• is

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"

• as!

• as?

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)")
    }
}

• Any

不仅仅能够容括class类型 同时还可以适用于包括struct 和enum在内的所有类型

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)" }) 

• AnyObject

本身就是一个接口

protocol AnyObject { }

而且所有的class都隐式的实现了这个接口，这也限制了AnyObject是只适用于Class类型的原因。

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

NOTE

The Any type represents values of any type, including optional types. Swift gives you a warning if you use an optional value where a value of type Any is expected. If you really do need to use an optional value as an Any value, you can use the as operator to explicitly cast the optional to Any, as shown below.

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