code: private function prepareHydrateData(array $data)
      {
          /*
           * unflatten tree
           */
          $items = $data['items'];
          $tree = [ '__root__' => array_shift($items) ];
  
          foreach ($items as $item) {
              $parent = substr($item['current'], 0, strrpos($item['current'], '-'));
              $tree[$parent][] = $item;
          }
  
          $this->hydrateData = $tree;
      }
docs: Prepares the form values for hydrating.
  
  @internal
  Populates the {@link hydrateData} array
  
  @param array $data