- (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;

}

viewDid 方法中调用：_navBarHairlineImageView = [self findHairlineImageViewUnder:self.navigationController.navigationBar];

will方法中

_navBarHairlineImageView.hidden = YES;

viewDisapp方法中

_navBarHairlineImageView.hidden = No;