18.类型转换 Type Casting Swift官方文档——

//: Playground - noun: a place where people can play

import UIKit

// # 为类型转换定义类层次
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")
] // 注意:library的类型被推断为[MediaItem].而事实上,library存储的项目在后台依旧是各自的类型的实例.但是遍历这个数组取出的将会是MediaItem类型

// # 类型检查
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")
library[0] is MediaItem
// another example
var controller = UITableViewController()
controller is CustomDebugStringConvertible

// # 向下类型转换
// as 用于消除二义性
let num1 = 42 as CGFloat
let num2 = 42 as Int
let num3 = 42 as Double
class Animal {}
class Cat: Animal {}
let cat = Cat()
let animal = cat as Animal // as is used to make an instance become its father class instance--this will never fail.
let kitty = animal as! Cat // as! is used to downcast a father instance--this can fail.

// as & protocol
protocol Facial {
    var faceSize: Double { get set }
}
class Human: Facial {
    var faceSize: Double = 1
}
class Dog: Facial {
    var faceSize: Double = 0.5
}
let creatureOne: Facial = Human()
let humanOne = creatureOne as! Human
let creatureTwo = humanOne as Facial

// as?返回可选项,as!强制转换
// 情况1: 左操作数非可选,存疑在于其能不能进行转换
for item in library {
    if let movie = item as? Movie {
        print("Movie: '\(movie.name)', director \(movie.director)")
    } else if let song = item as? Song {
        print("Song: '\(song.name)', by \(song.artist)")
    }
}
// 情况2: 左操作数是可选,存疑在于存不存在以及存在时能不能转换
var itemMaybe: MediaItem? = Movie(name: "I Love You", director: "Berg")
let songMaybe = itemMaybe as? Song
let movieMaybe = itemMaybe as? Movie
let movieMaybeTwo = itemMaybe as! Movie

// 注意:也可以将类转化为基类,但是基类仍然保留了子类数据
let movie = Movie(name: "Casablanca", director: "Michael Curtiz")
let ss = movie as MediaItem
ss as? Movie

// # Any和AnyObject的类型转换
// AnyObject 可以表示任何类类型实例
// Any 可以表示任何类型,包括函数类型
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)" })
// trick: is 和 as 灵活使用
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")
    }
}
let optionalNumber: Int? = 3
things.append(optionalNumber)        // Warning
things.append(optionalNumber as Any) // No warning


// 返回不同的类型
func function() -> MediaItem {
    return Movie(name: "Whatever", director: "Kevin")
}
let some = function()
some.name
let dd = some as! Movie
dd.director