- 使用场景：
- 判断实例的类型
- 转换实例的类型
- 检查类型：
is - 转换类型：
as - 检查是否遵循某个协议
为类型转换定义类层次
- 在类和子类的层次结构上，检查特定类实例的类型并且转换这个类实例的类型成为这个层次结构中的其他类型。
- 基类
MediaItem。这个类为任何出现在数字媒体库的媒体项提供基础功能
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)
}
}
- Swift 的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[MediaItem]类作为library的类型：
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")
]
// 数组 library 的类型被推断为 [MediaItem]
-
library里存储的媒体项依然是Movie和Song类型的 - 遍历取出的实例会是
MediaItem类型的，而不是Movie和Song类型
检查类型
-
操作符（
is）：检查一个实例是否属于特定子类型 - 属于子类型，返回
true，否则返回false
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”
向下转(子类)型
用类型转换操作符（
as?或as!）向下转到它的子类型条件形式
as?返回向下转型的可选值，转型失败，返回 nil强制形式
as!，在as?继承上进行强制解包，失败触发运行时错误。
- 事前你不知道每个
item的真实类型，所以这里使用条件形式的类型转换（as?）去检查
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
注意
转换没有真的改变实例或它的值。根本的实例保持不变；只是简单地把它作为它被转换成的类型来使用。
Any 和 AnyObject 的类型转换
-
使用场景：为不确定类型提供了两种特殊的类型别名
- 如新建一个存放数据的数组；
- 网络获取的不确定类型的数据
Any可以表示任何类型，包括函数类型AnyObject表示任何类（class）类型的实例用
Any类型来和混合的不同类型一起工作
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)" })
- 可在
switch表达式的case中，用is和as找出Any或AnyObject类型的常量或变量的具体类型
case is Double:
print("some other double value that I don't want to print")
case let someString as String:
- 确定 any 类型数组元素的真实类型
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
// a movie called Ghostbusters, dir. Ivan Reitman
// Hello, Michael
注意
Any类型可表示所有类型的值，包括可选类型。Swift 会在你用
Any类型来表示一个可选值的时候，给你一个警告。如果你确实想使用
Any类型来承载可选值，你可以使用as操作符显式转换为Any，如下所示：
let optionalNumber: Int? = 3
things.append(optionalNumber) // 警告
things.append(optionalNumber as Any) // 没有警告
