1. as? ，返回了一個(gè)你將要向下類型轉(zhuǎn)換的值的可選項(xiàng)。
2. as! ，則將向下類型轉(zhuǎn)換和強(qiáng)制展開結(jié)合為一個(gè)步驟
3. is: 來(lái)檢查一個(gè)實(shí)例是否屬于一個(gè)特定的子類乱投，如果實(shí)例是該子類類型，類型檢查操作符返回 true 顷编，否則返回 false 戚炫。

父子轉(zhuǎn)化

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")
]
// "library" 的類型被推斷為[MediaItem]

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"

向下類型轉(zhuǎn)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)")
    }
}
 
// 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 和 AnyObject 的類型轉(zhuǎn)換

• AnyObject 可以表示任何類類型的實(shí)例。
• Any 可以表示任何類型媳纬，包括函數(shù)類型双肤。

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

let optionalNumber: Int? = 3
things.append(optionalNumber)        // Warning
things.append(optionalNumber as Any) // No warning

let optionalNumber: Int? = 3
things.append(optionalNumber)        // Warning
things.append(optionalNumber as Any) // No warning