iOS如何去掉导航栏（UINavigationBar）下方的横线

[self.navigationController.navigationBar setBackgroundImage:[[UIImage alloc] init] forBarMetrics:UIBarMetricsDefault];self.navigationController.navigationBar.shadowImage = [[UIImage alloc] init];

**UINavigationBar****--[ 1] _UIBarBackground****----[ 2] UIImageView****----[ 2] UIVisualEffectView****------[ 3] _UIVisualEffectBackdropView****------[ 3] _UIVisualEffectFilterView****--[ 1] UINavigationButton****----[ 2] UIButtonLabel****--[ 1] _UINavigationBarBackIndicatorView**

NSArray *allSubviews(UIView *aView) { NSArray *results = [aView subviews]; for (UIView *eachView in aView.subviews) {   NSArray *subviews = allSubviews(eachView);   if (subviews)     results = [results arrayByAddingObjectsFromArray:subviews]; } return results;}

@property (nonatomic, strong) UIImageView *shadowImage;

- (void)viewWillAppear:(BOOL)animated{  [super viewWillAppear:animated];  NSArray *subViews = allSubviews(self.navigationController.navigationBar);  for (UIView *view in subViews) {    if ([view isKindOfClass:[UIImageView class]] && view.bounds.size.height<1){     //实践后发现系统的横线高度为0.333      self.shadowImage = (UIImageView *)view;    }  }  self.shadowImage.hidden = YES;}

- (void)viewWillDisappear:(BOOL)animated{  [super viewWillDisappear:animated];  self.shadowImage.hidden = NO;}