为类型转换定义类的层次

<aside> 🌀 如下代码，library 中的元素分别为 Movie 和 Song 类型，但是当你迭代访问这些元素时，会被推断为 MediaItem 类型，即它们的父类。当你需要用这些元素自身的类型来访问它们时，就需要进行类型转换。

</aside>

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")
]
// 数组 library 的类型被推断为 [MediaItem]

类型检查

<aside> 🌀 使用 is 判断一个实例对象的类型是否是某个子类。

</aside>

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")
// 打印“Media library contains 2 movies and 3 songs”

向下转换类型

<aside> 🌀 as! 与 as?，一个会强制解包，另一个会返回一个可选值。

</aside>

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 的类型转换

<aside> 🌀 Any 可以表示任何类型，包括函数类型。 AnyObject 可以表示任何类类型的实例。

</aside>

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

<aside> 🌀 Any 类型可以表示所有类型的值，包括可选类型。Swift 在你用 Any 类型来表示一个可选值时，会产生一个警告。如果确实想要用 Any 类型来承载可选值，可以用 as 操作符将其显示转换为 Any：

</aside>

let optionalNumber: Int? = 3
things.append(optionalNumber)        // 警告
things.append(optionalNumber as Any) // 没有警告