code: public function getLayerOfDescendants($id)
      {
          $child = $this->getSubLevel($id);
          $data = collect([]);
          if ($child) {
              foreach ($child as $val) {
                  $val->child = $this->getLayerOfDescendants($val->{$this->getTreeNodeIdName()});
                  $data->put($val->{$this->getTreeNodeIdName()}, $val);
              }
          }
  
          return $data;
      }
docs: 获取指定节点的所有后代（分层级）.
  
  @param mixed $id
  
  @return \Illuminate\Support\Collection