Type Casting

• 当数组中存有同一父类的不同子类时，该数组内容会被统一认定为是父类
• is和as的区别：is用来判断，返回Bool；as用来转换，返回可选类型
• 使用is来判断是否是某一子类

  for item in library {
    if item is Movie {
        movieCount += 1
    } else if item is Song {
        songCount += 1
    }
  }
  

• 在某一类可能实际上是其子类，并需要访问子类中属性时使用downcasting。由于转换可能会失败，使用as?表示可选，使用as!表示强制拆包

  //Downcasting
  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)")
    }
  }
  

• swift中可以使用any代表所有类型实例，包括函数；可以使用anyobject代表所有类的实例

  //Type Casting for Any and AnyObject
  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.init(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 thar 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")
    }
  }
  

• any可以代表任何类型包括可选，但是表示可选时加上as any来消除警告

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

• Forced cast from 'Data?' to 'Data' only unwraps optionals; did you mean to use '!'?

  let originImage = UIImage(data: area.image as! Data)
  

  当已知area.image为Data类型时请不要使用as!进行强制转换。只需要使用!强拆即可。

  let originImage = UIImage(data: area.image!)