Swift從入門到精通第十五篇 - 型別轉換 初識
阿新 • • 發佈：2019-09-21
型別轉換（學習筆記）
環境Xcode 11.0 beta4 swift 5.1
- 型別轉換
- 型別轉換是檢查例項型別的一種方法，或者檢查來自類層級不同的父類或子類一個例項，用
is和as操作符
- 型別轉換是檢查例項型別的一種方法，或者檢查來自類層級不同的父類或子類一個例項，用
- 為型別轉換定義層級結構
示例
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" 型別編譯器自動推導為： [MediaItem]
- 型別檢查
示例
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"
- 型別向下轉換
可以用
as?或as!實現型別轉換，型別轉換有可能失敗，as?返回一個可選型別，as!嘗試型別轉換且強制解包，如果型別轉換不不正確，會觸發執行時錯誤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
AnyAnyObject型別轉換Swift為非指定型別提供了兩種特殊型別Any可以代表例項型別，包括函式AnyObject可以代表任何類型別的例項
swift 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
- 注意
如果你想用可選值作為
Any值，那麼要用as操作符顯示的將型別轉換為Anylet optionalNumber: Int? = 3 things.append(optionalNumber) // Warning things.append(optionalNumber as Any) // No warning