Flutter中用Riverpod和HTTP解析JSON失败，求排查

问题描述 #

参考Flutter官方文档实现后台JSON解析，请求能正常获取响应内容，但在映射到实体类阶段停止执行，仅打印到“genderData out 2 parsed”，相关代码如下：

class NameClass {
  final String name;
  NameClass({required this.name});
}

var nameList = [
  'Martine',
  'Els',
  'Dirk',
  'Alain',
];

class GenderData {
  final String gender;
  final int count;
  final String name;
  final double prob;
  const GenderData({
    required this.gender,
    required this.count,
    required this.name,
    required this.prob,
  });
  factory GenderData.fromJson(Map<String, dynamic> json) {
    return GenderData(
      gender: json['gender'] as String,
      count: json['count'] as int,
      name: json['name'] as String,
      prob: json['prob'] as double,
    );
  }
}

var _names = "name[]=" + nameList.join('&name[]=').toString();

GenderData? out;
Future<List<GenderData>> getGenderData() async {
  print("getGenderData out");
  final response =
      await get(Uri.parse('https://api.genderize.io/?${_names}'));
  print("response.body");
  print(response.body);
  return compute(parsePhotos, response.body);
}

List<GenderData> parsePhotos(String responseBody) {
  print("genderData out 1");
  final parsed = jsonDecode(responseBody).cast<Map<String, dynamic>>();
  print("genderData out 2 parsed");
  print(parsed);
  out = parsed.map<GenderData>((json) => GenderData.fromJson(json)).toList();
  print("genderData out 3");
  return (out);
}

问题根源 #

核心是类型不匹配导致的隐式运行错误：

• 全局变量out的类型是GenderData?（单个可空对象），但parsed.map(...).toList()返回的是List<GenderData>（对象列表），将列表赋值给单个对象类型的变量会直接抛出类型错误，代码执行到这一行就中断，无法继续打印“genderData out 3”。
• parsePhotos函数声明返回List<GenderData>，但最后返回的是out（GenderData?类型），类型完全不兼容，即使前面的赋值不报错，这里也会出现编译错误。

修复方案 #

修改parsePhotos函数，删除多余的全局变量，直接处理并返回列表：

// 删除全局的GenderData? out;变量

List<GenderData> parsePhotos(String responseBody) {
  print("genderData out 1");
  final parsed = jsonDecode(responseBody) as List<dynamic>;
  print("genderData out 2 parsed");
  print(parsed);
  final genderList = parsed.map<GenderData>((json) => GenderData.fromJson(json as Map<String, dynamic>)).toList();
  print("genderData out 3");
  return genderList;
}

额外优化建议 #

为避免API返回字段类型不符合预期导致的崩溃，在GenderData.fromJson中增加空安全与类型兼容处理：

factory GenderData.fromJson(Map<String, dynamic> json) {
  return GenderData(
    gender: json['gender'] as String? ?? 'unknown',
    count: json['count'] as int? ?? 0,
    name: json['name'] as String? ?? '',
    prob: (json['prob'] as num?)?.toDouble() ?? 0.0,
  );
}

内容的提问来源于stack exchange，提问作者Hadi Majidi