Swift 4:向下转换回调函数问题

class Animal {
    func makeSound() {
        print("Animal makes sound")
    }
}

class Dog: Animal {
    override func makeSound() {
        print("Dog barks")
    }
}

class Cat: Animal {
    override func makeSound() {
        print("Cat meows")
    }
}

func callback(animal: Animal) {
    if let dog = animal as? Dog {
        dog.makeSound()
    } else if let cat = animal as? Cat {
        cat.makeSound()
    }
}

let animal = Animal()
let dog = Dog()
let cat = Cat()

callback(animal: animal)  // Output: Animal makes sound
callback(animal: dog)     // Output: Dog barks
callback(animal: cat)     // Output: Cat meows