json - flutter-类型 '(dynamic, dynamic) => Null'不是 '(dynamic) => dynamic'的 'f'类型的子类型

Question

我只是尝试从Firebase实时数据库解析数据。
但在转换为模型时有问题
我正在尝试从Flutter上解析Firebase数据库中的数据。
但是一个错误说

MY Complete QUIZ: {-M5-R3BqTajbCFk5mQuQ: {coins: 434, isSubmit: true,questions: [{answer: sddsd, name: Why do we use it?, options:[established, adwada, adawda, sddsd], select: }, {answer: adawda,name: Where can I get some?, options: [established, adwada, adawda,sddsd], select: sddsd}, {answer: adwada, name: Lorem Ipsum is simplydummy text of the printing?, options: [established, adwada, adawda,sddsd], select: established}], quizId: YItWgbYjHm},-M50HhYPnuR7tSC-9ajw: {isSubmit: true, questions: [{answer: dadada, name: Where does it come from?, options: [vvvv, dadada, dsdsdssd,bbbbbb], select: dadada}], quizId: 9pdzphz0x8}}

I/flutter ( 6768): type '(dynamic, dynamic) => Null' is not a subtype of type '(dynamic) => dynamic' of 'f'

在这里，数据库结构
fetchMyQuiz()的以下功能

     Future<Quiz> fetchMyQuiz(String uid) async {
        Quiz _quiz;
    
        var dio = Dio();
        dio.options
          ..baseUrl = Constant.baseUrl
          ..connectTimeout = 5000 //5s
          ..receiveTimeout = 5000
          ..validateStatus = (int status) {
            return status > 0;
          }
          ..headers = {
            HttpHeaders.userAgentHeader: 'dio',
            'common-header': 'xx',
          };
    
        _isLoadingUser = true;
        notifyListeners();
    
        List<Quiz> _fetchedQuiz = [];
    
        try {
          var response = await dio.get(
            Constant.userParam + '/$uid' + Constant.quiz + Constant.jsonExt,
            options: Options(
              contentType: Headers.formUrlEncodedContentType,
            ),
          );
    
          print("MY Complete QUIZ: ${response.data}");
    
          if (response.statusCode == 200) {
            var responseData = response.data;
            responseData.forEach((String id, dynamic json) {
              if (responseData != null) {
                _quiz = Quiz.fromJson(id, json);
                _fetchedQuiz.add(_quiz);
              }
            });
          } else {
            print("FETCH QUIZ error: ${response.data}");
          }
        } catch (e) {
          print(e);
        }
    
        _myQuizList = _fetchedQuiz;
    
        _isLoadingUser = false;
        notifyListeners();
    
        return _quiz;
      }

 class Quiz {
      String id;
      String quizId;
      int coins;
      bool isSubmit;
      List<Questions> questions;
    
      Quiz({this.id, this.quizId, this.coins, this.isSubmit, this.questions});
    
      Quiz.fromJson(String idQuiz, Map<String, dynamic> json) {
        id = idQuiz;
        quizId = json['quizId'];
        coins = json['coins'];
        isSubmit = json['isSubmit'] == null ? false : json['isSubmit'];
        if (json['questions'] != null) {
          questions = new List<Questions>();
          json['questions'].forEach((idQuest, vQuest) {
            questions.add(new Questions.fromJson(idQuest, vQuest));
          });
        }
      }
    
      Map<String, dynamic> toJson() {
        final Map<String, dynamic> data = new Map<String, dynamic>();
        data['id'] = this.id;
        data['quizId'] = this.quizId;
        data['coins'] = this.coins;
        data['isSubmit'] = this.isSubmit;
        if (this.questions != null) {
          data['questions'] = this.questions.map((v) => v.toJson()).toList();
        }
        return data;
      }
    }
    
    class Questions {
      String id;
      String name;
      String select;
      String answer;
      // bool isSave;
      List<String> options;
    
      Questions(
          {this.id,
          this.name,
          this.select,
          this.answer,
          // this.isSave,
          this.options});
    
      Questions.fromJson(String id, Map<String, dynamic> json) {
        id = id;
        name = json['name'];
        select = json['select'] == null ? '' : json['select'];
        answer = json['answer'];
        // isSave = false;
        if (json['options'] != null) {
          options = json['options'].cast<String>();
        }
      }
    
      Map<String, dynamic> toJson() {
        final Map<String, dynamic> data = new Map<String, dynamic>();
        data['id'] = this.id;
        data['name'] = this.name;
        data['select'] = this.select;
        data['answer'] = this.answer;
        // data['isSave'] = this.isSave;
        data['options'] = this.options;
        return data;
      }
    }

任何答案都会得到体现。

Answer 1

我认为您的问题出在解析过程中。我使用quicktype生成了模型。在下面检查

class Quiz {
    final int coins;
    final bool isSubmit;
    final List<Question> questions;
    final String quizId;

    Quiz({
        this.coins,
        this.isSubmit,
        this.questions,
        this.quizId,
    });

    Quiz copyWith({
        int coins,
        bool isSubmit,
        List<Question> questions,
        String quizId,
    }) => 
        Quiz(
            coins: coins ?? this.coins,
            isSubmit: isSubmit ?? this.isSubmit,
            questions: questions ?? this.questions,
            quizId: quizId ?? this.quizId,
        );

    factory Quiz.fromRawJson(String str) => Quiz.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory Quiz.fromJson(Map<String, dynamic> json) => Quiz(
        coins: json["coins"] == null ? null : json["coins"],
        isSubmit: json["isSubmit"] == null ? null : json["isSubmit"],
        questions: json["questions"] == null ? null : List<Question>.from(json["questions"].map((x) => Question.fromJson(x))),
        quizId: json["quizId"] == null ? null : json["quizId"],
    );

    Map<String, dynamic> toJson() => {
        "coins": coins == null ? null : coins,
        "isSubmit": isSubmit == null ? null : isSubmit,
        "questions": questions == null ? null : List<dynamic>.from(questions.map((x) => x.toJson())),
        "quizId": quizId == null ? null : quizId,
    };
}

class Question {
    final String answer;
    final String name;
    final List<String> options;
    final dynamic select;

    Question({
        this.answer,
        this.name,
        this.options,
        this.select,
    });

    Question copyWith({
        String answer,
        String name,
        List<String> options,
        dynamic select,
    }) => 
        Question(
            answer: answer ?? this.answer,
            name: name ?? this.name,
            options: options ?? this.options,
            select: select ?? this.select,
        );

    factory Question.fromRawJson(String str) => Question.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory Question.fromJson(Map<String, dynamic> json) => Question(
        answer: json["answer"] == null ? null : json["answer"],
        name: json["name"] == null ? null : json["name"],
        options: json["options"] == null ? null : List<String>.from(json["options"].map((x) => x)),
        select: json["select"],
    );

    Map<String, dynamic> toJson() => {
        "answer": answer == null ? null : answer,
        "name": name == null ? null : name,
        "options": options == null ? null : List<dynamic>.from(options.map((x) => x)),
        "select": select,
    };
}