一. 豆瓣数据

这里我们使用豆瓣的请求地址：

https://douban.uieee.com/v2/movie/top250?start=0&count=20

在浏览器中请求，获取到的数据如下：

注意：这里我使用了一个格式化插件：FeHelper，所以结构看起来很清晰

这个数据还是比较复杂的：

如果我们希望在Flutter代码中使用，直接将JSON转成Map来使用也可以，但是非常麻烦，而且类型会不容易确定，并且不安全；
所以对于面向对象开发的语言，我们通常都会将它转成模型对象，之后使用一个个模型对象；

我们一起来探究一下，目前Flutter中比较常见的将JSON转成模型的方式。

二. 手动转化

JSON转模型，必然可以通过手动来进行转化：

优点：完全是自己可控的，并且需要哪些字段就转化哪些字段，对于不需要的，忽略即可；并且继承关系也会一目了然
缺点：麻烦，并且容易出错；

下面是我之前针对上面的数据，写的JSON转Model的模型类：

class Person {
  String name;
  String avatarURL;
  Person.fromMap(Map<String, dynamic> json) {
    this.name = json["name"];
    this.avatarURL = json["avatars"]["medium"];
  }
}
class Actor extends Person {
  Actor.fromMap(Map<String, dynamic> json): super.fromMap(json);
}
class Director extends Person {
  Director.fromMap(Map<String, dynamic> json): super.fromMap(json);
}
int counter = 1;
class MovieItem {
  int rank;
  String imageURL;
  String title;
  String playDate;
  double rating;
  List<String> genres;
  List<Actor> casts;
  Director director;
  String originalTitle;
  MovieItem.fromMap(Map<String, dynamic> json) {
    this.rank = counter++;
    this.imageURL = json["images"]["medium"];
    this.title = json["title"];
    this.playDate = json["year"];
    this.rating = json["rating"]["average"];
    this.genres = json["genres"].cast<String>();
    this.casts = (json["casts"] asList<dynamic>).map((item) {
      return Actor.fromMap(item);
    }).toList();
    this.director = Director.fromMap(json["directors"][0]);
    this.originalTitle = json["original_title"];
  }
}

三. json_serializable

json_serializable是dart官方推荐和提供的JSON转Model的方式：

一个自动化源代码生成器来为你生成 JSON 序列化数据模板；
由于序列化数据代码不再需要手动编写或者维护，你可以将序列化 JSON 数据在运行时的异常风险降到最低；

第一步：添加相关的依赖

依赖分为项目依赖（dependencies），开发依赖（dev_dependencies）:

注意：需要执行flutter pub get确保我们的项目中有这些依赖

dependencies:
  json_annotation:^3.0.1
dev_dependencies:
  json_serializable:^3.2.5
  build_runner:^1.8.0

第二步：以json_serializable 的方式创建模型类

这里不以豆瓣数据为例，以一个简单的Json数据作为例子

final jsonInfo = {
    "nickname": "coderwhy",
    "level": 18,
    "courses": ["语文", "数学", "英语"],
    "register_date": "2222-2-22",
    "computer": {
      "brand": "MackBook",
      "price": 1000
    }
  };

创建对应的模型（以json_serializable 的方式，创建完成后代码是报错的）

1.part 'user.g.dart'

这个是之后json_serializable会自动帮助我们生成的文件

2.JsonSerializable()

注解：告诉json_serializable哪一个类需要进行转换

3.JsonKey

当映射关系不一样时，可以指定映射关系

4.另外，这里必须有我们的构造方法
5.需要有对应的工厂构造器

_UserFromJson(json)和_UserToJson(this)调用的该方法目前会报错，需要json_serializable来生成

6.toString方法不是必须的，是待会儿进行测试的

User类的代码：

import'package:json_annotation/json_annotation.dart';
import'model/computer.dart';
part'user.g.dart';
@JsonSerializable()
class User {
  String name;
  String email;
  @JsonKey(name: "register_date")
  String registerDate;
  List<String> courses;
  Computer computer;
  User(this.name, this.email, this.registerDate, this.courses, this.computer);
  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);
  @override
  String toString() {
    return'User{name: $name, email: $email, registerDate: $registerDate, courses: $courses, computer: $computer}';
  }
}

