//类型转换
import UIKit
/*类型转换（Type Casting）
1.类型转换 可以判断实例的类型，也可以将实例看做是其父类或者子类的实例。
2.类型转换使用 is 和 as 操作符实现。用这两个操作符来检查值的类型或者转换它的类型，也可以用它来检查一个类型是否实现了某个协议
检查类型（Checking Type）：用类型检查操作符"is"来检查一个实例是否属于特定类型,若实例属于那个类型，类型检查操作符返回 true，否则返回 false。
向下转型（Downcasting）：
1.某类型的一个常量或变量可能在幕后实际上属于一个子类。当确定是这种情况时，你可以尝试向下转到它的子类型，用类型转换操作符（as? 或 as!）
2.因为向下转型可能会失败，类型转型操作符带有两种不同形式。条件形式（conditional form）as? 返回一个你试图向下转成的类型的可选值（optional value）。强制形式 as! 把试图向下转型和强制解包（force-unwraps）转换结果结合为一个操作。
3.当你不确定向下转型可以成功时，用类型转换的条件形式（as?），转型成功返回一个可选值，失败则返回nil
4.只有你可以确定向下转型一定会成功时，才使用强制形式（as!），转型为一个不正确的类型时，则触发运行时错误
Any 和 AnyObject 叫做不确定类型，
1.AnyObject可以表示任何类class类型的实例，可以用is和as来检查和转换它为更明确的类型
2.Any可以表示任何类型，包括函数类型，可以用is和as来检查和转换它为更明确的类型
*/
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)
}
}
//类型检测器能够推断出 Movie 和 Song 有共同的父类 MediaItem，所以它推断出library 的类型为 [MediaItem]
//在幕后 library 里存储的媒体项依然是 Movie 和 Song 类型的
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")
]
var movieCount = 0
var songCount = 0
//=========== is 类型检查 ============
for item in library {
if item is Movie {
++movieCount
} else if item is Song {
++songCount
}
}
print("Media library contains \(movieCount) movies and \(songCount) songs")
// 打印 “Media library contains 2 movies and 3 songs”
//=============== as 类型转换（意思为：把它当作某类型来用） ========
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)")
}
}
//===========AnyObject不确定的类类型==========
let someObjects: [AnyObject] = [
Movie(name: "2001: A Space Odyssey", director: "Stanley Kubrick"),
Movie(name: "Moon", director: "Duncan Jones"),
Movie(name: "Alien", director: "Ridley Scott")
]
for movie in someObjects as! [Movie] {
print("Movie: '\(movie.name)', dir. \(movie.director)")
}
//===========Any不确定类型，可为所有类型========
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")
}
}