In Dart, is it possible to pass an argument in a singleton?

Question

class Peoples {
  late int id;
  late String name;

  static final Peoples _inst = Peoples._internal();

  Peoples._internal();

  factory Peoples() {
    return _inst;
  }
}

I have this singleton class. Which ensures that, only one instance of a class is ever created. So, even if someone tries to instantiate it, they will use the same instance. And i can create and set values, like:

  Peoples ps1 = Peoples();
  Peoples ps2 = Peoples();

  ps1.id = 1;
  ps1.name = "First";

  ps2.id = 2;
  ps2.name = "Second";

Is it possible to instantiate and set values like:

  Peoples ps1 = Peoples(1, "First");
  Peoples ps2 = Peoples(2, "Second");

So, now both "ps1" and "ps2" will have (2, "Second").

Answer 1

Sure! You need to pass the arguments to the factory method then you need to update the properties USING the referenced instance.

For example, you had

class Peoples {
  int id;
  String name;

  static final Peoples _inst = Peoples._internal();

  Peoples._internal();

  factory Peoples() {
    return _inst;
  }
}

If you apply my solution then you have

class Peoples {
  int id;
  String name;

  static final Peoples _inst = Peoples._internal();

  Peoples._internal();

  factory Peoples({int id, String name}) {
    _inst.id = id
    _inst.name = name
    return _inst;
  }
}

with this your question should be answered for more info about factory and params visit

https://dart.dev/guides/language/language-tour

Working Example

class Peoples {
  int id;
  String name;

  static final Peoples _inst = Peoples._internal();

  Peoples._internal();


  factory Peoples(int id, String name) {
    _inst.id = id;
    _inst.name = name;
    return _inst;
  }
}

void main() {
  print("Instance of = " + Peoples(0, "Dylan").name);
  print("Instance of = " + Peoples(1, "Joe").name);
  print("Instance of = " + Peoples(2, "Maria").name);
}

Answer 2

I'd like to answer showing a way to create a singleton by passing arguments to it and how to "lock" its values after creating it for the first time.

class People {
  static final People _inst = People._internal();
  People._internal();

  factory People(int id, String name) {
    assert(!_inst._lock, "it's a singleton that can't re-defined");
    _inst.id = id;
    _inst.name = name;
    _inst._lock = true;
    return _inst;
  }
  
  int id;
  String name;
  bool _lock = false;
}

void main() {
  var people = People(0, 'Dylan');
  try{
    print('Instance of = ' + People(0, 'Joe').name);
    print('Instance of = ' + People(1, 'Maria').name);
    print('Instance of = ' + People(2, 'Ete sech').name);
  } finally {
    print('Instance of = ' + people.name);
  }
}

Answer 3

The answer can not run on my machine, 29/3/2022

The computer says: “Non-nullable instance field 'id' must be initialized.”

I cannot comment, so write this answer:

Add keyword late before the member variables will help:

class Peoples {
  late int id;
  late String name;

  static final Peoples _inst = Peoples._internal();

  Peoples._internal();


  factory Peoples(int id, String name) {
    _inst.id = id;
    _inst.name = name;
    return _inst;
  }
}