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十七、类型绑定 Type Casting
1. 概述
Type Casting 的作用：
- 1）检查某个实例的类型。使用 is 关键字。
- 2）将某个实例的类型绑定为其他类型来访问。使用 as 关键字。
- 3）检查某种类型是否实现了协议的方法。
2. 引例，定义三个类：
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) } }
定义一个数组：
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]
2.1 检查实例的类型 Checking Type
使用类型检查操作符（type check operator） is 判断一个实例是否是子类类型。如果是子类类型返回 true，否则返回 false。
遍历上面的数组：
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"
2.2 向下绑定 Downcasting
一个类实例有时要当做它的子类实例来使用，使用类型绑定操作符（type cast operator） as 来向下绑定到子类类型。
如果你不确定转换是否会成功，使用可选值形式 as? ，如果失败，返回 nil 。
for item in library { 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)") } } // 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
注意：类型绑定并没有真的修改实例，仅仅只是把它当做它所绑定的类型来对待和访问。
3. 绑定为Any和AnyObject类型 Type Casting for Any and AnyObject
Swift为一些不明确的类型提供了两个特殊的类型别名：
- 1）AnyObject 可以表示任何类类型的实例（对象类型）。
- 2）Any 可以表示任何类型，包括函数类型。
3.1 AnyObject 类型
比如：
let someObjects: [AnyObject] = [ Movie(name: "2001: A Space Odyssey", director: "Stanley Kubrick"), Movie(name: "Moon", director: "Duncan Jones"), Movie(name: "Alien", director: "Ridley Scott") ]
因为数组中都是Movie的实例类型（对象类型），我们可以将他们向下绑定直接拆包为非可选的Movie类型
for object in someObjects { let movie = object as Movie println("Movie: ‘\(movie.name)‘, dir. \(movie.director)") } // Movie: ‘2001: A Space Odyssey‘, dir. Stanley Kubrick // Movie: ‘Moon‘, dir. Duncan Jones // Movie: ‘Alien‘, dir. Ridley Scott
也可以使用更简洁的方式——将数组 someObjects的[AnyObject]类型 向下绑定为 [Movie]类型：
for movie in someObjects as [Movie] { println("Movie: ‘\(movie.name)‘, dir. \(movie.director)") } // Movie: ‘2001: A Space Odyssey‘, dir. Stanley Kubrick // Movie: ‘Moon‘, dir. Duncan Jones // Movie: ‘Alien‘, dir. Ridley Scott
3.2 Any 类型
比如：
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)" })
遍历数组：
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)") case let stringConverter as String -> String: println(stringConverter("Michael")) 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 // Hello, Michael
注意：在上面的代码中使用的是强制绑定as而不是可选的绑定as?，因为在在switch的case语句中使用强制的as永远是安全的。
参考自：https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TypeCasting.html#//apple_ref/doc/uid/TP40014097-CH22-ID338
十七、类型绑定 Type Casting
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