Dart Singletons

singleton_way_to_go.dart

// (!) This implementation seems to work;
// Trying this in DartPad results in "s = false" and "f = true" as expected.
// This was taken from https://gist.github.com/theburningmonk/6401183
class FooBar {
  static final FooBar _instance = new FooBar._internal();

  factory FooBar() => _instance;
  
  FooBar._internal() {
    // initialization logic here
  }
}

class SpamEggs {}

void main() {
  var s1 = SpamEggs();
  var s2 = SpamEggs();
  var f1 = FooBar();
  var f2 = FooBar();
  print('s = ${identical(s1, s2)}');
  print('f = ${identical(f1, f2)}');
}

singleton_wrong_way.dart

// (!) This singleton implementation doesn't seem to work;
// Trying this in DartPad results in "s = false" and "f = false" which is unexpected.
class FooBar {
  static final FooBar _instance = FooBar.getInstance();
  
  FooBar.getInstance();
  
  factory FooBar() => _instance;
}

class SpamEggs {}

void main() {
  var s1 = SpamEggs();
  var s2 = SpamEggs();
  var f1 = FooBar.getInstance();
  var f2 = FooBar.getInstance();
  print('s = ${identical(s1, s2)}');
  print('f = ${identical(f1, f2)}');
}