height: 80.0,
              ),

              FutureBuilder(
                future:
                DefaultAssetBundle.of(context).loadString('assets/data.json'),
                builder: (context, snapshot) {
                  if(!snapshot.hasData) {
                    return CircularProgressIndicator();
                  }
                  var myData = json.decode(snapshot.data);
                  print(myData);
                  return ListView.builder(
                    shrinkWrap: true,

                    itemBuilder: (BuildContext context, index) {
                      print(_selectedDept.name);
                      print(myData[index]['Department']);
                      print(index);
                      String URL= myData[index]['URL'];
                      if (_selectedDept.name ==
                          myData[index]['Department']) {
                        return Card(
                          child: Column(
                            // crossAxisAlignment: CrossAxisAlignment.stretch,
                            children: <Widget>[

                              Text(myData[index]['Department']),
                              Text("HOD : " + myData[index]['HOD']),
                              Text("Contact :" + myData[index]['CONTACT']),
                              Text("Vision :" + myData[index]['VISION']),
                              new InkWell(
                                child: Text("Websites :" +URL),
                                onTap : ()=> launch(URL),
                              ),
                            ],

                          ),
                        );

                      } //if block

                      else {
                        return new Container();
                      }

                    }, // itembuilder
                    itemCount: myData == null ? 0 : myData.length,
                  );

                },
              ),



              // Text('Selected Department is : ${_selectedDept.name} '),
            ],



          ),


        ),
      ),
    );

  }
}