Swift 中向上铸造和向下铸造混淆？

Question

class Media {
    var name :String = ""
    init(name:String) {
        self.name = name
    }
}

class Song:Media {}

class Movie:Media{}

let s1 = Song(name :"Fireproof")
var m1 :Media = s1 //upcasting

//var s2 :Song = m1
var s2:Song = m1 as Song //down casting

// var x1 :Movie = m1 as Movie //

在行 var m1: Media = s1 你可以设置 m1 等于 s1 因为 m1 的类型是 [=14= 的超类]??
在线var s2: Song = m1 as Song，据说是"down casting"，是因为m1: Media而你是"casting"所以"as"一个Song类型才能匹配相同类型的s2？（如果这是真的，那为什么我们早点设置 m1 = s1，而 s1 的类型与 m1 不同？？）
所有这些向上转换和向下转换的意义何在？我已经阅读了苹果文档并设法让自己更加困惑:'(

Answer 1

On the line var m1: Media = s1 you can set m1 equal to s1 because m1's type is the superclass of s1??

是的。 s1 是一个 Song，这意味着根据定义它也是一个 Media。因此它可以赋值给 Song 或 Media.

类型的变量

On line var s2: Song = m1 as Song , it is supposedly "down casting", is that because m1: Media and you are "casting" it "as" a Song type in order to match the same type of s2? (If this is true, then why did we set m1 = s1 earlier, when s1 had a different type than m1??)

在实际代码中没有理由向上转换，然后立即向下转换。这只是向您展示您可以执行的操作的示例代码。

顺便说一句，简单的 "as" 语法是旧的 Swift - 你现在需要使用 as! 如果你确定它会在 100% 的时间内工作，或者 as? 如果你不确定。（Google "swift optional casting" 了解更多信息。）普通旧 "as" 现在仅对编译器知道将始终有效的强制转换有效，例如从 String 到 NSString.

What's the point of all this up casting and down casting? I've read the apple documentation and managed to confuse myself even more :'(

此代码中没有演示真正的要点。它通常用于与具有公共基础 class 的异构类型数组进行交互。例如，如果您有一组不同的视图，您可能希望将它们放在一个数组中并对所有视图执行某些操作：

class ViewController : UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let label = UILabel()
        let button = UIButton()
        let slider = UISlider()

        let views = [label, button, slider] // views is a [UIView]

        for view in views {
            // view is known to be a UIView, even though it is also really a label, button, etc.
            view.translatesAutoresizingMaskIntoConstraints = false
            self.view.addSubview(view)
        }
    }
}

如果您不确定是否需要了解这一点，暂时不要太担心。当你遇到一堆相似类型但又不完全相同的问题时，请回到这里。

Swift 中向上铸造和向下铸造混淆？

Up casting and down casting confusion in Swift?

swift

casting

type-conversion

downcast

upcasting