element-ui tree全部展开和全部收起功能的实现

怎样才能实现点击全部展开,只要有二级菜单的都自动展开 ,点击全部收起的时候所有的二级菜单全部收起,我的后台数据中一级菜单的ID和二级菜单的ID几乎是重复的,我获取了this.$refs.tree.data中的ID,但是我只能打开点前点击过的那一条记录,我该怎么修改.

for(var i=0;i<this.$refs.selectTree.store._getAllNodes().length;i++){
this.$refs.selectTree.store._getAllNodes()[i].expanded = false;
}

<el-tree :data="data"
                   :props="{ label: 'name' }"
                   highlight-current
                   default-expand-all
                   ref="selectTree"
                   :expand-on-click-node="false"
                   node-key="id"></el-tree>

                 let self = this;
      let treeList = this.data;
      for (let i = 0; i < treeList.length; i++) {
        self.$refs.selectTree.store.nodesMap[treeList[i].id].expanded = true
      }