How do you build a Singleton in Dart?

Here is a comparison of several different ways to create a singleton in Dart.

1. Factory constructor

class SingletonOne {

  SingletonOne._privateConstructor();

  static final SingletonOne _instance = SingletonOne._privateConstructor();

  factory SingletonOne() {
    return _instance;
  }

}

2. 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();
  
}

How to instantiate

The above singletons are instantiated like this:

SingletonOne one = SingletonOne();
SingletonTwo two = SingletonTwo.instance;
SingletonThree three = SingletonThree.instance;

Note:

I originally asked this as a question, but discovered that all of the methods above are valid and the choice largely depends on personal preference.

Thanks to Dart's factory constructors, it's easy to build a singleton:

class Singleton {
  static final Singleton _singleton = Singleton._internal();

  factory Singleton() {
    return _singleton;
  }

  Singleton._internal();
}

You can construct it like this

main() {
  var s1 = Singleton();
  var s2 = Singleton();
  print(identical(s1, s2));  // true
  print(s1 == s2);           // true
}