code: public function getCheckedValues()
      {
          if ($this->values === null) {
              $data = $this->config->getImageOptimizationRatios();
              if (!isset($data)) {
                  $data = '';
              }
              $this->values = explode(',', $data);
          }
          return $this->values;
      }
docs: Get checked values
  
  @return array|null