Add a block as the touchUpInside handler.

UIButton+BlockAction.swift

import UIKit

public extension UIButton {
    
    public typealias TouchUpInside = () -> Void
    
    public func setTouchUpInside(_ block: @escaping TouchUpInside) {
        touchUpInsideBlock = block
        addTarget(self, action: #selector(runTouchUpInside), for: .touchUpInside)
    }
    
    @objc private func runTouchUpInside() {
        touchUpInsideBlock?()
    }
    
    private var touchUpInsideBlock: TouchUpInside? {
        get {
            objc_sync_enter(self); defer { objc_sync_exit(self) }
            return objc_getAssociatedObject(self, &AssociatedKeys.targetBlockKey) as? TouchUpInside
        }
        set {
            objc_sync_enter(self); defer { objc_sync_exit(self) }
            objc_setAssociatedObject(self, &AssociatedKeys.targetBlockKey, newValue, .OBJC_ASSOCIATION_RETAIN)
        }
    }
    
}

private struct AssociatedKeys {
    static var targetBlockKey = "targetBlockKey"
}