Instance members can't be accessed from a factory constructor. Flutter error

Question

I get an error such as:

Instance members can't be accessed from a factory constructor. Try removing the reference to the instance member.

Any solutions?

class DepartureModel {
  String route;
  String departureTime;
  String arrivalTime;
  String tourType;
  List<String> daysOfWeek;

  DepartureModel({
    required this.route,
    required this.departureTime,
    required this.arrivalTime,
    required this.tourType,
    required this.daysOfWeek,
  });

  //method that assign values to respective datatype vairables

  factory DepartureModel.fromJson(Map<String, dynamic> json) {
    return DepartureModel(
      route: json['route'],
      departureTime: json['departureTime'],
      arrivalTime: json['arrivalTime'],
      tourType: json['tourType'],
      daysOfWeek: json["daysOfWeek"].forEach(
        (day) {
          daysOfWeek.add(day);
        },
      ),
    );
  }

Solution 1

You are trying to access daysOfWeek while assigning it, this is why the compiler is complaining as it doesn't yet know the value of daysOfWeek.

A really valid workaround would be to create a new list in the factory constructor and after you are done looping you assign that to daysOfWeek as such:

factory DepartureModel.fromJson(Map<String, dynamic> json) {
    final tempDaysOfWeek = [];
    json["daysOfWeek"].forEach((day) => tempDaysOfWeek.add(day));
    return DepartureModel(
      route: json['route'],
      departureTime: json['departureTime'],
      arrivalTime: json['arrivalTime'],
      tourType: json['tourType'],
      daysOfWeek: tempDaysOfWeek,
    );

You can also name tempDaysOfWeek as daysOfWeek as scope will take care of which variable is being called but this alleviates confusion.

Also a more concise usage without the forEach can be as follows:

factory DepartureModel.fromJson(Map<String, dynamic> json) {
    return DepartureModel(
      route: json['route'],
      departureTime: json['departureTime'],
      arrivalTime: json['arrivalTime'],
      tourType: json['tourType'],
      daysOfWeek: (json["daysOfWeek"] as List).cast<String>(),
    );

Solution 2

You need to make some changes to your code to make it work. Use below model or make change in daysofweek line in your model like below :

import 'dart:convert';

DepartureModel departureModelFromJson(String str) => DepartureModel.fromJson(json.decode(str));

String departureModelToJson(DepartureModel data) => json.encode(data.toJson());

class DepartureModel {
    DepartureModel({
        this.route,
        this.departureTime,
        this.arrivalTime,
        this.tourType,
        this.daysOfWeek,
    });

    String route;
    String departureTime;
    String arrivalTime;
    String tourType;
    List<String> daysOfWeek;

    factory DepartureModel.fromJson(Map<String, dynamic> json) => DepartureModel(
        route: json["route"],
        departureTime: json["departureTime"],
        arrivalTime: json["arrivalTime"],
        tourType: json["tourType"],
        daysOfWeek: List<String>.from(json["daysOfWeek"].map((x) => x)),
    );

    Map<String, dynamic> toJson() => {
        "route": route,
        "departureTime": departureTime,
        "arrivalTime": arrivalTime,
        "tourType": tourType,
        "daysOfWeek": List<dynamic>.from(daysOfWeek.map((x) => x)),
    };
}

Comments

[devfoz]

thank you very much !