Dart使用json_serializable解析到json或从json解析出库类 [英] Dart Parse to/from json a library class using json_serializable

问题描述

• 我正在扑朔迷离地使用地理编码器库.
• 当我得到一个地址时，我想保存在 firebase
  中firestore 作为用户(对象)的属性，但要做到这一点，我需要
  序列化地址(将其转换为地图).
• 当前，我正在使用 json_serializable 来解析/映射我的用户类别.
• 因为Address是地理编码器库中的一类，所以我很难将其解析到地图中/从地图中解析出来，因为这是由json_serializable处理的.

• I'm working with geocoder library in flutter.
• When I get an Address I want to save it in firebase
  firestore as an user (object) atribute but to do that I need to
  serialize the address (convert it to a map).
• Currently, I'm using json_serializable to parse from/to map my User class.
• Because Address is a class from geocoder library I'm having troubles to parse it to/from map because this is handled by json_serializable.

这是我的User类:

    import 'package:geocoder/model.dart';
    import 'package:json_annotation/json_annotation.dart';

    import './resource.dart';

    part 'user.g.dart';

    @JsonSerializable()
    class User extends Resource {
       User(this.email, this.name);

       String email;
       String name;
       List<Address> addresses;

       factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);

       Map<String, dynamic> toJson() => _$UserToJson(this);
    }

• 我尝试创建一个名为UserAddress的新类，其父类将为类似于以下地址，但出现错误

构造函数返回的类型"Address"不是预期的类型"UserAddress".dart(invalid_cast_new_expr)

The constructor returns type 'Address' that isn't of expected type 'UserAddress'.dart(invalid_cast_new_expr)

在命名的构造函数UserAddress.fromJson中:

in the named constructor UserAddress.fromJson:

import 'package:geocoder/geocoder.dart';
import 'package:json_annotation/json_annotation.dart';

part 'user-address.g.dart';

@JsonSerializable()
class UserAddress extends Address {
  UserAddress();

  factory UserAddress.fromJson(Map<String, dynamic> json) =>
      Address.fromMap(json);

  Map<String, dynamic> toJson() => this.toMap();
}

请给我一些选择，以实现与地图之间的解析.

Please give me some options to achieve parsing to/from map, please.

推荐答案

如果我答对了，您想用自己的修饰类.您可以使用 @JsonKey 批注设置(解码)库的 Address 对象的函数.

If I got you right, you want to decorate library's class with your own. You can use @JsonKey annotation to set functions for (en)decoding library's Address object.

import 'dart:convert';

import 'package:geocoder/geocoder.dart';
import 'package:json_annotation/json_annotation.dart';

part 'user_address.g.dart';

@JsonSerializable()
class UserAddress {
  UserAddress(this.address, this.oneMoreField);

  @JsonKey(fromJson: _addressFromJson, toJson: _addressToJson)
  final Address address;
  final int oneMoreField;

  factory UserAddress.fromJson(Map<String, dynamic> json) => _$UserAddressFromJson(json);
  Map<String, dynamic> toJson() => _$UserAddressToJson(this);
}

String _addressToJson(Address address) => jsonEncode(address.toMap());
Address _addressFromJson(String json) => Address.fromMap(jsonDecode(json));

示例用法:

final testUserAddress = UserAddress(Address(
  addressLine: 'test',
  coordinates: Coordinates(10, 20)
), 1);

final json = testUserAddress.toJson();
print(json);

final decoded = UserAddress.fromJson(json);

打印:

{address: {"coordinates":{"latitude":10.0,"longitude":20.0},"addressLine":"test","countryName":null,"countryCode":null,"featureName":null,"postalCode":null,"locality":null,"subLocality":null,"adminArea":null,"subAdminArea":null,"thoroughfare":null,"subThoroughfare":null}, oneMoreField: 1}

如您所见，所有嵌套对象都被正确编码.

As you can see, all nested objects are encoded fine.

UPD :通过继承进行操作时，您必须将所有基类的字段传递给超级构造函数，并使用 @JsonKey 批注覆盖嵌套对象的字段:

UPD: When doing that via inheritence, you'll have to pass all base class' fields to super constructor and to override nested objects fields with @JsonKey annotation:

@JsonSerializable()
class UserAddress extends Address {
  UserAddress({String addressLine, String countryName, this.coordinates, this.oneMoreField}) 
    : super(addressLine: addressLine, countryName: countryName);

  @override
  @JsonKey(fromJson: _coordinatesFromJson, toJson: _coordinatesToJson)
  final Coordinates coordinates;

  final int oneMoreField;

  factory UserAddress.fromJson(Map<String, dynamic> json) => _$UserAddressFromJson(json);
  Map<String, dynamic> toJson() => _$UserAddressToJson(this);
}

String _coordinatesToJson(Coordinates coordinates) => jsonEncode(coordinates.toMap());
Coordinates _coordinatesFromJson(String json) => Coordinates.fromMap(jsonDecode(json));

结果:

{addressLine: test, countryName: null, coordinates: {"latitude":10.0,"longitude":20.0}, oneMoreField: 1}

但是在这种情况下，我建议对继承使用合成.

But I recommend using composition over inheritence in this case.

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