Swift从入门到精通第十五篇

类型转换（学习笔记）

环境Xcode 11.0 beta4 swift 5.1

类型转换
- 类型转换是检查实例类型的一种方法，或者检查来自类层级不同的父类或子类一个实例，用 is 和 as 操作符

为类型转换定义层级结构

示例

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.init(name: name)

    }

}

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: "Rick Astley")

]

// "library" 类型编译器自动推导为： [MediaItem]

类型检查

示例

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"

类型向下转换

可以用 as?或 as! 实现类型转换，类型转换有可能失败，as?返回一个可选类型， as!尝试类型转换且强制解包，如果类型转换不不正确，会触发运行时错误

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)")

    }

}

//

// 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

Any AnyObject 类型转换

Swift 为非指定类型提供了两种特殊类型

Any 可以代表实例类型，包括函数

AnyObject 可以代表任何类类型的实例

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

注意

如果你想用可选值作为 Any 值，那么要用 as 操作符显示的将类型转换为 Any

let optionalNumber: Int? = 3

things.append(optionalNumber)        // Warning

things.append(optionalNumber as Any) // No warning

巴特西

Swift从入门到精通第十五篇 - 类型转换初识

类型转换（学习笔记）

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Swift从入门到精通第十五篇 - 类型转换初识