Different Ways of Create Singleton classes in dart

singleton.dart

//1. Factory constructor
class SingletonOne { 

  SingletonOne._privateConstructor();

  static final SingletonOne _instance = SingletonOne._privateConstructor();

  factory SingletonOne() {
    return _instance;
  }

}
//Static field with getter
class SingletonTwo {

  SingletonTwo._privateConstructor();

  static final SingletonTwo _instance = SingletonTwo._privateConstructor();

  static SingletonTwo get instance => _instance;

}
//3. Static field
class SingletonThree {

  SingletonThree._privateConstructor();

  static final SingletonThree instance = SingletonThree._privateConstructor();

}
//ways of init
SingletonOne one = SingletonOne();
SingletonTwo two = SingletonTwo.instance;
SingletonThree three = SingletonThree.instance;