Dart解析嵌套JSON数组为Freezed数据类时出现类型错误

错误根源 #

你遇到的类型错误，本质是解析嵌套的neighborhoods数组时，误将单个对象的解析函数（Neighborhood.fromJson）当作可迭代的JSON数组传入，导致类型不匹配。

正确的Freezed数据类定义 #

先定义Neighborhood类：

import 'package:freezed_annotation/freezed_annotation.dart';

part 'neighborhood.freezed.dart';
part 'neighborhood.g.dart';

@freezed
class Neighborhood with _$Neighborhood {
  const factory Neighborhood({
    required String id,
    required String name,
  }) = _Neighborhood;

  factory Neighborhood.fromJson(Map<String, dynamic> json) =>
      _$NeighborhoodFromJson(json);
}

再定义包含嵌套数组的City类：

import 'package:freezed_annotation/freezed_annotation.dart';
import 'neighborhood.dart';

part 'city.freezed.dart';
part 'city.g.dart';

@freezed
class City with _$City {
  const factory City({
    required String id,
    required String name,
    required List<Neighborhood> neighborhoods,
  }) = _City;

  factory City.fromJson(Map<String, dynamic> json) => _$CityFromJson(json);
}

正确的JSON解析代码 #

假设服务器返回的JSON结构如下：

{
  "cities": [
    {
      "id": "1",
      "name": "北京",
      "neighborhoods": [
        {"id": "1-1", "name": "朝阳区"},
        {"id": "1-2", "name": "海淀区"}
      ]
    }
  ]
}

解析整个城市数组的代码：

import 'dart:convert';

// 假设responseBody是服务器返回的JSON字符串
final jsonData = jsonDecode(responseBody) as Map<String, dynamic>;
final List<City> cities = (jsonData['cities'] as List<dynamic>)
    .map((cityJson) => City.fromJson(cityJson as Map<String, dynamic>))
    .toList();

避免常见错误 #

• 错误写法（会触发类型错误）：直接用单个对象解析函数处理数组

  // 错误示例：把数组当成单个Map传入
  neighborhoods: Neighborhood.fromJson(json['neighborhoods'])
  

• 正确写法：先将JSON转成List，再遍历解析每个元素

  // 若需手动处理嵌套数组（Freezed自动生成的代码已包含此逻辑，无需手动写）
  neighborhoods: (json['neighborhoods'] as List<dynamic>)
      .map((n) => Neighborhood.fromJson(n as Map<String, dynamic>))
      .toList()
  

内容的提问来源于stack exchange，提问作者DolDurma