override func viewDidLoad() { super.viewDidLoad() //类型转换运算符: is, as //可以用类型转换检查实例类型, 或者一个类型是否服从协议 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) } } //数组中有 Movie Class 类型,也有 Song Class 类型, 都继承自MediaItem, 编译器推断为 MediaItem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") ] // the type of "library" is inferred to be [MediaItem] //1. 检查类型 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" //2. 向下类型转换(Downcasting) //一个 Class 类型的常量或变量, 可能指向子类的实例. 这种情况下可以使用 as? 或 as! 进行向下类型转换 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)") } } //3. Any 和 AnyObject 的类型转换 //Any 可以代表任何类型的实例, 包括函数类型 //AnyObject 可以代表任何 Class 类型的实例 //能确定类型的时候,要尽量写具体类型 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") } let optionalNumber: Int? = 3 things.append(optionalNumber) // Warning things.append(optionalNumber as Any) // No warning } // 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 }
Swift- 类型转换
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转载自blog.csdn.net/leecsdn77/article/details/80741177
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