swift 类型转换

1. var movieCount = 0
2. var songCount = 0
4. for item in library {
5. if item is Movie {
6. movieCount += 1
7. } else if item is Song {
8. songCount += 1
9. }
10. }

1. for item in library {
2. if let movie = item as? Movie {
3. print("Movie: (movie.name), dir. (movie.director)")
4. } else if let song = item as? Song {
5. print("Song: (song.name), by (song.artist)")
6. }
7. }

Type Casting for Any and AnyObject

Swift provides two special types for working with nonspecific types:

• Any can represent an instance of any type at all, including function types.

• AnyObject can represent an instance of any class type.

To discover the specific type of a constant or variable that is known only to be of type Any or AnyObject, you can use an is or as pattern in a switch statement’s cases. The example below iterates over the items in the things array and queries the type of each item with a switch statement. Several of the switch statement’s cases bind their matched value to a constant of the specified type to enable its value to be printed:

1. for thing in things {
2. switch thing {
3. case 0 as Int:
4. print("zero as an Int")
5. case 0 as Double:
6. print("zero as a Double")
7. case let someInt as Int:
8. print("an integer value of (someInt)")
9. case let someDouble as Double where someDouble > 0:
10. print("a positive double value of (someDouble)")
11. case is Double:
12. print("some other double value that I don't want to print")
13. case let someString as String:
14. print("a string value of "(someString)"")
15. case let (x, y) as (Double, Double):
16. print("an (x, y) point at (x), (y)")
17. case let movie as Movie:
18. print("a movie called (movie.name), dir. (movie.director)")
19. case let stringConverter as (String) -> String:
20. print(stringConverter("Michael"))
21. default:
22. print("something else")
23. }
24. }
26. // zero as an Int
27. // zero as a Double
28. // an integer value of 42
29. // a positive double value of 3.14159
30. // a string value of "hello"
31. // an (x, y) point at 3.0, 5.0
32. // a movie called Ghostbusters, dir. Ivan Reitman
33. // Hello, Michael