用Riverpod通过http获取JSON数据时报错：预期Coord类型却得到_JsonMap

这个报错的根本原因是你在Madrid.fromJson方法中，没有将JSON解析得到的_JsonMap（也就是coord字段对应的Map结构）转换为Coord实体类实例，而是直接把Map赋值给了声明为Coord类型的属性，导致类型不匹配。

1. 确认所有嵌套实体类的fromJson方法已正确实现 #

首先检查Coord、Weather、Main、Wind、Clouds、Sys这些嵌套的实体类，都要实现对应的fromJson构造方法，以Coord类为例，正确写法参考：

class Coord {
  final double lon;
  final double lat;

  Coord({required this.lon, required this.lat});

  factory Coord.fromJson(Map<String, dynamic> json) {
    return Coord(
      lon: json['lon']?.toDouble() ?? 0.0,
      lat: json['lat']?.toDouble() ?? 0.0,
    );
  }
}

其他嵌套类按照相同规则实现fromJson构造即可。

2. 修正Madrid.fromJson中嵌套字段的赋值逻辑 #

不要直接把JSON返回的Map、List赋值给实体类属性，要调用对应嵌套类的fromJson做类型转换，数组类型的字段还要做遍历转换，正确的Madrid类参考写法：

class Madrid {
  final Coord coord;
  final List<Weather> weather;
  final String base;
  final Main main;
  final int visibility;
  final Wind wind;
  final Clouds clouds;
  final int dt;
  final Sys sys;
  final int timezone;
  final int id;
  final String name;
  final int cod;

  Madrid({
    required this.coord,
    required this.weather,
    required this.base,
    required this.main,
    required this.visibility,
    required this.wind,
    required this.clouds,
    required this.dt,
    required this.sys,
    required this.timezone,
    required this.id,
    required this.name,
    required this.cod,
  });

  factory Madrid.fromJson(Map<String, dynamic> json) {
    return Madrid(
      // 调用Coord.fromJson做类型转换，不要直接赋值json['coord']
      coord: Coord.fromJson(json['coord']),
      // 数组类型遍历转成对应实体类实例
      weather: (json['weather'] as List<dynamic>).map((item) => Weather.fromJson(item)).toList(),
      base: json['base'] ?? '',
      main: Main.fromJson(json['main']),
      visibility: json['visibility'] ?? 0,
      wind: Wind.fromJson(json['wind']),
      clouds: Clouds.fromJson(json['clouds']),
      dt: json['dt'] ?? 0,
      sys: Sys.fromJson(json['sys']),
      timezone: json['timezone'] ?? 0,
      id: json['id'] ?? 0,
      name: json['name'] ?? '',
      cod: json['cod'] ?? 0,
    );
  }
}

3. 修正代码笔误 #

你定义的FutureProvider里调用的是fetchMadridLocal()，但实际实现的请求方法名是fetchMadrid()，要改成一致才能正常触发请求：

final madridStateProvider = FutureProvider<Madrid>((ref) async {
  return fetchMadrid();
});

内容的提问来源于stack exchange，提问作者Diego jiménez