[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;
}