Computer类的代码：

import'package:json_annotation/json_annotation.dart';
part'computer.g.dart';
@JsonSerializable()
class Computer {
  String brand;
  double price;
  Computer(this.brand, this.price);
  factory Computer.fromJson(Map<String, dynamic> json) => _$ComputerFromJson(json);
  Map<String, dynamic> toJson() => _$ComputerToJson(this);
  @override
  String toString() {
    return'Computer{brand: $brand, price: $price}';
  }
}

第三步：生成JSON序列化代码

在项目终端运行下面的指令：

该指令是生成一次JSON序列化的代码

flutter pub run build_runner build

或运行下面的指令：

会监听文件的改变，重新生成JSON序列化的代码

flutter pub run build_runner watch

第四步：测试代码

final jsonInfo = {
  "nickname": "coderwhy",
  "level": 18,
  "courses": ["语文", "数学", "英语"],
  "register_date": "2222-2-22",
  "computer": {
    "brand": "MackBook",
    "price": 1000
  }
};
final user = User.fromJson(jsonInfo);
print(user);

更多资料，请查看下面的资源：

dart:convert 和 JsonCodec 文档
Pub 中的 json_serializable package
GitHub 中的 json_serializable 例子

四. 网页转换

目前有一些网页，可以直接将JSON转成Model

网页推荐：https://javiercbk.github.io/json_to_dart/

我们这里以网页版本为例，非常简单：

注意：可能因为豆瓣的数据过于复杂，所以在生成的时候发现少了一个Directors类
这里我重新复制对应的JSON，再次生成了一下

class MovieItem {
  Rating rating;
  List<String> genres;
  String title;
  List<Casts> casts;
  List<String> durations;
  int collectCount;
  String mainlandPubdate;
  bool hasVideo;
  String originalTitle;
  String subtype;
  List<Directors> directors;
  List<String> pubdates;
  String year;
  Avatars images;
  String alt;
  String id;
  MovieItem(
      {this.rating,
        this.genres,
        this.title,
        this.casts,
        this.durations,
        this.collectCount,
        this.mainlandPubdate,
        this.hasVideo,
        this.originalTitle,
        this.subtype,
        this.directors,
        this.pubdates,
        this.year,
        this.images,
        this.alt,
        this.id});
  MovieItem.fromJson(Map<String, dynamic> json) {
    rating =
    json['rating'] != null ? new Rating.fromJson(json['rating']) : null;
    genres = json['genres'].cast<String>();
    title = json['title'];
    if (json['casts'] != null) {
      casts = newList<Casts>();
      json['casts'].forEach((v) {
        casts.add(new Casts.fromJson(v));
      });
    }
    durations = json['durations'].cast<String>();
    collectCount = json['collect_count'];
    mainlandPubdate = json['mainland_pubdate'];
    hasVideo = json['has_video'];
    originalTitle = json['original_title'];
    subtype = json['subtype'];
    if (json['directors'] != null) {
      directors = newList<Directors>();
      json['directors'].forEach((v) {
        directors.add(new Directors.fromJson(v));
      });
    }
    pubdates = json['pubdates'].cast<String>();
    year = json['year'];
    images =
    json['images'] != null ? new Avatars.fromJson(json['images']) : null;
    alt = json['alt'];
    id = json['id'];
  }
  Map<String, dynamic> toJson() {
    finalMap<String, dynamic> data = newMap<String, dynamic>();
    if (this.rating != null) {
      data['rating'] = this.rating.toJson();
    }
    data['genres'] = this.genres;
    data['title'] = this.title;
    if (this.casts != null) {
      data['casts'] = this.casts.map((v) => v.toJson()).toList();
    }
    data['durations'] = this.durations;
    data['collect_count'] = this.collectCount;
    data['mainland_pubdate'] = this.mainlandPubdate;
    data['has_video'] = this.hasVideo;
    data['original_title'] = this.originalTitle;
    data['subtype'] = this.subtype;
    if (this.directors != null) {
