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!)
```

Type Casting

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