_TypeError（类型 'List<dynamic>' 不是类型 'Map<String, dynamic>' 的子类型）

Question

我正在学习 Dart 和 Flutter。现在我正在品尝 JSON 作为一种持久性方法。我收到很多错误，都与类型和东西有关。这是我遇到的最新错误：_TypeError (type 'List<dynamic>' is not a subtype of type 'Map<String, dynamic>')。

这是课程：

import './topic.dart';

class Subject {
  String name;
  int order;
  bool isMajor;
  List<Topic> topics;

  Subject({this.name, this.order, this.isMajor, this.topics});

  factory Subject.fromJSON(Map<String, dynamic> json) {
    if (json != null) {
      return Subject(
          name: json['name'],
          order: json['order'],
          isMajor: json['isMajor'],
          topics: [Topic.fromJSON(json['topics'])]);
    } else {
      return null;
    }
  }
}

主题类是这样的：

import './content.dart';

class Topic {
  String name;
  int order;
  List<Content> contents;

  Topic({this.name, this.order, this.contents});

  factory Topic.fromJSON(Map<String, dynamic> json) {
    if (json != null) {
      return Topic(
          name: json['name'],
          order: json['order'],
          contents: [Content.fromJSON(json['contents'])]);
    } else {
      return null;
    }
  }
}

此处出现错误：[Topic.fromJSON(json['topics'])]。

有人可以帮忙吗？谢谢！

Answer 1

主题应该是

topics: List<Topic>.from(json["topics"].map((x) => Topic.fromJson(x))),

因为你没有提供Content 类，我假设它有name 和order 属性
可以使用Subject subject = subjectFromJson(jsonString);解析jsonString

完整的相关类

// To parse this JSON data, do
//
//     final subject = subjectFromJson(jsonString);

import 'dart:convert';

Subject subjectFromJson(String str) => Subject.fromJson(json.decode(str));

String subjectToJson(Subject data) => json.encode(data.toJson());

class Subject {
    String name;
    int order;
    bool isMajor;
    List<Topic> topics;

    Subject({
        this.name,
        this.order,
        this.isMajor,
        this.topics,
    });

    factory Subject.fromJson(Map<String, dynamic> json) => Subject(
        name: json["name"],
        order: json["order"],
        isMajor: json["isMajor"],
        topics: List<Topic>.from(json["topics"].map((x) => Topic.fromJson(x))),
    );

    Map<String, dynamic> toJson() => {
        "name": name,
        "order": order,
        "isMajor": isMajor,
        "topics": List<dynamic>.from(topics.map((x) => x.toJson())),
    };
}

class Topic {
    String name;
    int order;
    List<Content> contents;

    Topic({
        this.name,
        this.order,
        this.contents,
    });

    factory Topic.fromJson(Map<String, dynamic> json) => Topic(
        name: json["name"],
        order: json["order"],
        contents: List<Content>.from(json["contents"].map((x) => Content.fromJson(x))),
    );

    Map<String, dynamic> toJson() => {
        "name": name,
        "order": order,
        "contents": List<dynamic>.from(contents.map((x) => x.toJson())),
    };
}

class Content {
    String name;
    int order;

    Content({
        this.name,
        this.order,
    });

    factory Content.fromJson(Map<String, dynamic> json) => Content(
        name: json["name"],
        order: json["order"],
    );

    Map<String, dynamic> toJson() => {
        "name": name,
        "order": order,
    };
}