- 類型轉(zhuǎn)換 可以判斷實例的類型脖镀。
- 類型轉(zhuǎn)換在 Swift 中使用 is和 as操作符實現(xiàn)饲做。
- 類型轉(zhuǎn)換可以檢查一個類型是否實現(xiàn)了某個協(xié)議线婚。
定義類型轉(zhuǎn)換的類層次（Defining a Class Hierarchy for Type Casting）
- 可以將類型轉(zhuǎn)換用在類和子類的層次結(jié)構(gòu)上，檢查特定類實例的類型并且轉(zhuǎn)換這個類實例的類型成為這個層次結(jié)構(gòu)中的其他類型盆均。
檢查類型（Checking Type）
- 用類型檢查操作符（is）來檢查一個實例是否屬于特定子類型塞弊。若實例屬于那個子類型，類型檢查操作符返回 true泪姨，否則返回 false游沿。
向下轉(zhuǎn)型（Downcasting）
- 某類型的一個常量或變量可能在幕后實際上屬于一個子類。當(dāng)確定是這種情況時肮砾，你可以嘗試向下轉(zhuǎn)到它的子類型诀黍，用類型轉(zhuǎn)換操作符（as?或 as!）。
多用as仗处？眯勾，少用或者不用as！疆柔；
- 轉(zhuǎn)換沒有真的改變實例或它的值咒精。根本的實例保持不變镶柱；只是簡單地把它作為它被轉(zhuǎn)換成的類型來使用旷档。
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")
]
// 數(shù)組 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")
// 打印 “Media library contains 2 movies and 3 songs”
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（Type Casting for Any and AnyObject）
- AnyObject可以表示任何類類型的實例。
- Any可以表示任何類型歇拆，包括函數(shù)類型鞋屈。
AnyObject
- 相當(dāng)于NSObject，是Cocoa API的基類故觅，Object-C中用id的地方可以用AnyObject來替換厂庇，僅僅適用于類，不能用于結(jié)構(gòu)體输吏，枚舉等值類型
Any
- 混合的不同類型一起工作权旷，包括函數(shù)類型和非類類型。
- 將類贯溅，結(jié)構(gòu)體拄氯，枚舉躲查，基礎(chǔ)類型等混合在一起，裝到一個數(shù)組中
不要這樣用译柏，Swift是類型安全的镣煮。下面的例子僅僅說明語法，不推薦在實際中使用鄙麦。
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)) // 這句編譯不過典唇，原因不明
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 // 編譯不過，不會輸出
// a movie called 'Ghostbusters', dir. Ivan Reitman
// Hello, Michael
