用翻转过渡改变leftBarButtonItem？

let firstBarButtonItem = UIBarButtonItem(title: "First", style: .plain, target: self, action: #selector(changeBarButtonItem))
let secondBarButtonItem = UIBarButtonItem(title: "Second", style: .plain, target: self, action: #selector(changeBarButtonItem))

@objc func changeBarButtonItem() {
    let currentBarButtonItem = navigationItem.leftBarButtonItem
    navigationItem.leftBarButtonItem = (currentBarButtonItem == firstBarButtonItem) ? secondBarButtonItem : firstBarButtonItem
}

navigationItem.leftBarButtonItem = firstBarButtonItem

class ViewController: UIViewController, UINavigationControllerDelegate {
    // ...
    override func viewDidLoad() {
        super.viewDidLoad()
        navigationController?.delegate = self
    }
    // ...
}

func navigationController(_ navigationController: UINavigationController, animationControllerFor operation: UINavigationController.Operation, from fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
    return FlipTransition()
}

class FlipTransition: NSObject, UIViewControllerAnimatedTransitioning {
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.5
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        let containerView = transitionContext.containerView
        guard let fromView = transitionContext.view(forKey: .from), let toView = transitionContext.view(forKey: .to) else { return }
        
        containerView.addSubview(toView)
        toView.alpha = 0
        
        let duration = transitionDuration(using: transitionContext)
        
        UIView.animate(withDuration: duration, animations: {
            fromView.transform = CGAffineTransform(rotationAngle: -CGFloat.pi/2)
            toView.alpha = 1
        }) { _ in
            fromView.transform = .identity
            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        }
    }
}