vue2+element UI 树形只有两级， 第一级只能上下移动，第二级不能成为第一级，只能拖到别的第一级和在同一级排序

<template>
    <el-tree draggable :allow-drop="dropAllow" @node-drop="dragSuccess" ref="tree" :data="dataList"
      :props="defaultProps" default-expand-all node-key="id" highlight-current></el-tree>
   
</template>
<script>
  export default {
    data() {
      return {
        dataList: [{
            id: 1,
            label: "一级菜单",
            children: [{
              id: 5,
              label: "1-1",
            }, ],
          },
          {
            id: 2,
            label: "二级菜单",
            children: [{
                id: 6,
                label: "2-1",
              },
              {
                id: 7,
                label: "2-2",
              },
            ],
          },
          {
            id: 3,
            label: "三级菜单",
            children: [{
                id: 8,
                label: "3-1",
              },
              {
                id: 9,
                label: "3-2",
              },
            ],
          },
          {
            id: 4,
            label: "四级菜单",
          },
        ],
        defaultProps: {
          label: "label", //指定节点标签为节点对象的某个属性值
          children: "children", //指定子树为节点对象的某个属性值
        },
      };
    },
    methods: {
      dropAllow(draggingNode, dropNode, type) {
        if (draggingNode.level === dropNode.level) { //同级
          if (draggingNode.parent.id === dropNode.parent.id) {
            // 向上拖拽 || 向下拖拽
            return type === "prev" || type === "next"
          }
        } else { //不同级
          if (draggingNode.level != 1) { //不是第一级
            return type === "inner" //插入
          } else {
            return false;
          }
        }
      },
      dragSuccess(draggingNode, dropNode, type, event) {
        console.log(draggingNode, dropNode, type, event);
      },
    }
  };
</script>