Dart 列表<；动态>'；不是类型为'；列表<；Obj>'；内铸

我正在尝试创建动态下拉列表，它的值是使用micro服务从一个查找表填充的，但我在许多方面都尝试过相同的方法，直到现在我还没有成功地使其工作。因为我对飞镖/飞镖完全不熟悉 所以，任何人都可以请确定我在下面的代码中做错了什么

下面的代码用于调用Web服务

Future<List<CountryDropDownList>> getCountriesDetails(String url) async{
    HttpClient httpClient = new HttpClient();
    HttpClientRequest request = await httpClient.getUrl(Uri.parse(url));
    // request.headers.set('content-type', 'application/json');
    print("request data "+request.toString());
    HttpClientResponse microServicesResponse= await request.close();
    String microServicesResponseString = await microServicesResponse.transform(utf8.decoder).join();
    final parsed = await json.decode(microServicesResponseString).cast<Map<String, dynamic>>();
    httpClient.close();
    print("Data Recieved   "+microServicesResponseString.toString());
    return parsed
        .map<CountryDropDownList>(
            (json) => CountryDropDownList.fromJson(json))
        .toList();
  }

    class CenterFoundationSubmission extends StatefulWidget  {
      CenterFoundationSubmission({Key key}) : super(key: key);

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

    class _CenterFoundationSubmissionState extends State<CenterFoundationSubmission> {

      NetworkUtil _netUtil = new NetworkUtil();

      var url = "SomeUrl";

      @override
      void initState() {
        super.initState();
        setState(() {
        });
      }


      @override
      Widget build(BuildContext context) {
        var futureBuilder = new FutureBuilder(
          future: _getData(),
          builder: (BuildContext context, AsyncSnapshot snapshot) {
            switch (snapshot.connectionState) {
              case ConnectionState.none:
              case ConnectionState.waiting:
                return new Text('loading...');
              default:
                if (snapshot.hasError)
                  return new Text('Exception here is : ${snapshot.error}');
                else
                  return createView(context, snapshot);
            }
          },
        );

        return new Scaffold(
          appBar: new AppBar(
            title: new Text("Center Foundation"),
          ),
          body: futureBuilder,
        );

      }

      Future<List<CountryDropDownList>> _getData() async {
        List<CountryDropDownList> values = new List<CountryDropDownList>();
        values.addAll(_netUtil.getCountriesDetails(url) as List<CountryDropDownList>);

/*Error Added here , i am getting error while casting list to my object type*/


        await new Future.delayed(new Duration(seconds: 10));

        return values;
      }

      Widget createView(BuildContext context, AsyncSnapshot snapshot) {
        List<CountryDropDownList> values = snapshot.data;
        return new ListView.builder(
          itemCount: values.length,
          itemBuilder: (BuildContext context, int index) {
            return new Column(
              children: <Widget>[
                new ListTile(
                  title: new Text(values[index].countryName),
                  subtitle:  new Text(values[index].stateDropDownList[index].statesName),
                  trailing:   new Text(values[index].stateDropDownList[index].districtDropDownList[index].districtsName),
                ),
                new Divider(height: 2.0,),
              ],
            );
          },
        );
      }
    }

Future getCountriesDetails（字符串url）异步{
HttpClient HttpClient=新HttpClient（）；
HttpClientRequest请求=等待httpClient.getUrl（Uri.parse（url））；
//request.headers.set（'content-type'，'application/json'）；
打印（“请求数据”+request.toString（））；
HttpClientResponse microServicesResponse=等待请求。关闭（）；
String microServicesResponseString=等待microServicesResponse.transform（utf8.decoder.join（）；
final parsed=wait json.decode（microservicessresponsestring.cast（）；
httpClient.close（）；
打印（“接收的数据”+microServicesResponseString.toString（））；
返回解析
.地图(
（json）=>CountryDropDownList.fromJson（json））
.toList（）；
}

这是我的对象类型

class CountryDropDownList{

  String countryName;
  List<StateDropDownList> stateDropDownList;


  CountryDropDownList({this.countryName, this.stateDropDownList});

  factory CountryDropDownList.fromJson(Map<String, dynamic> json) {
    return CountryDropDownList(
      countryName: json['countryName'] as String,
      stateDropDownList: json['states'] as List<StateDropDownList>,
    );
  }
}

classcountrydropdownlist{
字符串countryName；
列表状态下拉列表；
CountryDropDownList（{this.countryName，this.stateDropDownList}）；
factory CountryDropDownList.fromJson（映射json）{
返回国家下拉列表(
countryName:json['countryName']作为字符串，
stateDropDownList:json['states']作为列表，
);
}
}

