Dart singletons

main.dart

class SingletonValueNoFactory {
  static SingletonValueNoFactory _instance;
  
  final dynamic value;
  
  SingletonValueNoFactory._internal(this.value);

  static SingletonValueNoFactory getInstance(value) {
    return _instance ??= SingletonValueNoFactory._internal(value);
  }
}


class SingletonValue {
  static SingletonValue _instance;
  
  final dynamic value;
  
  SingletonValue._internal(this.value);
  
  factory SingletonValue(value) {
    return _instance ??= SingletonValue._internal(value);
  }
}

void main() {
  print(SingletonValueNoFactory.getInstance("no factory").value);
  print(SingletonValueNoFactory.getInstance("no factory - changed").value);
  print(SingletonValueNoFactory._internal("no factory - breaking singleton").value);
  
  print(SingletonValue("factory").value);
  print(SingletonValue("factory - changed").value);
  print(SingletonValue._internal("factory - breaking singleton").value);
}