为什么使用选择器需要@objc标记? [英] Why is the @objc tag needed to use a selector?

问题描述

我正在尝试使用选择器来查看某个协议是否可以执行操作.当我这样尝试时:

I am trying to use a selector to see if a certain protocol can perform an action. When I try it like this:

protocol Test {
    func hello()
    func goodBye(a: String)
}

class Tester: NSObject, Test {        
    override init() {}
    func hello() { }        
    func goodBye(a: String) { }
}

let a: Test = Tester()

let result = a.responds(to: Selector("goodByeWithA:"))

在这种情况下，result的计算结果为false.

In this case, result evaluates to false.

但是，如果我将@objc标签添加到协议中，则其评估结果为true.

But if I add the @objc tag to the protocol, it evaluates as true.

@objc protocol Test {
    func hello()
    func goodBye(a: String)
}

这是为什么?

另一方面，我知道现在建议使用#selector语法并远离使用字符串，但是出于各种原因，在这种情况下，我必须使用字符串.

On a side note, I know that it is now recommended to use the #selector syntax and to move away from using strings, but for various reasons, I have to use a string in this case.

只有当我将项目迁移到Swift 4.2后，这种情况才开始发生

This only started happening once I migrated my project to Swift 4.2

推荐答案

默认情况下，Swift生成仅对其他Swift代码可用的代码，但是如果您需要与Objective-C运行时进行交互–例如所有UIKit –您需要告诉Swift该怎么做.

By default Swift generates code that is only available to other Swift code, but if you need to interact with the Objective-C runtime – all of UIKit, for example – you need to tell Swift what to do.

这就是@objc属性的来源:将其应用于类或方法时，它会指示Swift将这些内容提供给Objective-C和Swift代码.因此，每次您要从UIBarButtonItem或Timer调用方法时，都需要使用@objc标记该方法，以便将其公开-这两个以及许多其他都是Objective-C代码.

That’s where the @objc attribute comes in: when you apply it to a class or method it instructs Swift to make those things available to Objective-C as well as Swift code. So, any time you want to call a method from a UIBarButtonItem or a Timer, you’ll need to mark that method using @objc so it’s exposed – both of those, and many others, are Objective-C code.

不用担心:如果您忘记了在需要时添加@objc，则代码根本就不会编译-这不是您会偶然忘记并引入错误的东西.

Don’t worry: if you forget to add @objc when it’s needed, your code simply won’t compile – it’s not something you can forget by accident and introduce a bug.

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