解决隐藏导航栏过渡动画出现的白板bug，导航栏隐藏过度动画

A界面push到B界面，在B界面需要隐藏导航栏，直接使用self.navigationController.navigationBar.hidden = YES; 过渡动画明显出现白板现象

解决方案

```

- (void)viewWillAppear:(BOOL)animated {

    [super viewWillAppear:animated];

     [self.navigationController setNavigationBarHidden:YES animated:animated];

}

  - (void)viewWillDisappear:(BOOL)animated {

    [super viewWillDisappear:animated];

     [self.navigationController setNavigationBarHidden:NO animated:animated];

}

```