Slightly improved version of Beau Nouvelle's UIBarButtonItem closure extension. I hid all the helper classes within the extension and made them private, so as not to pollute the global namespace. Original article: https://medium.com/@BeauNouvelle/adding-a-closure-to-uibarbuttonitem-24dfc217fe72

UIBarButtonItem+Closure.swift

import Foundation
import UIKit

public extension UIBarButtonItem {
    public typealias TargetClosure = (UIBarButtonItem) -> ()
    
    public convenience init(title: String?, style: UIBarButtonItem.Style = .plain, closure: @escaping TargetClosure) {
        self.init(title: title, style: style, target: nil, action: nil)
        
        targetClosure = closure
        action = #selector(UIBarButtonItem.closureAction)
    }
    
    private struct AssociatedKeys {
        static var targetClosure = "targetClosure"
    }
    
    private class ClosureWrapper: NSObject {
        let closure: TargetClosure
        
        init(_ closure: @escaping TargetClosure) {
            self.closure = closure
        }
    }
    
    private var targetClosure: TargetClosure? {
        get {
            guard let closureWrapper = objc_getAssociatedObject(self, &AssociatedKeys.targetClosure) as? ClosureWrapper else {
                return nil
            }
            return closureWrapper.closure
        }
        
        set {
            guard let newValue = newValue else {
                return
            }
            objc_setAssociatedObject(self, &AssociatedKeys.targetClosure, ClosureWrapper(newValue), objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
    
    @objc private func closureAction() {
        targetClosure?(self)
    }
}