第二种方法：1）声明UIImageView变量,存储底部横线

@implementation MyViewController {

UIImageView *navBarHairlineImageView;

2）在viewDidLoad中加入：

navBarHairlineImageView = [self findHairlineImageViewUnder:navigationBar];

3）实现找出底部横线的函数

- (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;

}

4）最后在viewWillAppear，viewWillDisappear中处理

- (void)viewWillAppear:(BOOL)animated {

  [super viewWillAppear:animated];

  navBarHairlineImageView.hidden = YES;

}

- (void)viewWillDisappear:(BOOL)animated {

  [super viewWillDisappear:animated];

  navBarHairlineImageView.hidden = NO;

}