一、当A页面要push到B页面，需要将B页面的导航栏隐藏时，我们只需要在A页面中重写以下两个方法：
override func viewWillAppear(animated: Bool) {
super.viewWillAppear(animated)
self.navigationController?.setNavigationBarHidden(true, animated: true)
}
override func viewWillDisappear(animated: Bool) {
super.viewWillDisappear(animated)
