我试图使依赖的多级下拉列表首先包含州列表，其次包含城市列表，所有这些数据都是从API获取的。最初，我加载状态下拉列表，当我选择状态时，然后加载该状态的城市（如果我选择城市，则成功选择了城市），但是当我更改状态值时，就会发生错误。如果将在第一个下拉列表中进行更改，重新加载第二个下拉列表的正确方法是什么？

错误：应该只有一个具有[DropdownButton]值的项目：“城市”的实例。检测到零个或两个或两个以上具有相同值的[DropdownMenuItem]

Future _state;
Future _city;

@override
  void initState() {
    super.initState();
    _state = _fetchStates();
  }



Future<List<StateModel>> _fetchStates() async {
    final String stateApi = "https://dummyurl/state.php";
    var response = await http.get(stateApi);

    if (response.statusCode == 200) {
      final items = json.decode(response.body).cast<Map<String, dynamic>>();

      List<StateModel> listOfUsers = items.map<StateModel>((json) {
        return StateModel.fromJson(json);
      }).toList();

      return listOfUsers;
    } else {
      throw Exception('Failed to load internet');
    }
  }



  Future<List<City>> _fetchCities(String id) async {
    final String cityApi = "https://dummyurl/city.php?stateid=$id";
    var response = await http.get(cityApi);

    if (response.statusCode == 200) {
      final items = json.decode(response.body).cast<Map<String, dynamic>>();
      print(items);
      List<City> listOfUsers = items.map<City>((json) {
        return City.fromJson(json);
      }).toList();

      return listOfUsers;
    } else {
      throw Exception('Failed to load internet');
    }
  }

状态下拉

FutureBuilder<List<StateModel>>(
                                        future: _state,
                                        builder: (BuildContext context,
                                            AsyncSnapshot<List<StateModel>> snapshot) {
                                          if (!snapshot.hasData)
                                            return CupertinoActivityIndicator(animating: true,);
                                          return DropdownButtonFormField<StateModel>(
                                            isDense: true,
                                            decoration: spinnerDecoration('Select your State'),
                                            items: snapshot.data
                                                .map((countyState) => DropdownMenuItem<StateModel>(
                                              child: Text(countyState.billstate),
                                              value: countyState,
                                            ))
                                                .toList(),
                                            onChanged:(StateModel selectedState) {
                                              setState(() {
                                                stateModel = selectedState;
                                                _city = _fetchCities(stateModel.billstateid);
                                              });
                                            },
                                            value: stateModel,
                                          );
                                        }),

城市下拉

FutureBuilder<List<City>>(
                                        future: _city,
                                        builder: (BuildContext context,
                                            AsyncSnapshot<List<City>> snapshot) {
                                          if (!snapshot.hasData)
                                            return CupertinoActivityIndicator(animating: true,);
                                          return DropdownButtonFormField<City>(
                                            isDense: true,
                                            decoration: spinnerDecoration('Select your City'),
                                            items: snapshot.data
                                                .map((countyState) => DropdownMenuItem<City>(
                                              child: Text(countyState.billcity)
                                                .toList(),
                                            onChanged: (City selectedValue) {
                                              setState(() {
                                                cityModel = selectedValue;
                                              });
                                            },
                                            value: cityModel,
                                          );
                                        }),



class StateModel {
  String billstateid;
  String billstate;
  String billcountryid;

  StateModel({this.billstateid, this.billstate, this.billcountryid});

  StateModel.fromJson(Map<String, dynamic> json) {
    billstateid = json['billstateid'];
    billstate = json['billstate'];
    billcountryid = json['billcountryid'];
  }
}



class City {
  String billcityid;
  String billcity;
  String billstateid;

  City({this.billcityid, this.billcity, this.billstateid});

  City.fromJson(Map<String, dynamic> json) {
    billcityid = json['billcityid'];
    billcity = json['billcity'];
    billstateid = json['billstateid'];
  }