Allows to set a Swift closure to a control instead of Objective-C selector

UIControl+onTouchUpInside.swift

import UIKit

public extension UIControl {

    typealias OnTouchUpInside = () -> Void

    private struct Constants {
        static var callbackKey = "UIControl.OnTouchUpInsideAssociatedKey"
    }

    @objc
    var onTouchUpInside: OnTouchUpInside? {
        get {
            if let savedObject = objc_getAssociatedObject(self, &Constants.callbackKey) as? WrappedValue {
                return savedObject.block
            }
            return nil
        }
        set {
            let kSelector = #selector(UIControl._uicontrol_plus_touch_up_inside_block_handler)

            removeTarget(self, action: kSelector, for: .touchUpInside)
            if newValue != nil {
                addTarget(self, action: kSelector, for: .touchUpInside)
            }
            objc_setAssociatedObject(self,
                                     &Constants.callbackKey,
                                     WrappedValue(newValue),
                                     objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
        }
    }

    @objc private func _uicontrol_plus_touch_up_inside_block_handler() {
        gtuOnTouchUpInside?()
    }

    private class WrappedValue: NSObject {

        let block: OnTouchUpInside

        init?(_ block: OnTouchUpInside?) {
            guard let block = block else {
                return nil
            }
            self.block = block
            super.init()
        }
    }
}