Using Dart's factory constructor to implement the singleton pattern

singleton.dart

class MySingleton {
  String var1;
  int var2;
  
  static MySingleton _instance;
  
  factory MySingleton({String var1, int var2}) {
    _instance == null
      ? print('New instance!')
      : print('Old instance!');
    _instance ??= MySingleton._internal(var1, var2); // if _instance is null then init it
    return _instance;
  }

  MySingleton._internal([this.var1, this.var2]){
    // Todo: Your logic stuff
  }

}

final instanceA = MySingleton(var1: 'Yo!', var2: 1); // prints "New instance"
final instanceB = MySingleton(); // prints "Old instance"
final instanceC = MySingleton(); // prints "Old instance"