code: public function build()
      {
          $filter = $this->getMetaModel()->getEmptyFilter();
  
          if ($this->configuration->getFilter()) {
              $this->calculateSubfilter(
                  array
                  (
                      'operation' => 'AND',
                      'children' => $this->configuration->getFilter()
                  ),
                  $filter
              );
          }
  
          return $filter;
      }
docs: Prepare a filter and return it.
  
  @return IFilter