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;
}

Nice trick, but you are going to loose the variables when trying to access Person() again. It will set will set them back to null, so you will have to check for null before changing the variable of the instance.

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.