导航栏隐藏意外？

Question

这是我的问题gif： question GIF 我在第一个VC中使用此代码

override func viewWillAppear(animated: Bool) {
    self.navigationController?.setNavigationBarHidden(true, animated: animated)
    super.viewWillAppear(animated)
    print("VC1 viewWillAppear")   
}

override func viewWillDisappear(animated: Bool) {
    self.navigationController?.setNavigationBarHidden(false, animated: animated)
    super.viewWillDisappear(animated)
    print("VC1 viewWillDisappear")
}

它指的是how to hidden when push to a vc