列出< dynamic>不是List< Option>的子类型 [英] List<dynamic> is not a subtype of List<Option>

问题描述

我有一个Cloud Firebase数据库，其中包含一个 questions 集合.每个问题都有一个地图选项的列表.我正在使用Flutter，并具有以下用于 question 和 option 的类:

I have a Cloud Firebase database with a questions collection. Each question has a list of map options. I'm using Flutter and have the following classes for question and option:

class Question {
  final String text;
  final List<Option> options; // I have tried changing this to List<dynamic> but it doesn't help
  final String reference;

  Question(this.text, this.options, this.reference);

  Question.fromMap(Map<String, dynamic> map, {this.reference}) : 
    text = map['text'],
    options = map['options']; // here the error happens

  Question.fromSnapshot(DocumentSnapshot snapshot)
     : this.fromMap(snapshot.data, reference: snapshot.documentID);
}

和 option

class Option {
  final String text;
  final int votes;
  bool isSelected;

  Option(this.text, this.votes, this.isSelected);

  Option.fromMap(Map<String, dynamic> map) : 
    text = map['text'],
    votes = map['votes'];
}

我真的不知道该如何解决.不过，这种错误似乎在互联网上无处不在.任何帮助表示赞赏.

I really have no idea how I can fix this. This kind of error seems to be everywhere on the internet, though. Any help is appreciated.

更新

供您参考: options 是Cloud Firestore中的一组地图.

For your information: options is an array of map in Cloud Firestore.

我根据以下两个答案将代码更改为:

I changed my code based on the two answers below to:

factory Question.fromMap(Map<String, dynamic> map) {

    var options = map['options'];
    var text = map['text'];
    var reference = map['documentID'];
    List<Option> optionsList = options.cast<Option>();

    return new Question(
      text = text,
      options = optionsList,
      reference = reference
    );
  }

  factory Question.fromSnapshot(DocumentSnapshot snapshot) {
    return Question.fromMap(snapshot.data);
  }

仍然出现此错误: type'_InternalLinkedHashMap< dynamic，dynamic>'不是强制类型转换中选项"类型的子类型

我已经看到并尝试了很多答案，而且所有人似乎都说相同的话.我只是想不通.

I have seen and tried so many answers and all seem to say the same. I just can't figure this out.

Cloud Firestore的屏幕截图:

Screenshot of Cloud Firestore:

推荐答案

我最终遵循了 @chipli onat 的建议，并将其与 @harsh 答案混合，最后得到了它去工作.这是我的代码现在的样子:

I ended up following @chipli onat advice and mix it with @harsh answer and finally got it to work. This is how my code looks like now:

class Question {
  final String text;
  final String reference;
  final List<Map> options;

  Question(this.text, this.options, this.reference);

  factory Question.fromMap(Map<String, dynamic> map, String reference) {

    var options = map['options'];
    var text = map['text'];
    List<Map> optionsList = options.cast<Map>();

    return new Question(
      text = text,
      options = optionsList,
      reference = reference
    );
  }

  factory Question.fromSnapshot(DocumentSnapshot snapshot) {
    return Question.fromMap(snapshot.data, snapshot.documentID);
  }
}

和

class Option {
  final String text;
  final int votes;
  bool isSelected;

  Option(this.text, this.votes, this.isSelected);

  Option.fromMap(Map<dynamic, dynamic> map) : // notice Map<dynamic, dynamic> here
    text = map['text'],
    votes = map['votes'];
}

我敢肯定，这个答案会有所改善，但是老实说，我不完全理解为什么它必须如此困难.希望这个答案对您有所帮助.

I'm sure this answer could be improved, but to be honest I don't understand entirely why it has to be so difficult. I hope this answer helps you.

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