Flutter中如何为含嵌套rating字段的API数据创建正确数据模型

你当前的代码存在两个核心错误：

• 错误将嵌套在rating下的rate、count字段定义为Product的顶层属性，这两个字段实际属于rating对象，不应该出现在Product的构造参数中。
• 给rating声明的Map<double, int>类型和实际结构完全不匹配：你传入的{rate: 2.1, count: 430}是key为字符串、value包含多种类型的结构，Dart的Map要求key类型统一、value类型统一，这种固定字段名的嵌套结构不能用强类型Map直接接收。

和TypeScript中定义嵌套interface的逻辑一致，Dart中遇到固定结构的嵌套对象，需要单独为嵌套结构定义对应的数据类。

1. 首先定义Rating类，承接rate和count两个字段：

class Rating {
  final double? rate;
  final int? count;

  Rating({this.rate, this.count});
}

2. 修改Product类，移除冗余的顶层rate、count字段，将rating的类型改为Rating?：

class Product {
  final int? id;
  final String? title;
  final double? price;
  final String? description;
  final String? category;
  final String? image;
  final Rating? rating;

  Product({
    this.id,
    this.title,
    this.price,
    this.description,
    this.category,
    this.image,
    this.rating,
  });
}

3. 实例化Product时，给rating传入Rating类的实例即可，不会再触发报错：

List<Product> productsList = [
  Product(
      id: 1,
      title: "Blue Bag",
      price: 100,
      description: " this is a description",
      category: "Clothes",
      image: "https://fakestoreapi.com/img/81fPKd-2AYL._AC_SL1500_.jpg",
      rating: Rating(rate: 2.1, count: 430)
  )
];

如果后续需要解析接口返回的JSON数据，可以给两个类添加fromJson工厂构造函数，直接把Map格式的JSON数据转成类型安全的模型对象：

class Rating {
  final double? rate;
  final int? count;

  Rating({this.rate, this.count});

  factory Rating.fromJson(Map<String, dynamic> json) {
    return Rating(
      // 调用toDouble()兼容接口返回int类型分值的情况
      rate: json['rate']?.toDouble(),
      count: json['count'] as int?,
    );
  }
}

class Product {
  final int? id;
  final String? title;
  final double? price;
  final String? description;
  final String? category;
  final String? image;
  final Rating? rating;

  Product({
    this.id,
    this.title,
    this.price,
    this.description,
    this.category,
    this.image,
    this.rating,
  });

  factory Product.fromJson(Map<String, dynamic> json) {
    return Product(
      id: json['id'] as int?,
      title: json['title'] as String?,
      price: json['price']?.toDouble(),
      description: json['description'] as String?,
      category: json['category'] as String?,
      image: json['image'] as String?,
      // rating字段不为空时才调用Rating的fromJson方法解析
      rating: json['rating'] != null ? Rating.fromJson(json['rating']) : null,
    );
  }
}

注意：如果硬要用Map类型接收rating字段，只能声明为Map<String, dynamic>，但这种写法会丢失静态类型校验，取字段时很容易出现拼写错误、类型不匹配问题，生产环境不推荐使用。

内容的提问来源于stack exchange，提问作者Oren Papa