iOS 设置导航条下方的黑线隐藏
{
// 导航条下方的黑Line 添加一个全局变量用来放找到的黑线
UIImageView * _navigationBarLine;
}

// 用这种方法可以保持bar的 translucent 属性  并且可以找到导航条中的黑线 
-(UIImageView * )fineTheLineBelowNavigationBar:(UIView *)view
{
  if ([view isKindOfClass:[UIImageView class]] && view.frame.size.height <=1.0) {
      return (UIImageView *)view;
  }
  for (UIView * sub in view.subviews) {
      UIImageView * image = [self fineTheLineBelowNavigationBar:sub];
      if (image) {
          return  image;
      }
  }
  return  nil;
}

- (void)viewDidLoad {
   [super viewDidLoad];
   _navigationBarLine = [self fineTheLineBelowNavigationBar:self.navigationController.navigationBar];
}

-(void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    _navigationBarLine.hidden = YES;
}

-(void)viewDidDisappear:(BOOL)animated
{
    [super viewDidDisappear:animated];
    _navigationBarLine.hidden = NO;
}