enum AnimalType {
    case dog
    case cat
}

protocol Animal: AnyObject {
    var description: String { get }
}

class Dog: Animal {
    var description: String { return "Dog" }
}

class Cat: Animal {
    var description: String { return "Cat" }
}

class Foo {
    weak var animal: Animal?
}

func f1(animalType: AnimalType) {
    let animal: Animal
    switch animalType {
    case .dog:
        animal = Dog()
    case .cat:
        animal = Cat()
    }
    
    let foo = Foo()
    foo.animal = animal
  
    if let a = foo.animal {
        print(a.description) // Debug
    } else {
        print("animal has been released") // Release
    }
}

f1(animalType: .dog)