NavigationBar 下面的横线是怎么来的? 如何去除?

<code class="hljs php has-numbering"><span class="hljs-comment">//自定义一个NaVIgationBar</span> [<span class="hljs-keyword">self</span>.navigationController.navigationBar setBackgroundImage:[UIImage <span class="hljs-keyword">new</span>] forBarMetrics:UIBarMetricsDefault]; <span class="hljs-comment">//消除阴影</span>    <span class="hljs-keyword">self</span>.navigationController.navigationBar.shadowImage = [UIImage <span class="hljs-keyword">new</span>];</code>