Visitor Pattern

main.dart

//#region Zoo
class Zoo {
  final List<Dog> dogs;
  final List<Cat> cats;
  final List<Bear> bears;
  
  const Zoo({
    this.dogs, 
    this.cats, 
    this.bears,
  });
  
  int get totalAnimalInZoo => dogs.length + cats.length + bears.length;
  
  Iterator<Animal> makeIteratorBySpecies() {
    List<Animal> animals = [];
    animals.addAll(bears);
    animals.addAll(cats);
    animals.addAll(dogs);
    return animals.iterator;
  }
  
  Iterator<Animal> makeIteratorByName() {
    List<Animal> animals = [];
    animals.addAll(bears);
    animals.addAll(cats);
    animals.addAll(dogs);
    animals.sort((firstAnimal, secondAnimal) {
      return firstAnimal.name.compareTo(secondAnimal.name);
    });
    return animals.iterator;
  }
}
//#endregion

//#region Composite
abstract class Animal {
  String get name;
  int get weight;
  DateTime get birthday;
  
  void accept(Visitor visitor);
}


class Cat implements Animal {
  final String name;
  final int weight;
  final DateTime birthday;
  
  const Cat({this.name, this.weight, this.birthday});
  
  void accept(Visitor visitor) {
    visitor.visitCat(this);
  }
}

class Dog implements Animal {
  final String name;
  final int weight;
  final DateTime birthday;
  
  const Dog({this.name, this.weight, this.birthday});
  
  void accept(Visitor visitor) {
    visitor.visitDog(this);
  }
}

class Bear implements Animal {
  final String name;
  final int weight;
  final DateTime birthday;
  
  const Bear({this.name, this.weight, this.birthday});
  
  void accept(Visitor visitor) {
    visitor.visitBear(this);
  }
}
//#endregion

//#region Visitor
abstract class Visitor {
  void visitCat(Cat cat);
  void visitDog(Dog dog);
  void visitBear(Bear bear);
}

class HealthCheckVisitor implements Visitor {
  void visitCat(Cat cat) {
    if (cat.weight < 2) {
      print("  Cat ${cat.name} is underweight.");
    } else if (cat.weight < 10) {
      print("  Cat ${cat.name} is healthy.");
    } else {
      print("  Cat ${cat.name} is obese.");
    }
  }
  void visitDog(Dog dog) {
    if (dog.weight < 2) {
      print("  Dog ${dog.name} is underweight.");
    } else if (dog.weight < 10) {
      print("  Dog ${dog.name} is healthy.");
    } else {
      print("  Dog ${dog.name} is obese.");
    }
  }
  void visitBear(Bear bear) {
    if (bear.weight < 20) {
      print("  Bear ${bear.name} is underweight.");
    } else if (bear.weight < 100) {
      print("  Bear ${bear.name} is healthy.");
    } else {
      print("  Bear ${bear.name} is obese.");
    }
  }
}

class HappyBirthdayVisitor implements Visitor {
  void checkBirthday(Animal animal) {
    if (animal.birthday.day == DateTime.now().day &&
       animal.birthday.month == DateTime.now().month) {
      print("");
      print("  Happy birthday ${animal.name}...");
      print("  Happy birthday ${animal.name}...");
      print("  Happy birthday dear ${animal.name}...");
      print("  Happy birthday to you...");
      print("");
    } else {
      print("  It's not ${animal.name}'s birthday");
    }
  }
  void visitCat(Cat cat) {
    checkBirthday(cat);
  }
  void visitDog(Dog dog) {
    checkBirthday(dog);
  }
  void visitBear(Bear bear) {
    checkBirthday(bear);
  }
}
//#endregion

void walkAround(Iterator<Animal> iterator, Visitor visitor) {
  while (iterator.moveNext()) {
    iterator.current.accept(visitor);
  }
}

void main() {
  Zoo zoo = Zoo(
    cats: [
      Cat(name: "Kibum", weight: 15, birthday: DateTime(2005, 1, 15)),
    ],
    dogs: [
      Dog(name: "Apple", weight: 5, birthday: DateTime(2019, 11, 11)),
      Dog(name: "Dalmatian", weight: 12, birthday: DateTime(1995, 5, 4)),
    ],
    bears: [
      Bear(name: "Teddy", weight: 10, birthday: DateTime(2018, 1, 30)),
      Bear(name: "Lotso", weight: 75, birthday: DateTime(1992, 5, 14)),
    ],
  );
  
  print("Performing health check");
  print("-----------------------");
  final healthCheckVisitor = HealthCheckVisitor();
  walkAround(zoo.makeIteratorByName(), healthCheckVisitor);
  print("");
  
  print("Performing birthday check");
  print("-----------------------");
  final happyBirthdayVisitor = HappyBirthdayVisitor();
  walkAround(zoo.makeIteratorByName(), happyBirthdayVisitor);
  print("");
}