Dart null-safety singleton pattern with parameters.

main.dart

void main() {
  Person personA = Person();
  Person personB = Person();
  print(personA);
  print(personB);
  print(identical(personA, personB)); // if this return `true`, that mains this class is singleton.
}

singleton.dart

class Person {
  static final Person _instance = Person._internal();

  Person._internal();

  factory Person({
    String? name,
    String? lastName,
    int? age,
  }) {
    _instance.name = name;
    _instance.lastName = lastName;
    _instance.age = age;
    return _instance;
  }

  String? name;
  String? lastName;
  int? age;
}

Hi @icnahom I'm sorry for the delay answer,
You are absolutely right.

For simplicity I think if you want instance variables as non-null, remove ? mark, it'll ask for the constructer, to solve that you should declare variables with late keyword for later assignment.

Note: For each instance you should initialize variables other way you'll got an initialization error.

Conclusion Using late keyword is more helpful if your constructer has no optional parameters.