類型轉換

1. 類型判斷操做符
   1. ==，！=判斷倆個值類型是否相等
   2. ===，！==判斷兩個引用是不是一個內存地址
   3. is判斷某個對象是不是某個類型

class MediaItem {
    var name: String
    init(name: String) {
        self.name = name
    }
}

class Movie: MediaItem {
    var director: String
    init(director: String, name:String) {
        self.director = director
        super.init(name: name)
    }
}

class Song: MediaItem {
    var artist: String
    init(artist: String, name: String) {
        self.artist = artist
        super.init(name: name)
    }
}

//library被推斷出來存儲的類型是MediaItem類型，因此遍歷時候取出來的每個item類型默認都是MediaItem類型
let library = [
    Movie(director: "1", name: "1"),
    Song(artist: "2", name: "2"),
    Song(artist: "3", name: "3"),
    Movie(director: "4", name: "4"),
    Song(artist: "5", name: "5"),
]

var movieCount = 0, songCount = 0
for  item in library {
    //item是MediaItem類型
    if item is Movie {
        movieCount += 1
    }else if item is Song{
        songCount += 1
    }
}
print("moveCount = \(movieCount) songCount = \(songCount)")
複製代碼

2. 向下轉型
   1. 當不肯定轉型是否會是肯定的子類類型時候要用as？
   2. 當肯定轉型會是肯定的子類類型時候要用as!
   3. 向上轉型老是會成功，沒有意義的！沒有關聯的對象轉型老是會失敗，也是沒意義的。因此只是存在向下轉型。轉成更加肯定的類型。

class MediaItem {
    var name: String
    init(name: String) {
        self.name = name
    }
}

class Movie: MediaItem {
    var director: String
    init(director: String, name:String) {
        self.director = director
        super.init(name: name)
    }
}

class Song: MediaItem {
    var artist: String
    init(artist: String, name: String) {
        self.artist = artist
        super.init(name: name)
    }
}

//library被推斷出來存儲的類型是MediaItem類型，因此遍歷時候取出來的每個item類型默認都是MediaItem類型
let library = [
    Movie(director: "1", name: "1"),
    Song(artist: "2", name: "2"),
    Song(artist: "3", name: "3"),
    Movie(director: "4", name: "4"),
    Song(artist: "5", name: "5"),
]

var movieCount = 0
var songCount = 0
for item in library {
    //item默認是MediaItem類型
    if let _ = item as? Movie{
        movieCount += 1
    } else if let _ = item as? Song{
        songCount += 1
    }
}

print("movieCount = \(movieCount) songCount = \(songCount)")
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3. Any與AnyObject區別

   1. AnyObject是Any的子類
   2. AnyObject只是表示對象類型
   3. Any能夠表示全部任意類型，包括值，對象，可選，函數類型等。
   4. 遇見Any類型時候可能編譯器給出警報，這時候能夠用 as Any來消除警報.

4. as,as?,as!

   1. 若是涉及到向上轉型，值類型，case匹配對象，消除可選類型使用時候的警告用as。
   2. 若是涉及到向下轉型，可能會失敗用as？。
   3. 若是涉及到向下轉型，必定成功用as!。

class Animal {}
class Dog: Animal {}

/*** as使用場景 ***/
//1. as值類型
var a = 4
let b = a as Int

//2. as向上轉型
var c = Dog()
let d = c as Animal

//3. switch,case匹配對象
switch d {
case  _ as Dog :
    break
default:
    break
}

//4. 消除Any類型警報
var g: Int?
print(g as Any)

var h: Int? = 5
print(h!)
print(h as Any)

/*** as？使用場景 ***/
//1. 向下轉型可能會失敗
var e = d as? Dog

/*** as!使用場景 ***/
//1. 向下轉型確保必定成功
var f = d as! Dog
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5. 因爲使用了可選類型可能致使警報，這時候若是肯定可選類型必定有值。那麼能夠用！強制解包，也能夠用 a as Any來解決警報。 這時候若是多是缺省值那麼就只能用a as Any來解決警報。