Swift4 类型转换,例子代码
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类型转换
苹果官方指南 Type Casting
苹果官方指南翻译 类型转换
类型转换可判断实例类型,也可将实例在雷层次中视为父类或子类的实例。
操作符为 is 和 as 。
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")
]
// "library" 的类型被推断为[MediaItem]
类型检查
类型检查操作符 is 来检查一个实例是否属于一个特定的子类。是返回 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")
// Prints "Media library contains 2 movies and 3 songs"
向下类型转换
类型转换操作符 as? 或 as! 将它向下类型转换至其子类类型或父类类型。
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
Any 和 AnyObject 的类型转换
Swift 为不确定的类型提供了两种特殊的类型别名:
- AnyObject 可以表示任何类类型的实例。
- 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)" })
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
let optionalNumber: Int? = 3
things.append(optionalNumber) // Warning
things.append(optionalNumber as Any) // No warning
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