Dart Error: Can't access 'this' in a field initializer-CodePudding

I am having some trouble with class inheritance in dart and I'm not sure what I'm doing wrong. I am inheriting a class with the extends keyword, but when I try to make the constructor for that class I am getting some errors.

Here is my code:

@JsonSerializable(constructor: '_', fieldRename: FieldRename.snake)
class ParentClass {
  final DateTime time;
  final String bar;

  ParentClass ._(
      {required this.time,
      required this.bar});

  factory ParentClass.fromRecord(MyRecord record, String bar) {
    return ParentClass._(
        time: record.time.toUtc(),
        bar: bar);
  }

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

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

@JsonSerializable(constructor: '_', fieldRename: FieldRename.snake)
class ChildClass extends ParentClass {
  final DateTime time;
  final String bar;
  final int id;

  ChildClass._(
      {required this.time,
      required this.bar,
      required this.id});

  factory ChildClass.fromRecord(MyRecord2 record, String bar) {
    return TimingEvent._(
        id: record.id,
        time: record.time.toUtc(),
        bar: bar);
  }

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

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

When I run this, I get the error: Error: The superclass, 'ParentClass', has no unnamed constructor that takes no arguments. This error is being encountered at the line where the ChildClass is constructed: ChildClass._(

I then try to resolve that error by specifying the constructor for the superclass within the child class. I do this by changing the code from this:

   ChildClass._(
      {required this.time,
      required this.bar,
      required this.id});

to this:

   ChildClass._(
      {required this.id})
      : super._(time: this.time, bar: this.bar);

However, after making this change, I then get another error message: Error: Can't access 'this' in a field initializer. How am I supposed to pass the values through to the superclass constructor without using this?

CodePudding user response：

If you are using dart 2.17 or higher, the correct syntax for the constructor should be:

  ChildClass._({required super.time, required super.bar, required this.id})
      : super._();

Or if you using a version of dart lower than 2.17:

  ChildClass._({required DateTime time, required String bar, required this.id})
      : super._(time: time, bar: bar);

Note that you should also remove time and bar as properties from the ChildClass. These properties are already defined in the ParentClass, defining them again in ChildClass is redundant.