• 最近有個需求，懶加載樹形結構，默認展開二級節點。試了獲取節點設置節點的expanded屬性，發現不生效this.$refs.tree.store.nodesMap[data.id].expanded = true
• 看了下官方文檔利用default-expanded-keys屬性，懶加載第一級的時候將第一級節點的key放到定義的默認展開數組即default-expanded-keys動態綁定的數組就可以實現改工能
• 部分代碼入下圖

//css
<el-tree
      ref="menuTree"
       :props="defaultProps"
       node-key="id"
       :key="treeKey"
       lazy
       :default-expanded-keys="treeExpandelKeys"
       :load="loadDeptNode"
       :highlight-current="true"
       @node-contextmenu="rightClick"
       @node-expand="nodeExpand"
        ></el-tree>
//js
loadDeptNode(node, resolve, cb) {
      this.resolve = resolve;
      // 查詢參數
      let reqData = {
        roleTypeId: this.roleTypeId,
      };
      if (node.level === 0) {
        this.rootNode = node;
        this.rootNode_resolve = resolve; //同上，把node.level == 0的resolve也存起來
        resolve([this.rootNodeData]);
        this.treeExpandelKeys.push(this.rootNodeData.id)
        this.fristNodeKeys.push(this.rootNodeData.id)
      } else if (node.level > 0) {
        // 向后臺發送查詢請求
        getNodesService({ params: { node: node.data.id } })
          .then((res) => {
            // 請求成功,且查詢出有數據時
            if (res && res.data) {
              let resData = res.data || [];
              var newN = [];
              resData.forEach((it) => {
                let newIt = Object.assign({}, it, {
                  leaf: it.leaf == "1" ? true : false,
                });
                newN.push(newIt);
              });
              if(node.level == 1){
                this.secondNodeKeys = [];
                newN.forEach(item=>{
                    this.secondNodeKeys.push(item.id)
                })
              }
              resolve(newN);
            }
            // 請求失敗的情況,或者是查無數據的時候
            else {
              resolve([]);
            }
          })
          .catch((e) => {
            if (node.level === 0) {
              resolve(treeData);
            } else {
              resolve([]);
            }
          });
      }
    },