List<dynamic>' 不是类型转换中类型 'List<Obj>' 的子类型

Posted 2023-02-23

【中文标题】List<dynamic>\' 不是类型转换中类型 \'List<Obj>\' 的子类型

【英文标题】：List<dynamic>' is not a subtype of type 'List<Obj>' in type castList<dynamic>' 不是类型转换中类型 'List<Obj>' 的子类型

【发布时间】：2019-01-13 01:24:30

【问题描述】：

我正在尝试创建动态下拉列表，其值是使用微服务从一个查找表中填充的，但我在很多方面都尝试过相同的方法，直到现在我还没有成功让它工作。因为我对飞镖/颤振完全陌生 那么任何人都可以在下面的代码中找出我做错了什么

以下代码用于调用webservice

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();
  

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>,
    );
  

仅用于显示目前正在运行的以下代码

    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,),
              ],
            );
          ,
        );
      
    

我所做的我尝试过以多种方式调用 web 服务来调用它，尝试使用 JSON 字符串响应直接转换服务响应，可能我已经尝试过。

请您在这里提供帮助，我们将不胜感激。

【问题讨论】：

您能突出显示错误发生的那一行吗？

用他的“在这里添加错误”完成搜索

检查是否改变“.toList();”到“.toList();”解决问题。

否，运行时没有这种方法

试试这个：values.addAll(await _netUtil.getCountriesDetails(url) as List);

【参考方案1】：

你可以试试List.from() 构造函数吗？这是一个相同的链接：

https://api.dart.dev/stable/2.8.4/dart-core/List/List.from.html

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