导航栏平滑过渡

在开发过程中，经常遇到导航栏隐藏的需求，当导航栏隐藏界面跳转以及返回有导航栏界面时，需要一个平滑的过渡效果，上代码：

-(void)viewWillAppear:(BOOL)animated{
    [super viewWillAppear:animated];
    [self.navigationController setNavigationBarHidden:YES animated:animated];
}

-(void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    [self.navigationController setNavigationBarHidden:YES animated:NO];
}