Flutter解析Rick and Morty API数据时触发_TypeError类型错误

我正在使用Rick and Morty API，发送GET请求后返回如下格式的分页数据：

{
    "info": {
        "count": 826,
        "pages": 42,
        "next": "https://rickandmortyapi.com/api/character?page=2",
        "prev": null
    },
    "results": [
        {
            "id": 1,
            "name": "Rick Sanchez",
            "status": "Alive",
            "species": "Human",
            "type": "",
            "gender": "Male",
            "origin": {
                "name": "Earth (C-137)",
                "url": "https://rickandmortyapi.com/api/location/1"
            },
            "location": {
                "name": "Citadel of Ricks",
                "url": "https://rickandmortyapi.com/api/location/3"
            },
            "image": "https://rickandmortyapi.com/api/character/avatar/1.jpeg",
            "episode": [
                "https://rickandmortyapi.com/api/episode/1",
                ...
            ],
            "url": "https://rickandmortyapi.com/api/character/1",
            "created": "2017-11-04T18:48:46.250Z"
        }
        // 更多角色数据...
    ]
}

该数据包含info分页信息对象和results角色列表。我定义了Character类映射角色对象：

class Character {
  final int id;
  final String name;
  final String status;
  final String species;
  final String type;
  final String gender;
  final CharacterLocation origin;
  final CharacterLocation location;
  final String image;
  final List<String> episode;
  final String url;
  final DateTime created;

  Character({
    required this.id,
    required this.name,
    required this.status,
    required this.species,
    required this.type,
    required this.gender,
    required this.origin,
    required this.location,
    required this.image,
    required this.episode,
    required this.url,
    required this.created,
  });

  factory Character.fromJson(Map<String, dynamic> json) => Character(
        id: json["id"],
        name: json["name"],
        status: json["status"],
        species: json["species"],
        type: json["type"],
        gender: json["gender"],
        origin: CharacterLocation.fromJson(json["origin"]),
        location: CharacterLocation.fromJson(json["location"]),
        image: json["image"],
        episode: List<String>.from(json["episode"].map((x) => x)),
        url: json["url"],
        created: DateTime.parse(json["created"]),
      );
}

class CharacterLocation {
  CharacterLocation({
    required this.name,
    required this.url,
  });

  String name;
  String url;

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

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

同时定义了CharacterPage类映射整个分页数据：

import 'package:flutter_learn_practice/ram_character_models.dart';

class CharacterPage {
  Info info;
  List<Character> results;

  CharacterPage({
    required this.info,
    required this.results});
  
  factory CharacterPage.fromJson(Map<String, dynamic> json) => CharacterPage(
    info: Info.fromJson(json["info"]),
    results: List<Character>.from(json["results"].map((x) => x))
    );
}

class Info {
  int count;
  int pages;
  String? next;
  String? prev;

  Info({
    required this.count,
    required this.pages,
    this.next = '',
    this.prev = '',
  });

  factory Info.fromJson(Map<String, dynamic> json) => Info(
    count: json["count"],
    pages: json["pages"],
    next: json["next"],
    prev: json["prev"],
  );
}

目前info相关字段（如count、pages）可正常使用，但访问results列表时抛出错误：

Exception has occurred.
_TypeError (type '_Map<String, dynamic>' is not a subtype of type 'Character')

解决方案 #

错误根源在于CharacterPage.fromJson方法中，results的映射逻辑没有将JSON对象转换为Character实例。原代码直接把json["results"]中的每个_Map<String, dynamic>对象当作Character类型使用，导致类型不匹配。

修改CharacterPage.fromJson中的results赋值代码，调用Character.fromJson(x)将每个JSON对象转换为Character实例：

factory CharacterPage.fromJson(Map<String, dynamic> json) => CharacterPage(
    info: Info.fromJson(json["info"]),
    results: List<Character>.from(json["results"].map((x) => Character.fromJson(x)))
);

这样处理后，json["results"]中的每个Map都会被正确转换为Character对象，List<Character>的类型匹配，错误即可解决。

内容的提问来源于stack exchange，提问作者Doğan Erarslan