隐藏navigationBar底部的黑线shadowImage

1. 直接设置navigationBar的shadowImage和backgroundImage, 需先让translucent属性设置为NO

   -(void)removeBlackLine {
       [self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarPosition:UIBarPositionAny barMetrics:UIBarMetricsDefault]; 
       [self.navigationController.navigationBar setShadowImage:[UIImage new]];  
   }

2. 循环遍历navigationBar的子视图，找到高度小于1的黑线, 在appear是隐藏, disappear时再显示（推荐）

   @implementation ViewController {
       UIImageView *navBarHairlineImageView;
   }
   - (void)viewWillAppear:(BOOL)animated {
   	[super viewWillAppear:animated];
       navBarHairlineImageView.hidden = YES;
   }
   - (void)viewWillDisappear:(BOOL)animated {
   	[super viewWillDisappear:animated];
       navBarHairlineImageView.hidden = NO;
   } 
   - (void)viewDidLoad {
       [super viewDidLoad];
       navBarHairlineImageView= [self findHairlineImageViewUnder:self.navigationController.navigationBar];
       // Do any additional setup after loading the view, typically from a nib.
   }
   - (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) {  
               return imageView;  
           }  
       }  
       return nil;  
   }  

3. 切去超出部分, 此方法会把状态栏给切掉

   -(void)useClipsToBoundsRemoveBlackLine { self.navigationController.navigationBar.clipsToBounds = YES;  
   }