对象为空时从 Internet 错误中获取数据

Question

我有这个模型按预期工作，除非 InfoA 返回 null：

class ModelGTA {
  ModelGTA({
    required this.infoA,
    required this.infoB,
  });

  InfoA? infoA;
  InfoB? infoB;

  ModelGTA.fromJson(Map<String, dynamic> json) {
    infoA = InfoA.fromJson(json['infoA']);
    infoB = InfoB.fromJson(json['infoB']);
  }

  Map<String, dynamic> toJson() {
    final _data = <String, dynamic>{};
    _data['infoA'] = infoA!.toJson();
    _data['infoB'] = infoB!.toJson();
    return _data;
  }
}

class InfoA {
  InfoA({
    required this.data,
    required this.property,
  });

  Data? data;
  Property? property;


  InfoA.fromJson(Map<String, dynamic> json) {
    data = Data.fromJson(json['data']);
    property = Property.fromJson(json['property']);
  }

  Map<String, dynamic> toJson() {
    final _data = <String, dynamic>{};
    _data['data'] = data!.toJson();
    _data['property'] = property!.toJson();
    return _data;
  }
}

class Data {
  Dados({
    required this.number,
    required this.type,
  });

  String? number;
  String? type;

  Dados.fromJson(Map<String, dynamic> json) {
    numero = json['number'] ?? "";
    tipo = json['type'] ?? "";
  }

  Map<String, dynamic> toJson() {
    final _data = <String, dynamic>{};
    _data['number'] = number;
    _data['type'] = type;
    return _data;
  }
}

问题是当 infoA 返回 null 时，代码停止工作。我得到这个错误：

E/flutter ( 9958): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: NoSuchMethodError: The method '[]' was called on null.
E/flutter ( 9958): Receiver: null
E/flutter ( 9958): Tried calling: []("data")

我想知道是否有任何方法可以返回一个对象、带有空字符串的数据或防止程序在 InfoA 为空时尝试访问数据。

Answer 1

为此更改您的方法 ModelGTA.fromJson

 ModelGTA.fromJson(Map<String, dynamic> json) {
    infoA =
(json['infoA']==null)?[]: InfoA.fromJson(json['infoA']);
    infoB = InfoB.fromJson(json['infoB']);
  }