swift2 类型转换

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 = [    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]

检查类型

var movieCount = 0var songCount = 0for item in library {    if item is Movie {        ++movieCount    } else if item is Song {        ++songCount    }}print("Media library contains \(movieCount) movies and \(songCount) songs")// prints "Media library contains 2 movies and 3 songs"

向下转型

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)")    }}// Movie: 'Casablanca', dir. Michael Curtiz// Song: 'Blue Suede Shoes', by Elvis Presley// Movie: 'Citizen Kane', dir. Orson Welles// Song: 'The One And Only', by Chesney Hawkes// Song: 'Never Gonna Give You Up', by Rick Astley

Any与AnyObject类型转换

let someObjects: [AnyObject] = [    Movie(name: "2001: A Space Odyssey", director: "Stanley Kubrick"),    Movie(name: "Moon", director: "Duncan Jones"),    Movie(name: "Alien", director: "Ridley Scott")]for object in someObjects {    let movie = object as! Movie    print("Movie: '\(movie.name)', dir. \(movie.director)")}// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick// Movie: 'Moon', dir. Duncan Jones// Movie: 'Alien', dir. Ridley Scott

for movie in someObjects as! [Movie] {    print("Movie: '\(movie.name)', dir. \(movie.director)")}// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick// Movie: 'Moon', dir. Duncan Jones// Movie: 'Alien', dir. Ridley Scott

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")    }}// 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