class MediaItem {
    var name: String
    init(name: String) {
        self.name = name
    }
}

class Movie: MediaItem {
    var director: String
    init(name: String, director: String) {
        self.director = director
        super.init(name: name)
    }
}

class Song: MediaItem {
    var artist: String
    init(name: String, artist: String) {
        self.artist = artist
        super.init(name: name)
    }
}

let library = [ // library는 타입 추론에 의해 MediaItem 타입의 배열이다.
    Movie(name: "기생충", director: "봉준호")
    Song(name: "Lemon Tree", artist: "Post Malone")
    Movie(name: "올드보이", director: "박찬욱")
    Song(name: "Ballin", artist: "Parisalexa")
    Song(name: "Peter Pan Was Right", artist: "Anson Seabra")
]

var movieCount = 0
var songCount = 0

for item in library {
    if item is Movie {
        movieCount += 1
    } else if item is Song {
        songCount += 1
    }
}

print("Media library contains \(movieCount) movies and \(songCount) songs")

for item in library {   // library 배열의 모든 항목
    if let movie = item as? Movie { // as?는 옵셔널 값을 반환한다.
        print("Movie: \(movie.name), dir. \(movie.director)")
    } else if let song = item as? Song {
        print("Song: \(song.name), by \(song.artist)")
    }
}