import UIKit

    extension UIBarButtonItem {

        convenience init (image: UIImage?, target: Any?, action: Selector?) {
            let button = UIButton(type: .custom)
            button.imageView?.contentMode = .scaleAspectFit
            button.frame = CGRect(x: 0.0, y: 0.0, width: 44.0, height: 44.0)
            button.addTarget(target, action: action!, for: .touchUpInside)
            button.setImage(image, for: UIControlState.normal)
            button.imageEdgeInsets = UIEdgeInsets(top: 7.0, left: 7.0, bottom: 7.0, right: 7.0)

            if #available(iOS 9.0, *) {
                button.widthAnchor.constraint(equalToConstant: 44.0).isActive = true
                button.heightAnchor.constraint(equalToConstant: 44.0).isActive = true
            }
            self.init(customView: button)
        }
    }

let rightButton = UIBarButtonItem(image: UIImage(named: "example"), target: self, action: #selector(exampleAction(sender:)))
self.navigationItem.rightBarButtonItem = rightButton