Swift之路 —— 类型转换

Author Avatar
xiaoLit Created: Aug 28, 2019 Updated: Oct 11, 2019

前言:
学习后整理,大多记录Swift区别于Objective-C的特性,来源官方文档。

一、类型转换

不同于OC的指针星号强转和isKindOfClassisNumberClass的类型判断,swift有自己更简易的类型转换和判断。

二、检查类型

用类型检查操作符(is)来检查一个实例是否属于特定子类型。若实例属于那个子类型,类型检查操作符返回 true,否则返回 false

例子:

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")
]
// 数组 library 的类型被推断为 [MediaItem]


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")
// 打印“Media library contains 2 movies and 3 songs”

三、向下转型

某类型的一个常量或变量可能在幕后实际上属于一个子类。当确定是这种情况时,你可以尝试用类型转换操作符(as?as!)向下转到它的子类型。

例子:

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

注意:
转换没有真的改变实例或它的值。根本的实例保持不变;只是简单地把它作为它被转换成的类型来使用。

四、Any 和 AnyObject 的类型转换

Swift 为不确定类型提供了两种特殊的类型别名:

  • Any 可以表示任何类型,包括函数类型。
  • AnyObject 可以表示任何类类型的实例。

只有当你确实需要它们的行为和功能时才使用 Any 和 AnyObject。最好还是在代码中指明需要使用的类型。

例子:

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:
Any 类型可以表示所有类型的值,包括可选类型。Swift 会在你用 Any 类型来表示一个可选值的时候,给你一个警告。如果你确实想使用 Any 类型来承载可选值,你可以使用 as 操作符显式转换为 Any,如下所示:

let optionalNumber: Int? = 3
things.append(optionalNumber)        // 警告
things.append(optionalNumber as Any) // 没有警告