el-tree树点击全选按钮，全部展开并且全选

 // 全部选中
    qxClick() {
      this.isQx = !this.isQx;
      // 判断按钮的状态
      this.expandAll();
      if (this.isQx) {
        console.log(this.isQx, "-------------------------------", this.datas);
        // 设置
        this.$refs.treeForm_mxs.setCheckedNodes(this.datas);
        for (
          var i = 0;
          i < this.$refs.treeForm_mxs.getCheckedNodes().length;
          i++
        ) {}
      } else {
        this.$refs.treeForm_mxs.setCheckedNodes([]);
      }
    },
    // 全部展开
    expandAll() {
      if (this.isQx) {
        for (var i in this.$refs.treeForm_mxs.store.nodesMap) {
          this.$refs.treeForm_mxs.store.nodesMap[i].expanded = true;
        }
      } else {
        for (var j in this.$refs.treeForm_mxs.store.nodesMap) {
          this.$refs.treeForm_mxs.store.nodesMap[j].expanded = false;
        }
      }
    },