main() {
  print(new Dog.with(breed: "Golden Retriever"));
  print(new Dog.with(breed: "Labrador Retriever"));
  print(new Dog.with(breed: "American Pit Bull Terrier"));
  print(new Dog.with(breed: "Australian Shepherd"));

  // Ok, so try registering the type
  OpenMindedDogFactory.register(
    when:       (breed) => breed == "American Pit Bull Terrier",
    thenReturn: () => new AmericanPitBullTerrier());

  print("Ok, now we registered a new Dog type ... trying again");

  print(new Dog.with(breed: "Golden Retriever"));
  print(new Dog.with(breed: "Labrador Retriever"));
  print(new Dog.with(breed: "American Pit Bull Terrier"));
  print(new Dog.with(breed: "Australian Shepherd"));
}

interface Dog factory OpenMindedDogFactory {
  Dog.with([String breed]);
}

// Simple interface factory.
// Switches on something simple to determine the implementing class to use.
class CloseMindedDogFactory {
  factory Dog.with([String breed = null]) {
    switch (breed) {
      case "Golden Retriever":   return new GoldenRetriever();
      case "Labrador Retriever": return new LabradorRetriever();
      default:                   return new RegularDog();
    }
  }
}

typedef bool DogChooser(String breed);
typedef Dog DogFactory();

// Interface factory that stores a list of functions that can be registered so
// they will be called whenever someone calls the interface's constructor.
//
// This allows other users to register their own callbacks with your interface
// so they can return their own concrete types from your interface.
class OpenMindedDogFactory {
  static List<DogChooser> _dogChoosers;
  static List<DogFactory> _dogFactories;

  static void register([DogChooser when = null, DogFactory thenReturn = null]) {
    if (_dogChoosers == null) {
      _dogChoosers  = new List<DogChooser>();
      _dogFactories = new List<DogFactory>();
    }
    _dogChoosers.add(when);
    _dogFactories.add(thenReturn);
  }

  factory Dog.with([String breed = null]) {
    if (_dogChoosers != null) {
      for (int i = 0; i < _dogChoosers.length; i++) {
        if (_dogChoosers[i](breed))
          return _dogFactories[i]();
      }
    }
    return new RegularDog();
  }
}

class RegularDog implements Dog {}
class GoldenRetriever implements Dog {}
class LabradorRetriever implements Dog {}
class AmericanPitBullTerrier implements Dog {}