类型检查在@H_502_2@ Swift @H_502_2@中使用@H_502_2@is@H_502_2@和@H_502_2@ as@H_502_2@操作符实现。这两个操作符提供了一种简单达意的方式去检查值的类型或者转换它的类型。@H_502_2@

定义一个类层次作为例子@H_502_2@

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]

library@H_502_2@的储存的是@H_502_2@Movie@H_502_2@和@H_502_2@Song@H_502_2@的实例@H_502_2@,@H_502_2@若你迭代它@H_502_2@,@H_502_2@则取出的会是其父类@H_502_2@MediaItem@H_502_2@类型@H_502_2@,@H_502_2@而不是@H_502_2@Movie@H_502_2@和@H_502_2@Song@H_502_2@类型。为了让它们作为它们本来的类型工作，你需要检查它们的类型或者向下转换它们的类型到其它类型。@H_502_2@

通过操作符@H_502_2@is@H_502_2@来检查一个实例是否属于特定的子类型@H_502_2@,@H_502_2@返回@H_502_2@true@H_502_2@则属于@H_502_2@

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

向下转型@H_502_2@

通过类型检查操作符@H_502_2@as@H_502_2@可向下转型@H_502_2@.@H_502_2@但是由于向下转型可能会失败@H_502_2@

故有两种形式①可选形式@H_502_2@as? @H_502_2@②强制形式@H_502_2@as!@H_502_2@

当你确定转型一定会成功的时候使用@H_502_2@as!@H_502_2@@H_502_2@

for item in library {
    //由于item是一个MediaItem类型的实例,它可能是一个Movie,故使用as?
    if let movie = item as? Movie {
        println("Movie: '\(movie.name)',dir. \(movie.director)")
    } else if let song = item as? Song {
        println("Song: '\(song.name)',by \(song.artist)")
    }
}

Any@H_502_2@和@H_502_2@AnyObject@H_502_2@类型检查@H_502_2@

Swift@H_502_2@为不确定类型提供了两种特殊类型别名：@H_502_2@

①@H_502_2@AnyObject@H_502_2@可以代表任何@H_502_2@class@H_502_2@类型的实例。@H_502_2@

②@H_502_2@Any@H_502_2@可以表示任何类型，除了方法类型（@H_502_2@function types@H_502_2@）。@H_502_2@

注意：@H_502_2@只有当你明确的需要它的行为和功能时才使用@H_502_2@Any@H_502_2@和@H_502_2@AnyObject@H_502_2@。在你的代码里使用你期望的明确的类型总是更好的。@H_502_2@

AnyObject@H_502_2@类型@H_502_2@

let someObjects: [AnyObject] = [
    Movie(name: "2001: A Space Odyssey",director: "Stanley Kubrick"),Movie(name: "Moon",director: "Duncan Jones"),Movie(name: "Alien",director: "Ridley Scott")
]

由于这个数组中只包含@H_502_2@Movie@H_502_2@实例@H_502_2@,@H_502_2@所以你可以直接使用@H_502_2@as!@H_502_2@来进行强制解包@H_502_2@

for object in someObjects {
    let movie = object as! Movie
}

或者直接将@H_502_2@[AnyObject]@H_502_2@强转成@H_502_2@[Movie]@H_502_2@

for movie in someObjects as! [Movie] {
    println("Movie: '\(movie.name)',dir. \(movie.director)")
}

Any@H_502_2@类型@H_502_2@

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@H_502_2@中包含@H_502_2@2@H_502_2@个@H_502_2@Int,2@H_502_2@个@H_502_2@Double,1@H_502_2@个@H_502_2@String,1@H_502_2@个元组@H_502_2@ (Double,Double),@H_502_2@一个@H_502_2@Movie@H_502_2@对象@H_502_2@

for thing in things {
    switch thing {
    case 0 as Int:
        println("zero as an Int")
    case 0 as Double:
        println("zero as a Double")
    case let someInt as Int:
        println("an integer value of \(someInt)")
    case let someDouble as Double where someDouble > 0:
        println("a positive double value of \(someDouble)")
    case is Double:
        println("some other double value that I don't want to print")
    case let someString as String:
        println("a string value of \"\(someString)\"")
    case let (x,y) as (Double,Double):
        println("an (x,y) point at \(x),\(y)")
    case let movie as Movie:
        println("a movie called '\(movie.name)',dir. \(movie.director)")
    default:
        println("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

类型嵌套@H_502_2@

一个类型中嵌套另一个类型，将需要嵌套的类型的定义写在被嵌套类型的区域@H_502_2@{}@H_502_2@内，而且可以根据需要定义多级嵌套@H_502_2@

struct BlackjackCard {
    // 嵌套定义枚举型Suit
    enum Suit: Character {
        case Spades = "♠",Hearts = "♡",Diamonds = "♢",Clubs = "♣"
    }
    // 嵌套定义枚举型Rank
    enum Rank: Int {
        case Two = 2,Three,Four,Five,Six,Seven,Eight,Nine,Ten
        case Jack,Queen,King,Ace
        struct Values {
            let first: Int,second: Int?
        }
        var values: Values {
            switch self {
            case .Ace:
                return Values(first: 1,second: 11)
            case .Jack,.Queen,.King:
                return Values(first: 10,second: nil)
            default:
                return Values(first: self.toRaw(),second: nil)
            }
        }
    }
    // BlackjackCard 的属性和方法
    let rank: Rank,suit: Suit
    var description: String {
        var output = "suit is \(suit.toRaw()),"
        output += " value is \(rank.values.first)"
        if let second = rank.values.second {
            output += " or \(second)"
        }
        return output
    }
}

let theAceOfSpades = BlackjackCard(rank: .Ace,suit: .Spades)
println("theAceOfSpades: \(theAceOfSpades.description)")
// 打印出 "theAceOfSpades: suit is ♠,value is 1 or 11"

let heartsSymbol = BlackjackCard.Suit.Hearts.toRaw()
// 红心的符号 为 "♡"