swift2.0学习之类型转换

（一）类型转换（type casting）

类型转换是检查类型实例的方式，也是把类型实例作为子类或者父类的方式

主要有三个关键词，is，as?，as!(as)，is表示是某种类型(比如父类和子类同时出现，选择父类，就是表示层级比较大的那个类型)，as表示的是向下转换（downcast）到子类本身，问号表示可选，叹号表示强拆包

举个例子：

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)
}
}
<pre name="code" class="plain">        for item in library {

if let movie = item as? Movie {
print("name:\(movie.name), director:\(movie.director)")
}else if let song = item as? Song {
print("name:\(song.name), artist:\(song.artist)")
}
}

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

let library = [Movie(name: "张学友", director: "麦兆辉"),Song(name: "一路上有你", artist: "张学友"),Movie(name: "周星驰", director: "周星驰"),Song(name: "夕阳醉了", artist: "张学友"),Movie(name: "赌神", director: "王晶")]
for item in library {

if item is Movie {
movieCount++
}else if item is Song {
songCount++
}

}

print("Media library contains \(movieCount) movies and \(songCount) songs")

解释：

如果当前的MediaItem（item）是Movie的实例的话，就返回true，否则就返回false

而上面的item实际上是MediaItem类型，而不是真正的Movie类型，也就说你访问不到Movie的director的属性，那我们怎么才能让item实例真正是Movie本身的类型呢？这时我们就用到关键字as了，进行向下拆包

as有两种形式，一个是as?，表示返回的类型可选，一个是as!强制拆包，用这个的时候你要确保你解析的东西是正确有值的，可以解包成功的

for item in library {

if let movie = item as? Movie {
print("name:\(movie.name), director:\(movie.director)")
}else if let song = item as? Song {
print("name:\(song.name), artist:\(song.artist)")
}
}

因为item是MediaItem的实例，向下可能是Movie的实例，所以用as？可选，再用一个可选绑定，如果成功，就可以访问director属性了

注意：转换实际上没有改变这个实例或者它的值，潜在的实例还是保持原来的值，它仅仅是被转换的类型的实例

（二）Any和AnyObject

AnyObject：代表任意类类型（class type）的实例

Any：可以代表任意类型的实例，除了函数类型

a.AnyObject

let someObjects: [AnyObject] = [Movie(name: "大圣归来", director: "未知"), Movie(name: "速度与激情7", director: "温世仁"), Movie(name: "无间道", director: "刘伟强")]

for item in someObjects {

let movie = item as! Movie
print("name: \(movie.name),dir.\(movie.director)")
}

这里你已经确定item就是Movie的实例类型，直接强拆，用一个常量接收

还有一个办法，就是既然你已经确定someObjects是一个[Movie]类型的数组，那么就可以这么写:

for movie in someObjects as! [Movie] {

print("name: \(movie.name),dir.\(movie.director)")
}

b.Any

既然Any除了函数类型都能代表，那么我们就定义一个可变数组，Any型的，装它几种类型

var things = [Any]()
things.append(0)
things.append(0.0)
things.append(42)
things.append(3.14159)
things.append("hell0")
things.append((3.0, 5.0))
things.append(Movie(name: "恐怖游轮", director: "未知"))
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 double value of \(someDouble)")
case is Double:
print("some other double 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 is 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("xiaoyu"))
default:
print("something else")

}
}