如何解决 Flutter/Dart 中的 List<dynamic> 类型错误

Question

我对扑扑和飞镖很陌生。试图从这个未来返回一个从 api 中提取数据的列表，但我一直收到这个错误。这里有人通过转换为列表帮助我解决了类似的错误，因为我在返回可迭代的数组上调用 map 函数，但在这种情况下，我不确定需要做什么。

type List<dynamic> is not a subtype of type FutrueOr<ListCity>>

收到的数据如下：

{
 "data": {
        "127287": {
            "Feature_int_id": "127287",
            "Admin1_str_code": "US06",
            "Country_str_code": "US",
            "Feature_str_name": "Acampo",
            "Feature_dec_lat": "38.194",
            "Feature_dec_lon": "-121.25"
        },
        "116496": {
            "Feature_int_id": "116496",
            "Admin1_str_code": "US06",
            "Country_str_code": "US",
            "Feature_str_name": "Acton",
            "Feature_dec_lat": "34.49",
            "Feature_dec_lon": "-118.22"
        },
        "124284": {
            "Feature_int_id": "124284",
            "Admin1_str_code": "US06",
            "Country_str_code": "US",
            "Feature_str_name": "Adelanto",
            "Feature_dec_lat": "34.665",
            "Feature_dec_lon": "-117.512"
        },
}

下面是未来的代码：

Future<List<City>> fetchCitiesByProvince(provinceCode) async {
  final response = await http.get(Uri.https('localhost/msd', 'api/cities/' + provinceCode));
  final responseJson = json.decode(response.body);
  final dataMap =  responseJson['data'];

  if (response.statusCode == 200) {
 
    List citiesList = [];
    for (var city in dataMap.keys) {
      if (dataMap[city]['Admin1_str_code'] == provinceCode) {
        citiesList.add(
          {
            'cityCode': dataMap[city]['Feature_int_id'],
            'cityName': dataMap[city]['Feature_str_name']
          }
        );
      }
    }
    return citiesList;
  } else {
    throw Exception('Failed to load cities');
  }
}

城市等级：

class City {
  final String cityCode;
  final String cityName;

  City({@required this.cityCode, @required this.cityName});

  factory City.fromJson(Map<String, dynamic> json) {

    return City(
      cityCode: json['Feature_int_id'],
      cityName: json['Feature_str_name']
    );
  }
}

Answer 1

您需要在您的方法中返回List<City>。因此，更改以下代码：

List citiesList = [];

到

List<City> citiesList = [];

--- 更新 ---

您需要使用您的 City 构造函数或工厂从 json 生成项目，如下所示：

City.fromJson(dataMap[city]);

// Or

City(cityCode: dataMap[city]['Feature_int_id'],
        cityName: dataMap[city]['Feature_str_name']
    );

这里是更新的示例代码：

Future<List<City>> fetchCitiesByProvince(provinceCode) async {
  final response = await http.get(Uri.https('localhost/msd', 'api/cities/' + provinceCode));
  final responseJson = json.decode(response.body);
  final dataMap =  responseJson['data'];

  List<City> citiesList = [];

  if (response.statusCode == 200) {
    
    for (var city in dataMap.keys) {
      if (dataMap[city]['Admin1_str_code'] == provinceCode) {
        citiesList.add(City.fromJson(dataMap[city]));
      }
    }
  } else {

    throw Exception('Failed to load cities');
    // either throwing an error or return empty list.
  }

  return citiesList;
}