element ui tree组件的节点全展开与收缩

```javascript
onExpandAll() {
  // 遍历tree结构修改展开与折叠是个异步行为，会耗时间，所以要写promise
  let task = new Promise((resolve, reject) => {
    this.expandAllStatus = !this.expandAllStatus;
    resolve();
  }).then(() => {
    this.treeNodeExpand(this.expandAllStatus)
  })
},
treeNodeExpand(status) {
  // 这是个耗时操作
  for(var i=0;i<this.$refs.myTree.store._getAllNodes().length;i++){
    this.$refs.myTree.store._getAllNodes()[i].expanded = status;
  }
},
// expandAllStatus: 是控制展开与折叠的状态(true/false)
// myTree: 自己的Tree ref名

转载：https://segmentfault.com/q/1010000012032027