so_myservice.dart

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

  factory MyService() => _instance;
  int _myVariable = 0;

  MyService._internal() {
    _myVariable = 0;
  }

  int get myVariable => _myVariable;
  set myVariable(int value) => myVariable = value;

  void incrementMyVariable() => _myVariable++;
}