仅用于显示当前正在运行的以下代码

Future<List<CountryDropDownList>> getCountriesDetails(String url) async{
    HttpClient httpClient = new HttpClient();
    HttpClientRequest request = await httpClient.getUrl(Uri.parse(url));
    // request.headers.set('content-type', 'application/json');
    print("request data "+request.toString());
    HttpClientResponse microServicesResponse= await request.close();
    String microServicesResponseString = await microServicesResponse.transform(utf8.decoder).join();
    final parsed = await json.decode(microServicesResponseString).cast<Map<String, dynamic>>();
    httpClient.close();
    print("Data Recieved   "+microServicesResponseString.toString());
    return parsed
        .map<CountryDropDownList>(
            (json) => CountryDropDownList.fromJson(json))
        .toList();
  }

    class CenterFoundationSubmission extends StatefulWidget  {
      CenterFoundationSubmission({Key key}) : super(key: key);

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

    class _CenterFoundationSubmissionState extends State<CenterFoundationSubmission> {

      NetworkUtil _netUtil = new NetworkUtil();

      var url = "SomeUrl";

      @override
      void initState() {
        super.initState();
        setState(() {
        });
      }


      @override
      Widget build(BuildContext context) {
        var futureBuilder = new FutureBuilder(
          future: _getData(),
          builder: (BuildContext context, AsyncSnapshot snapshot) {
            switch (snapshot.connectionState) {
              case ConnectionState.none:
              case ConnectionState.waiting:
                return new Text('loading...');
              default:
                if (snapshot.hasError)
                  return new Text('Exception here is : ${snapshot.error}');
                else
                  return createView(context, snapshot);
            }
          },
        );

        return new Scaffold(
          appBar: new AppBar(
            title: new Text("Center Foundation"),
          ),
          body: futureBuilder,
        );

      }

      Future<List<CountryDropDownList>> _getData() async {
        List<CountryDropDownList> values = new List<CountryDropDownList>();
        values.addAll(_netUtil.getCountriesDetails(url) as List<CountryDropDownList>);

/*Error Added here , i am getting error while casting list to my object type*/


        await new Future.delayed(new Duration(seconds: 10));

        return values;
      }

      Widget createView(BuildContext context, AsyncSnapshot snapshot) {
        List<CountryDropDownList> values = snapshot.data;
        return new ListView.builder(
          itemCount: values.length,
          itemBuilder: (BuildContext context, int index) {
            return new Column(
              children: <Widget>[
                new ListTile(
                  title: new Text(values[index].countryName),
                  subtitle:  new Text(values[index].stateDropDownList[index].statesName),
                  trailing:   new Text(values[index].stateDropDownList[index].districtDropDownList[index].districtsName),
                ),
                new Divider(height: 2.0,),
              ],
            );
          },
        );
      }
    }

class CenterFoundationSubmission扩展了StatefulWidget{
CenterFoundationSubmission（{Key}）：超级（Key:Key）；
@凌驾
_CenterFoundationSubmissionState createState（）=>新建CenterFoundationSubmissionState（）；
}
类_CenterFoundationSubmissionState扩展状态{
NetworkUtil _netUtil=新的NetworkUtil（）；
var url=“SomeUrl”；
@凌驾
void initState（）{
super.initState（）；
设置状态（）{
});
}
@凌驾
小部件构建（构建上下文）{
var futureBuilder=新的futureBuilder(
未来：_getData（），
生成器：（BuildContext上下文，异步快照）{
交换机（快照.连接状态）{
案例连接状态。无：
案例连接状态。正在等待：
返回新文本（'加载…'）；
违约：
if（snapshot.hasError）
返回新文本（'此处的异常为：${snapshot.error}'）；
其他的
返回createView（上下文、快照）；
}
},
);
归还新脚手架(
appBar：新的appBar(
标题：新文本（“中心基金会”），
),
正文：未来建设者，
);
}
Future\u getData（）异步{
列表值=新列表（）；
addAll（_netUtil.getCountriesDetails（url）作为列表）；
/*此处添加错误，将列表强制转换为对象类型时出错*/
等待新的未来。延迟（新的持续时间（秒：10））；
返回值；
}
小部件createView（构建上下文上下文，异步快照快照）{
列表值=snapshot.data；
返回新的ListView.builder(
itemCount:values.length，
itemBuilder:（构建上下文，int索引）{
返回新列(
儿童：[
新ListTile(
标题：新文本（值[索引].countryName），
字幕：新文本（值[index].stateDropDownList[index].statesName），
尾随：新文本（值[index].stateDropDownList[index].districtDropDownList[index].districtsName），
),
新隔板（高度：2.0，），
],
);
},
);
}
}

我所做的就是尝试用多种方式调用webservices，尝试直接使用JSON字符串响应调用services响应，可能是我尝试过的事情

---

请在此提供帮助，我们将不胜感激。

您能尝试一下

List.from（）

构造函数吗？这里有一个相同的链接：

stateDropDownList:List.from（json['states']）；

您能突出显示错误发生的那一行吗？完成只需使用他的“错误添加在此处”进行搜索，检查是否更改.toList（）；“to”.toList（）；”就可以解决问题。否，在运行时没有这样的方法尝试此方法：values.addAll（等待_netUtil.getCountriesDetails（url）作为列表）；