初始化形式参数不能在工厂构造函数中使用 [英] Iniliziating formal parameters can't be used in factory constructors

问题描述

我正在尝试在类中添加单例模式，但是我收到初始化形式参数无法在工厂构造函数中使用错误。这是我尝试的方法：

i am trying add singleton pattern in my class but i receive Iniliziating formal parameters can't be used in factory constructors error. Here is what i tried:

import 'package:json_annotation/json_annotation.dart';

part 'user.g.dart';

@JsonSerializable()
class User {
  final String email;
  final String token;
  final bool wordtestCompleted;


  User.forJson({this.email, this.token, this.wordtestCompleted})
  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);



  static final User _singleton = User._internal();

  factory User({this.email, this.token, this.wordtestCompleted}) {
    return _singleton;
  }

  User._internal();
}

如何解决？

推荐答案

在构造函数参数中使用 this的语法糖。仅在普通构造函数中起作用，而在中不起作用factory 构造函数。 （一个工厂构造函数没有这个对象！）

The syntactic sugar of using this. in constructor parameters to initialize members works only in normal constructors, not in factory constructors. (A factory constructor has no this object!)

您将改为需要将工厂构造函数的参数手动转发到实际的构造函数。例如：

You instead will need to manually forward your factory constructor's arguments to an actual constructor. For example:

class User {
  static User _singleton;

  final String email;
  final String token;
  final bool wordtestCompleted;

  User._internal({this.email, this.token, this.wordtestCompleted});

  factory User({String email, String token, bool wordtestCompleted}) {
    return _singleton ??= User._internal(
      email: email,
      token: token,
      wordtestCompleted: wordtestCompleted,
    );
  }
}

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