Language Guide --- Type Casting

swift中的类型转换使用is 和 as 操作符。

Checking Type

使用is操作符判断一个实例是否是特定字类。

var movieCount = 0
var songCount = 0
 
for item in library {
    if item is Movie {
        movieCount += 1
    } else if item is Song {
        songCount += 1
    }
}

Downcasting

某个类型的常量和变量可能属于某种特定类型。可以使用as! 或 as?操作符向下转型为特定字类。
向下转型可能会失败，所以类型转换操作符有两种形式，as?返回某个字类的可选型值；as!，强制转换成某种字类类型。
当不确定是否能转换成功的时候使用as?；确定能转换成功的时候使用as!，如果转换失败会导致运行时错误。

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)")
    }
}

Type Casting for Any and AnyObject

swift提供了两种特殊的类型：

• Any 可以表示任何类型，包括方法。
• AnyObject 可以表示任何类类型。

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)" })

判断Any 或 AnyObject 类型变量的确切类型可以在switch中使用as 或 is 匹配模式。

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")
    }
}

Any 也可以表示可选类型，把一个可选类型加入数据会产生警告。可以使用as操作符把它明确转成Any类型。

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