_TypeError（类型'List'不是类型'Map'的子类型）颤动

Question

我在这里做了一个类似的帖子：problem，但我也做了同样的事情，我一直有问题......

我有我的课：

class PollQuestionsDto {
  int id;
  String entitled;
  int difficulty;
  List<Answer> answers;

  PollQuestionsDto({this.id, this.entitled, this.difficulty, this.answers});

  PollQuestionsDto.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    entitled = json['entitled'];
    difficulty = json['difficulty'];
    if (json['answers'] != null) {
      answers = new List<Answer>();
      json['answers'].forEach((v) {
        answers.add(new Answer.fromJson(v));
      });
    }
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['id'] = this.id;
    data['entitled'] = this.entitled;
    data['difficulty'] = this.difficulty;
    if (this.answers != null) {
      data['answers'] = this.answers.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

询问API方法：

Future<List<PollQuestionsDto>> getPollQuestions() async {
    String url = 'http://10.0.2.2:3000/v1/api/poll/40/question?page=0';
    //String url = 'http://10.0.2.2:3000/v1/api/poll/$idPoll/question?page=0';
    String token = await Candidate().getToken();
    final response = await http.get(url, headers: {
      'Content-Type': 'application/x-www-form-urlencoded',
      'Content-Type': 'application/json; charset=utf-8',
      'Accept': 'application/json',
      'Authorization': 'Bearer $token',
    });
    if (response.statusCode == 200) {
      print(jsonDecode(response.body));
      /*List pollsList = jsonDecode(response.body) ;
      List<PollQuestionsDto> polls = [];
      for (var themeMap in pollsList) {
        polls.add(PollQuestionsDto.fromJson(themeMap));
      }
      print(polls);
      print('Get Poll ${response.body}');*/
    } else {
      throw Exception('Failed get poll questions');
    }
  }

当我询问我的 API 时，请求的结果是：

{结果：[{id：2，标题：Le langage utilisé sous Flutter est le dart？，难度：1，答案：[{id：9，标题：Oui，isCorrect：true}，{id：10，标题：Non，isCorrect：false}]}]，pageCount：3，下一个：http://localhost:3000/v1/api/poll/40/question?page=1，上一个：}

当我想在列表中生成结果时，我有同样的错误：

_TypeError（类型'List'不是类型'Map'的子类型）

为什么？

Answer 1

如果结果是

{
  "results": [
    {
      "id": 2,
      "entitled": "Le langage utilisé sous Flutter est le dart ?",
      "difficulty": 1,
      "answers": [
        {
          "id": 9,
          "entitled": "Oui",
          "isCorrect": true
        },
        {
          "id": 10,
          "entitled": "Non",
          "isCorrect": false
        }
      ]
    }
  ],
  "pageCount": 3,
  "next": "http://localhost:3000/v1/api/poll/40/question?page=1"
}

你需要像这样添加['results']：

List pollsList = jsonDecode(jsonstr)['results'];

Answer 2

您需要将json['answers'] 转换为List 喜欢

    if (json['answers'] != null) {
      answers = (json['answers'] as List).map((v) => Answer.fromJson(v)).toList();
    }

除非您有特殊要求，否则最好使用https://pub.dev/packages/json_annotation 而不是手动实现toJson 和fromJson。

我能够解析以下字符串

    [
      {
        "id": 1,
        "entitled": "yes",
        "difficulty": 1,
        "answers": [
          {
            "value":"a"
          }
        ]        
      }
    ]

使用

class PollQuestionsDto {
  int id;
  String entitled;
  int difficulty;
  List<Answer> answers;

  PollQuestionsDto({this.id, this.entitled, this.difficulty, this.answers});

  PollQuestionsDto.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    entitled = json['entitled'];
    difficulty = json['difficulty'];
    if (json['answers'] != null) {
      answers = new List<Answer>();
      json['answers'].forEach((v) {
        answers.add(new Answer.fromJson(v));
      });
    }
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['id'] = this.id;
    data['entitled'] = this.entitled;
    data['difficulty'] = this.difficulty;
    if (this.answers != null) {
      data['answers'] = this.answers.map((v) => v.toJson()).toList();
    }
    return data;
  }

  @override String toString() => toJson().toString();

  static List<PollQuestionsDto> arrayOfQuestionsFromJson(String json) {
    return (jsonDecode(json) as List).map((e) => PollQuestionsDto.fromJson(e)).toList();
  }
}

这是您所期望的吗（您可能必须更改 json 以获取答案，因为我使用带有一个变量 value 的 Anser 类）？？