Swift 3 protocol extension using selector error

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

@objc protocol KeyboardDismissing { 
    func on(tap: UITapGestureRecognizer)
}

extension KeyboardDismissing where Self: UIViewController {

    func addDismissalGesture() {
        let tap = UITapGestureRecognizer(target: self, action: #selector(Self.on(tap:)))
        view.addGestureRecognizer(tap)
    }

    func on(tap: UITapGestureRecognizer) {
        dismissKeyboard()
    }

    func dismissKeyboard() {
        view.endEditing(true)
    }
}

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)

Ok fine, I add the tag:

@objc func on(tap: UITapGestureRecognizer) {
    dismissKeyboard()
}

But then, it throws a different compile error on this newly added @objc tag with the error message:

@objc can only be used with members of classes, @objc protocols, and concrete extensions of classes

with the suggestion to "fix it" by removing the exact same tag I was just told to add.

I originally thought adding @objc before my protocol definition would solve any #selector problems but apparently that's not the case, and these cyclical error messages/suggestions aren't helping in the slightest. I've gone down a wild goose chase of adding/removing @objc tags everywhere, marking methods as optional, putting methods in the protocol's definition, etc.

It also doesn't matter what I put in the protocol definition Leaving the extension the same, the following example does not work nor does any combination of the declared methods in the protocol's definition:

@objc protocol KeyboardDismissing { 
    func on(tap: UITapGestureRecognizer)
}

This tricks me into thinking it works by compiling as a stand alone protocol, but the second I try to add it to a view controller:

class ViewController: UIViewController, KeyboardDismissing {}

it spits back the original error.

Can someone explain what I'm doing wrong and how I can compile this?

Note:

I've looked at this question but it is for Swift 2.2 not Swift 3 nor does the answer compile as soon as you create a view controller class that inherits from the protocol defined in the example.

I've also looked at this question but the answer uses NotificationCenter which is not what I'm after.

If there are any other seemingly duplicate questions, please let me know.

Solution

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.