'List<dynamic>' 类型不是 'Map<dynamic, dynamic>' 类型的子类型

Question

当我尝试将 .json 映射到对象列表时遇到一些问题，首先是使用的 .json：

[{
"name": "Peru",
"capital": "Lima",
"region": "Americas",
"subregion": "South America",
"population": 31488700
}]

接下来是模型：

import 'dart:convert';

import 'package:clean_architecture_tdd_course/features/country_info/domain/entities/country_info.dart';
import 'package:meta/meta.dart';

List<CountryInfoModel> countryFromJson(String str) => new 
List<CountryInfoModel>.from(json.decode(str).map((x) => CountryInfoModel.fromJson(x)));
String countryToJson(List<CountryInfoModel> data) => json.encode(new List<dynamic>.from(data.map((x) 
  => x.toJson())));

class CountryInfoModel extends CountryInfo {
  CountryInfoModel({
  @required String name,
  @required String capital,
  @required String region,
  @required String subregion,
  @required int population,

}) : super(name:name, capital:capital, region:region, subregion:subregion, population:population);

factory CountryInfoModel.fromJson(Map<String, dynamic> json) {
  return CountryInfoModel(
    name: json['name'],
    capital: json['capital'],
    region: json['region'],
    subregion: json['subregion'],
    population: (json['population'] as num).toInt()
  );
}

Map<String, dynamic> toJson() {
  return {
    'name': name,
    'capital': capital,
    'region': region,
    'subregion': subregion,
    'population': population
  };
}

}

最后对此进行了测试，在这一部分中，我创建了用于资产的模型示例，如果 tojson 函数是正确的：

void main() {
  final tCountryInfoModel = CountryInfoModel(name: 'Peru', capital: 'Lima', region: "Americas",
      subregion: "South Americas", population: 31488700);


group('fromJson', () {
test(
  'should return a valid model',
  () async {
    // arrange
    final Map jsonMap =
        json.decode(fixture('country_info.json'));
    // act
   final result = countryFromJson(jsonMap.toString());
    // assert
    expect(result[0], tCountryInfoModel);
  },
);

当我运行测试时，它会抛出这个：

type 'List<dynamic>' is not a subtype of type 'Map<dynamic, dynamic>'
test\features\country_info\data\models\country_info_model_test.dart 27:19  main.<fn>.<fn>

Answer 1

你在测试中犯了一个简单的错误：

// arrange
final Map jsonMap =
    json.decode(fixture('country_info.json'));
// act
final result = countryFromJson(jsonMap.toString());

改成这样：

final result = countryFromJson(fixture('country_info.json'));

因为在countryFromJson里面，你又在解码了。