Flutter中对应的DropDownButtonFormFields

编辑
可以使用key: UniqueKey()int CityDropdownButtonFormField

DropdownButtonFormField<int>(
      key: UniqueKey(),
      decoration: InputDecoration(labelText: 'City'),

您可以在下面复制粘贴运行完整代码。
您可以将selectedStateId和设置selectedCityId为states的属性，
不能直接设置为，例如selectedStateId = 1，因为系统会比较地址
代码段

  int selectedStateId;
  int selectedCityId;

  @override
  void initState() {
    selectedStateId = states[0].stateId;
    selectedCityId = states[0].cities[0].cityId;
    super.initState();
  }

工作演示

完整的代码

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class City {
  int cityId;
  String cityName;
  City({this.cityId, this.cityName});
}

class CountryState {
  int stateId;
  String stateName;
  List<City> cities;
  CountryState({this.stateId, this.stateName, this.cities});
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  List<CountryState> states = [
    CountryState(stateId: 1, stateName: " York", cities: [
      City(cityId: 1, cityName: "York City"),
      City(cityId: 2, cityName: "Buffalo")
    ]),
    CountryState(stateId: 2, stateName: "A", cities: [
      City(cityId: 3, cityName: "A1"),
      City(cityId: 4, cityName: "A2")
    ])
  ];

  int selectedStateId;
  int selectedCityId;

  @override
  void initState() {
    selectedStateId = states[0].stateId;
    selectedCityId = states[0].cities[0].cityId;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            DropdownButtonFormField<int>(
              decoration: InputDecoration(labelText: 'State'),
              value: selectedStateId,
              items: states.map((CountryState state) {
                return DropdownMenuItem<int>(
                  value: state.stateId,
                  child: Text(states
                      .singleWhere((x) => x.stateId == state.stateId)
                      .stateName),
                );
              }).toList(),
              onChanged: (int StateId) {
                setState(() {
                  this.selectedCityId = states
                      .singleWhere((x) => x.stateId == StateId)
                      .cities[0]
                      .cityId; // set to first city for this state
                  this.selectedStateId = StateId;
                });
              },
            ),
            DropdownButtonFormField<int>(
              key: UniqueKey(),
              decoration: InputDecoration(labelText: 'City'),
              value: selectedCityId,
              items: states
                  .singleWhere((x) => x.stateId == selectedStateId)
                  .cities
                  .map((City city) {
                return DropdownMenuItem<int>(
                  value: city.cityId,
                  child: Text(states
                      .singleWhere((x) => x.stateId == selectedStateId)
                      .cities
                      .singleWhere((x) => x.cityId == city.cityId)
                      .cityName),
                );
              }).toList(),
              onChanged: (int CityId) {
                setState(() {
                  this.selectedCityId = CityId;
                });
              },
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}