Flutter中CompanyBranch.fromJson()返回空值的技术求助

问题描述 #

使用自研API获取JSON数据，服务器已返回序列化数据，但将响应映射为CompanyBranch对象时返回空值。

代码片段 #

Map<String, dynamic> resp = jsonDecode(response.body);
CompanyBranch companyBranch = CompanyBranch.fromJson(resp); // 返回null
LatLng coords = LatLng(companyBranch.lat, companyBranch.lng);
return coords;

服务器返回的JSON响应 #

{
  "id": 1,
  "name": "Fixing Now Surcursal",
  "outdoorNumber": 56,
  "internalNumber": 0,
  "street": "Si",
  "postalCode": 91193,
  "district": "No se que poner",
  "lat": -33.3017267,
  "lng": -66.3377522,
  "active": true,
  "creationDate": "2022-11-10T20:40:46.228762Z",
  "state": 1,
  "municipally": 1
}

映射用的CompanyBranch类代码 #

class CompanyBranch {
  int id;
  String name;
  int outdoorNumber;
  int? internalNumber;
  String street;
  int postalCode;
  String distric;
  String? state;
  String? municipality;
  double lat;
  double lng;
  bool active;
  DateTime creationDate;

  CompanyBranch({
    required this.id,
    required this.name,
    required this.outdoorNumber,
    this.internalNumber,
    required this.street,
    required this.postalCode,
    required this.distric,
    required this.lat,
    required this.lng,
    required this.active,
    required this.creationDate,
    this.state,
    this.municipality,
  });

  factory CompanyBranch.fromJson(Map<String, dynamic> json) {
    return CompanyBranch(
      id: json['id'] as int,
      name: json['name'] as String,
      outdoorNumber: json['outdoorNumber'] as int,
      internalNumber: json['internalNumber'] as int,
      street: json['street'] as String,
      postalCode: json['postalCode'] as int,
      distric: json['district'] as String,
      lat: json['lat'] as double,
      lng: json['lng'] as double,
      active: json['active'] as bool,
      creationDate: json['creationDate'] as DateTime,
      state: json['state'] as String,
      municipality: json['municipaly'] as String,
    );
  }
}

错误分析与修复方案 #

对象返回null的核心原因是fromJson方法中存在类型转换失败和字段拼写错误，导致初始化抛出异常，最终对象创建失败。

1. 字段拼写错误 #

• JSON中的键是municipally，但fromJson里写成了municipaly（少了字母l），无法正确取值。
• 类属性名distric建议改为district，和JSON键名保持一致，避免后续混淆。

2. 类型转换错误 #

• creationDate：JSON返回的是字符串，不能直接用as DateTime，必须通过DateTime.parse()解析。
• state：JSON返回的是数字类型（1），但类中定义为String?，直接as String会抛出类型转换异常，需转为字符串。
• internalNumber：类中定义为可选参数（int?），但as int无法处理null值，应改为as int?。
• lat/lng：JSON中的数值可能被解析为int或double，直接as double会报错，建议用(json['key'] as num).toDouble()兼容两种类型。

修复后的fromJson方法 #

factory CompanyBranch.fromJson(Map<String, dynamic> json) {
  return CompanyBranch(
    id: json['id'] as int,
    name: json['name'] as String,
    outdoorNumber: json['outdoorNumber'] as int,
    internalNumber: json['internalNumber'] as int?,
    street: json['street'] as String,
    postalCode: json['postalCode'] as int,
    distric: json['district'] as String, // 若类属性名改为district，这里也同步修改
    lat: (json['lat'] as num).toDouble(),
    lng: (json['lng'] as num).toDouble(),
    active: json['active'] as bool,
    creationDate: DateTime.parse(json['creationDate'] as String),
    state: json['state'].toString(),
    municipality: json['municipally'] as String?,
  );
}

额外建议 #

• 推荐使用json_serializable等代码生成库自动生成序列化/反序列化代码，避免手动编写时出现拼写和类型错误。
• 对可选参数添加null判断，比如json['internalNumber'] != null ? json['internalNumber'] as int : null，增强代码健壮性。

内容的提问来源于stack exchange，提问作者Arturo Montes de Oca Barrios