《Swift从入门到精通》（十六）：类型转换

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

      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