Flutter - null 安全和型号

Question

I have returned to Flutter after while (in the meantime null safety was introduced). I ended up with a need to update a project. Let's say in that project I have a User model. I managed to update most of the project but the part that gives me a headache is logout action. There is a need to clear the user (after logout) which I need to set up as null or empty it otherwise, but of course I am getting an error: Unhandled Exception: type 'Null' is not a subtype of type 'User'

So my question here is what is the best strategy to clear not only user but any other models I have for a redux state without hitting this problem with models not being able to be null?

User model:

class User {
  String id;
  String username;
  String email;
  String jwt;

  User({ required this.id, required this.username, required this.email, required this.jwt });

  factory User.fromJson(Map<String, dynamic> json) {
    return User (
      id: json['id'],
      username: json['username'],
      email: json['email'],
      jwt: json['jwt']
    );
  }
}

User actions:

/* User actions */
ThunkAction<AppState> getUserAction = (Store<AppState> store) async {
  final prefs = await SharedPreferences.getInstance();
  final String? storedUser = prefs.getString('user');
  final user = storedUser != null ? User.fromJson(json.decode(storedUser)) : null;

  if(user != null) {
    store.dispatch(GetUserAction(user));
  }
};


ThunkAction<AppState> logoutUserAction = (Store<AppState> store) async {
  final prefs = await SharedPreferences.getInstance();
  await prefs.remove('user');
  var user;
  store.dispatch(LogoutUserAction(user));
};


class GetUserAction {
  final User _user;
  User get user => this._user;
  GetUserAction(this._user);
}


class LogoutUserAction {
  final User _user;
  User get user => this._user;
  LogoutUserAction(this._user);
}

NOTE: see how I managed to go about the null in the getUserAction (login) part. I just don't dispatch the action if it is null, however I can not do this in the logout as I need explicitly to set the user to null (clear it) and that way log it out from the app.

Can I make a model accept null values? How would I go about this? Or is there any other way I should go about this? Thank you for your kind answer.

Answer 1

Change your model to:

class User {
  String? id;
  String? username;
  String? email;
  String? jwt;

  User({ required this.id, required this.username, required this.email, required this.jwt });

  factory User.fromJson(Map<String, dynamic> json) {
    return User (
      id: json['id'] ?? "",
      username: json['username'] ?? "",
      email: json['email'] ?? "",
      jwt: json['jwt'] ?? ""
    );
  }
}

And you need check null_safety for all variable with operation??

After logout you can check user null with user.id == "" or user == User()

ThunkAction<AppState> logoutUserAction = (Store<AppState> store) async {
  final prefs = await SharedPreferences.getInstance();
  await prefs.remove('user');
  var user = User();
  store.dispatch(LogoutUserAction(user));
};