隐藏UINavigationBar黑线

先直接上方法：

隐藏： 
- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [self.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
    self.navigationBar.shadowImage = [UIImage new];
}

显示：
- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    [self.navigationBar setBackgroundImage:nil forBarMetrics:UIBarMetricsDefault];
    self.navigationBar.shadowImage = nil;
}
 [self.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
 self.navigationBar.shadowImage = [UIImage new];

苹果官方文档上解释黑线的原因：