      data['directors'] = this.directors.map((v) => v.toJson()).toList();
    }
    data['pubdates'] = this.pubdates;
    data['year'] = this.year;
    if (this.images != null) {
      data['images'] = this.images.toJson();
    }
    data['alt'] = this.alt;
    data['id'] = this.id;
    return data;
  }
}
class Rating {
  int max;
  double average;
  Details details;
  String stars;
  int min;
  Rating({this.max, this.average, this.details, this.stars, this.min});
  Rating.fromJson(Map<String, dynamic> json) {
    max = json['max'];
    average = json['average'];
    details =
    json['details'] != null ? new Details.fromJson(json['details']) : null;
    stars = json['stars'];
    min = json['min'];
  }
  Map<String, dynamic> toJson() {
    finalMap<String, dynamic> data = newMap<String, dynamic>();
    data['max'] = this.max;
    data['average'] = this.average;
    if (this.details != null) {
      data['details'] = this.details.toJson();
    }
    data['stars'] = this.stars;
    data['min'] = this.min;
    return data;
  }
}
class Details {
  int i1;
  int i2;
  int i3;
  int i4;
  int i5;
  Details({this.i1, this.i2, this.i3, this.i4, this.i5});
  Details.fromJson(Map<String, dynamic> json) {
    i1 = json['1'];
    i2 = json['2'];
    i3 = json['3'];
    i4 = json['4'];
    i5 = json['5'];
  }
  Map<String, dynamic> toJson() {
    finalMap<String, dynamic> data = newMap<String, dynamic>();
    data['1'] = this.i1;
    data['2'] = this.i2;
    data['3'] = this.i3;
    data['4'] = this.i4;
    data['5'] = this.i5;
    return data;
  }
}
class Casts {
  Avatars avatars;
  String nameEn;
  String name;
  String alt;
  String id;
  Casts({this.avatars, this.nameEn, this.name, this.alt, this.id});
  Casts.fromJson(Map<String, dynamic> json) {
    avatars =
    json['avatars'] != null ? new Avatars.fromJson(json['avatars']) : null;
    nameEn = json['name_en'];
    name = json['name'];
    alt = json['alt'];
    id = json['id'];
  }
  Map<String, dynamic> toJson() {
    finalMap<String, dynamic> data = newMap<String, dynamic>();
    if (this.avatars != null) {
      data['avatars'] = this.avatars.toJson();
    }
    data['name_en'] = this.nameEn;
    data['name'] = this.name;
    data['alt'] = this.alt;
    data['id'] = this.id;
    return data;
  }
}
class Directors {
  Avatars avatars;
  String nameEn;
  String name;
  String alt;
  String id;
  Directors({this.avatars, this.nameEn, this.name, this.alt, this.id});
  Directors.fromJson(Map<String, dynamic> json) {
    avatars =
    json['avatars'] != null ? new Avatars.fromJson(json['avatars']) : null;
    nameEn = json['name_en'];
    name = json['name'];
    alt = json['alt'];
    id = json['id'];
  }
  Map<String, dynamic> toJson() {
    finalMap<String, dynamic> data = newMap<String, dynamic>();
    if (this.avatars != null) {
      data['avatars'] = this.avatars.toJson();
    }
    data['name_en'] = this.nameEn;
    data['name'] = this.name;
    data['alt'] = this.alt;
    data['id'] = this.id;
    return data;
  }
}
class Avatars {
  String small;
  String large;
  String medium;
  Avatars({this.small, this.large, this.medium});
  Avatars.fromJson(Map<String, dynamic> json) {
    small = json['small'];
    large = json['large'];
    medium = json['medium'];
  }
  Map<String, dynamic> toJson() {
    finalMap<String, dynamic> data = newMap<String, dynamic>();
    data['small'] = this.small;
    data['large'] = this.large;
    data['medium'] = this.medium;
    return data;
  }
}