UIBarButtonItem Closure

UIBarButtonItem Closure 1.swift

extension UIBarButtonItem {
    
    /// Typealias for UIBarButtonItem closure.
    private typealias UIBarButtonItemTargetClosure = (UIBarButtonItem) -> ()

    private class UIBarButtonItemClosureWrapper: NSObject {
        let closure: UIBarButtonItemTargetClosure
        init(_ closure: @escaping UIBarButtonItemTargetClosure) {
            self.closure = closure
        }
    }
    
    private struct AssociatedKeys {
        static var targetClosure = "targetClosure"
    }
    
    private var targetClosure: UIBarButtonItemTargetClosure? {
        get {
            guard let closureWrapper = objc_getAssociatedObject(self, &AssociatedKeys.targetClosure) as? UIBarButtonItemClosureWrapper else { return nil }
            return closureWrapper.closure
        }
        set(newValue) {
            guard let newValue = newValue else { return }
            objc_setAssociatedObject(self, &AssociatedKeys.targetClosure, UIBarButtonItemClosureWrapper(newValue), objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}

Awesome! Glad it was useful.
Your version is much better!