UIBarButtonItem在设置后未高亮显示

下面是固定代码

 class ViewController: UIViewController {

    var btnEdit : UIBarButtonItem!
    var btnSave : UIBarButtonItem!

    @objc func btnEditTapped (){
        btnSave = UIBarButtonItem(barButtonSystemItem: .save, target: self, action: #selector(btnSaveTapped));

      navigationItem.rightBarButtonItem = btnSave

    }


    @objc func btnSaveTapped (){
        btnEdit = UIBarButtonItem(barButtonSystemItem: .edit, target: self, action: #selector(btnEditTapped));        navigationItem.rightBarButtonItem = btnEdit


    }



    override func viewDidLoad() {
        super.viewDidLoad()
        btnSave = UIBarButtonItem(barButtonSystemItem: .save, target: self, action: #selector(btnSaveTapped))
        btnEdit = UIBarButtonItem(barButtonSystemItem: .edit, target: self, action: #selector(btnEditTapped))
        navigationItem.rightBarButtonItem = btnEdit

    }
}