iOS 11 -如何更改rightBarButtonItems的位置？

let customButtonItem = UIBarButtonItem(title: "Custom", style: .plain, target: self, action: #selector(customButtonTapped))
navigationItem.rightBarButtonItems = [customButtonItem]

let flexibleSpace = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)
navigationItem.rightBarButtonItems?.append(flexibleSpace)

if let items = navigationItem.rightBarButtonItems {
    if let index = items.firstIndex(of: flexibleSpace) {
        navigationItem.rightBarButtonItems?.insert(customButtonItem, at: index)
    }
}