layui导航栏二级菜单不显示问题及解决

 <ul class="layui-nav layui-nav-tree" lay-filter="test">
            <li class="layui-nav-item layui-nav-itemed">
              <a _href='#'>
                <i class="layui-icon layui-icon-home"></i>
                <cite>首页</cite>
              </a>
              <dl class="layui-nav-child">
                <dd>
                  <a  _index="0">项目介绍</a>
                </dd>
                <dd>
                  <a  _index="1">分析页</a>
                </dd>               
              </dl>
            </li>
            <li class="layui-nav-item">
              <a _href='#' v-on:click="chufa">
                <i class="layui-icon layui-icon-set-fill"></i>
                <span>系统管理</span>
              </a>
              <dl class="layui-nav-child">
                <dd>
                  <a href="javascript:;" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >用户管理</a>
                </dd>
                <dd>
                  <a href="javascript:;" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >角色管理</a>
                </dd>
                <dd>
                  <a href="javascript:;" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >菜单管理</a>
                </dd>
                <dd>
                  <a href="javascript:;" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >登录日志</a>
                </dd>
              </dl>
            </li>
          </ul>

mounted() {
this.$nextTick(() => {
  //导航栏依赖于 element 因此下面的代码是必须的否则二级导航内容就会不显示
  layui.use("element", function() {
    var element = layui.element;
  });    
});

<script>
        layui.use('element', function(){
            var element = layui.element;
            element.on('nav(filter)', function(elem){
                console.log(elem); //得到当前点击的DOM对象
            });
        });
    </script>

var index = layer.open({
    ……
});
layer.close(index)；/   layer.closeAll()

//当你在iframe页面关闭自身时
var index = parent.layer.getFrameIndex(window.name); //先得到当前iframe层的索引
parent.layer.close(index); //再执行关闭