override func viewDidLoad() {
        super.viewDidLoad()
        
        //型別轉換運算子: is, as
        //可以用型別轉換檢查例項型別, 或者一個型別是否服從協議
        
        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)
            }
        }
        //陣列中有 Movie Class 型別,也有 Song Class 型別, 都繼承自MediaItem, 編譯器推斷為 MediaItem型別
        let library = [
            Movie(name: "Casablanca", director: "Michael Curtiz"),
            Song(name: "Blue Suede Shoes", artist: "Elvis Presley"),
            Movie(name: "Citizen Kane", director: "Orson Welles"),
            Song(name: "The One And Only", artist: "Chesney Hawkes"),
            Song(name: "Never Gonna Give You Up", artist: "Rick Astley")
        ]
        // the type of "library" is inferred to be [MediaItem]
        
        //1. 檢查型別
        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")
        // Prints "Media library contains 2 movies and 3 songs"
        
        //2. 向下型別轉換(Downcasting)
        //一個 Class 型別的常量或變數, 可能指向子類的例項. 這種情況下可以使用 as? 或 as! 進行向下型別轉換
        for item in library {
            if let movie = item as? Movie {
                print("Movie: \(movie.name), dir. \(movie.director)")
            } else if let song = item as? Song {
                print("Song: \(song.name), by \(song.artist)")
            }
        }

        //3. Any 和 AnyObject 的型別轉換
        //Any 可以代表任何型別的例項, 包括函式型別
        //AnyObject 可以代表任何 Class 型別的例項
        //能確定型別的時候,要儘量寫具體型別
        
        var things = [Any]()
        
        things.append(0)
        things.append(0.0)
        things.append(42)
        things.append(3.14159)
        things.append("hello")
        things.append((3.0, 5.0))
        things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman"))
        things.append({ (name: String) -> String in "Hello, \(name)" })
        
        for thing in things {
            switch thing {
            case 0 as Int:
                print("zero as an Int")
            case 0 as Double:
                print("zero as a Double")
            case let someInt as Int:
                print("an integer value of \(someInt)")
            case let someDouble as Double where someDouble > 0:
                print("a positive double value of \(someDouble)")
            case is Double:
                print("some other double value that I don't want to print")
            case let someString as String:
                print("a string value of \"\(someString)\"")
            case let (x, y) as (Double, Double):
                print("an (x, y) point at \(x), \(y)")
            case let movie as Movie:
                print("a movie called \(movie.name), dir. \(movie.director)")
            case let stringConverter as (String) -> String:
                print(stringConverter("Michael"))
            default:
                print("something else")
            }
            
            let optionalNumber: Int? = 3
            things.append(optionalNumber)        // Warning
            things.append(optionalNumber as Any) // No warning
        }
        
        // zero as an Int
        // zero as a Double
        // an integer value of 42
        // a positive double value of 3.14159
        // a string value of "hello"
        // an (x, y) point at 3.0, 5.0
        // a movie called Ghostbusters, dir. Ivan Reitman
        // Hello, Michael
    }