swift基礎（十）：型別檢查操作符is、型別轉換操作符as(包括!及?的區別)、AnyObject和Any型別
參考文件：《The Swift programming
language 中文版 V1.2》
目錄:
1.型別檢查操作符is
2.型別轉換操作符as(包括!及?的區別)
3.AnyObject型別
4.Any型別
//1.型別檢查操作符:is
//用型別檢查操作符( is )來檢查一個例項是否屬於特定子型別。若例項屬於那個子型別,型別檢查操作符返回true ,否則返回 false 。
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
}
}
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:
]
var movieCount = 0
var songCount = 0
for item in library
{
if item is Movie
{
++movieCount
}
else if item is Song
{
++songCount
}
}
print("Media library contains \(movieCount) movies and \(songCount) songs")
//"Media library contains 2 movies and 3 songs\n"
//2.型別轉換操作符(as? 或 as!)
/*
因為向下轉型可能會失敗,型別轉型操作符帶有兩種不同形式。條件形式(conditional form) as? 返回一個你試圖向下轉成的型別的可選值(optional value)。強制形式 as! 把試圖向下轉型和強制解包(force-unwra ps)結果作為一個混合動作。
1.當你不確定向下轉型可以成功時,用型別轉換的條件形式( as? )。條件形式的型別轉換總是返回一個可選值(opt ional value),並且若下轉是不可能的,可選值將是 nil 。這使你能夠檢查向下轉型是否成功。
2.只有你可以確定向下轉型一定會成功時,才使用強制形式( as! )。當你試圖向下轉型為一個不正確的型別時,強制形式的型別轉換會觸發一個執行時錯誤。
*/
for item in library
{
if let movie = item as? Movie
{
print("Movie: '\(movie.name)', dir. \(movie.director)")//2 times
}
else if let song = item as? Song
{
print("Song: '\(song.name)', by \(song.artist)")//3 times
}
}
// 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
//3.AnyObject型別轉換
//當在工作中使用 Cocoa APIs,我們一般會接收一個 AnyObject型別的陣列,或者說“一個任何物件型別的陣列”。
let someObjects: [AnyObject] =
[
Movie(name: "2001: A Space Odyssey", director: "Stanley Kubrick"),
Movie(name: "Moon", director: "Duncan Jones"),
Movie(name: "Alien", director: "Ridley Scott")
]
for object insomeObjects
{
let movie = object as! Movie
print("Movie: '\(movie.name)', dir. \(movie.director)")
}
// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick
// Movie: 'Moon', dir. Duncan Jones
// Movie: 'Alien', dir. Ridley Scott
for movie in someObjects as! [Movie]//簡化處理
{
print("Movie: '\(movie.name)', dir. \(movie.director)")
}
// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick
// Movie: 'Moon', dir. Duncan Jones
// Movie: 'Alien', dir. Ridley Scott
//4.Any 型別轉換
//使用 Any 型別來和混合的不同型別一起工作,包括方法型別和非 class 型別
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