类型转换（学习笔记）

环境Xcode 11.0 beta4 swift 5.1

• 类型转换
  • 类型转换是检查实例类型的一种方法，或者检查来自类层级不同的父类或子类一个实例，用 is 和 as 操作符
• 为类型转换定义层级结构

  • 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" 类型编译器自动推导为： [MediaItem]

• 类型检查

  • 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"

• 类型向下转换

  • 可以用 as?或 as! 实现类型转换，类型转换有可能失败，as?返回一个可选类型， as!尝试类型转换且强制解包，如果类型转换不不正确，会触发运行时错误

    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 AnyObject 类型转换
  • Swift 为非指定类型提供了两种特殊类型
    • Any 可以代表实例类型，包括函数
    • AnyObject 可以代表任何类类型的实例
      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

  • 如果你想用可选值作为 Any 值，那么要用 as 操作符显示的将类型转换为 Any

    let optionalNumber: Int? = 3
    things.append(optionalNumber)        // Warning
    things.append(optionalNumber as Any) // No warning