第八篇：对protocol做extinction添加#selector的时候，出现@objc这种错误

I have what I thought to be a very simple protocol extension for my UIViewController
s providing the capability to dismiss a keyboard through a tap gesture. Here's my code:

protocol Tappable {
    func addTapGestureRecognizer()
    func tapGestureDetected(gesture:UITapGestureRecognizer)
}

extension Tappable where Self: UIView {
    func addTapGestureRecognizer() {
        let gesture = UITapGestureRecognizer(target: self, action:#selector(Tappable.tapGestureDetected(_:)))
        addGestureRecognizer(gesture)
    }
}

class TapView: UIView, Tappable {
    func tapGestureDetected(gesture:UITapGestureRecognizer) {
        print("Tapped")
    }
}

The problem is that the above code throws a compile error on this line:

let tap = UITapGestureRecognizer(target: self, action: #selector(Self.on(tap:)))

With the error message:
Argument of '#selector' refers to instance method 'on(tap:)' that is not exposed to Objective-C

with the suggestion to "fix it" by adding @objc
before func on(tap: UITapGestureRecognizer)

这个问题我搞了2天，才放弃了，下面是一个比较能说服我的理由：

This is a Swift protocol extension. Swift protocol extensions are invisible to Objective-C, no matter what; it knows nothing of them. But #selector is about Objective-C seeing and calling your function. That is not going to happen because your on(tap:) function is defined only in the protocol extension. Thus the compiler rightly stops you.
This question is one of a large class of questions where people think they are going to be clever with protocol extensions in dealing with Cocoa by trying to inject Objective-C-callable functionality (selector, delegate method, whatever) into a class via a protocol extension. It's an appealing notion but it's just not going to work.

翻译后是这个样子的：

这是一个快速的协议扩展。快速的协议扩展对于objective - c是看不见的，无论如何，它对它们一无所知。但是# selector是关于objective - c查看和调用你的函数。这不会发生，因为您的on(tap:)函数只在协议扩展中定义。因此编译器正确地阻止了你。
这个问题是一个很大的问题，人们认为在处理可可的过程中，他们会聪明地使用协议扩展，试图通过协议扩展将objective - c - callable功能(选择器、委托方法等)注入到类中。这是一个有吸引力的概念，但它是行不通的。

最后说一句：

在extinction协议的时候，尽量使用block，不要使用#selector了，使用回调虽然麻烦了一下，这也是一个曲线救国的方法吧。

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