如何解决这个响应类型'({bool growable}) => List<DropdownMenuItem<dynamic>>‘不是’List<DropdownMenuItem<String>>‘的子类型？

Question

我试图从API中检索数据，以显示在下拉列表中，但是出现了一个错误，我尝试根据我在google和论坛上寻找的内容进行了尝试，但问题仍然出现。我的代码写错了吗，谢谢

--这是调用api时执行的函数，响应为200。

class UtilFunction {
  String? _valMenu;
  var _dataMenu = [];

  Future getSemester() async {
    String url = Constant.baseURL;
    String token = await UtilSharedPreferences.getToken();
    final respose = await http.get(
      Uri.parse(
        '$url/auth/semester/get_smt',
      ),
      headers: {
        'Authorization': 'Bearer $token',
      },
    );
    await http
        .get(
          Uri.parse(
            '$url/auth/semester/get_smt',
          ),
        )
        .then((value) => (() {
              // respose = value; //untuk melakukan request ke webservice
              var listData =
                  jsonDecode(respose.body); //lalu kita decode hasil datanya
              _dataMenu = listData['data'];
              _valMenu = _dataMenu[0]['data'];
            }));
    print(respose.statusCode);
    print(respose.body);
    return _dataMenu;
  }
}

和我在这里接受上面的函数。

Container(
                child: FutureBuilder(
                  future: UtilFunction().getSemester(),
                  builder: (context, snapshot) {
                    if (snapshot.hasData) {
                      return Column(
                        crossAxisAlignment: CrossAxisAlignment.start,
                        children: [
                          DropdownButtonFormField(
                            value: _valMenu,
                            onChanged: (value) {},
                            items: snapshot.data
                                .map<DropdownMenuItem>(
                                    (project) => DropdownMenuItem(
                                          value: project['smt'],
                                          child: Text(project['smt']),
                                        ))
                                .toList,
                          ), ...

这个响应json

{
    "status": "success",
    "code": "200",
    "data": [
        {
            "id": "254dd6e9-791e-4a2b-959e-6ec5929f3104",
            "id_ta": "2b4d2dd1-ef8e-461b-b7c3-48409a13969e",
            "ta": "2022-2023",
            "smt": "GANJIL",
            "semester": "2022-2023 GANJIL",
            "periode_awal": "Senin, 01 Agustus 2022",
            "periode_akhir": "Minggu, 22 Januari 2023",
            "p_awal": "2022-08-01",
            "p_akhir": "2023-01-22",
            "period_smt": "Senin, 01 Agustus 2022 - Minggu, 22 Januari 2023",
            "created_at": "2022-08-10 05:02:18",
            "updated_at": "2022-09-06 03:57:00",
            "created_by": "Superadmin",
            "updated_by": "Dasep",
            "sts_hapus": 1
        }
    ]
}

Answer 1

在获取数据和如何打印数据方面存在一些问题。我使用了公共api，您可以更改url。

class UtilFunction {
  String? _valMenu;
  List<String> _dataMenu = [];

  Future getSemester() async {
    await http.get(Uri.parse("http://universities.hipolabs.com/search?country=United+States")).then((value) {
      // respose = value; //untuk melakukan request ke webservice
      var listData = jsonDecode(value.body); //lalu kita decode hasil datanya
      for (var i = 0; i < 10; i++) {
        _dataMenu.add(listData[i]["domains"][0]);
      }
      _valMenu = _dataMenu[0];
    });
    return _dataMenu;
  }
}

此外，我在FutureBuilder中做了一些修改。请注意，还需要更改api调用的返回类型，以提供下拉值的第一个元素。

FutureBuilder(
                future: UtilFunction().getSemester(),
                builder: (context, AsyncSnapshot snapshot) {
                  if (snapshot.hasData) {
                    val = snapshot.data[0];
             
                    return Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
    
                      DropdownButton<String>(
                        value: val,
                        onChanged: (value) {
                          val = value!;
                        },
                        items: snapshot.data
                            .map<DropdownMenuItem<String>>((String project) => DropdownMenuItem<String>(
                                  value: project,
                                  child: Text(project),
                                ))
                            .toList(),
                      ),
                    ]);
                  } else {
                    return Text("dsds");
                  }
                })

Answer 2

将List<DropdownMenuItem>更改为List<DropdownMenuItem<String>>

                        snapshot.data
                                .map<DropdownMenuItem<String>>(
                                    (project) => DropdownMenuItem<String>(
                                          value: project['smt'],
                                          child: Text(project['smt']),
                                        ))
                                .toList()

也是在解码中试试这个

             var listData =  jsonDecode(respose.body);  as Map<String, dynamic>
             if(listData.containsKey('data')){
                   _dataMenu = listData['data'];
                  _valMenu = _dataMenu[0]['data']; 
               } else {
                 // HANDLE NO DATA SCENARIO
               } 
}