navagationbar不显示底部线

有时我们需要实现如下场景， 
(1)在同一个界面中 显示与隐藏NavigationBar底部分割线 
(2)当离开这个ViewController的时候，仍然需要显示底部分割线。 
效果图如下所示：

图1

图2

(1)定义变量 
UIImageView*navBarHairlineImageView;

(2)创建findHairlineImageViewUnder方法，并在viewDidLoad中调用 
- (UIImageView*)findHairlineImageViewUnder:(UIView*)view { 
if([view isKindOfClass:UIImageView.class] && view.bounds.size.height<=1.0) { 
return(UIImageView*)view; 
} 
for(UIView*subview in view.subviews) { 
UIImageView*imageView = [self findHairlineImageViewUnder:subview]; 
if(imageView) { 
returnimageView; 
} 
} 
return nil; 
}

viewDidLoad中调用 
navBarHairlineImageView= [self findHairlineImageViewUnder:self.navigationController.navigationBar];

(3)在viewWillAppear方法中隐藏该分割线 
navBarHairlineImageView.hidden=YES;

(4)在viewWillDisappear方法中显示该分割线 
navBarHairlineImageView.hidden=NO;