/**
  * Batch print shipping labels for whole shipments.
  * Push pdf document with shipping labels to user browser
  *
  * @param AbstractCollection $collection
  * @return ResponseInterface|ResultInterface
  */
 protected function massAction(AbstractCollection $collection)
 {
     $labelsContent = [];
     if ($collection->getSize()) {
         /** @var \Magento\Sales\Model\Order\Shipment $shipment */
         foreach ($collection as $shipment) {
             $labelContent = $shipment->getShippingLabel();
             if ($labelContent) {
                 $labelsContent[] = $labelContent;
             }
         }
     }
     if (!empty($labelsContent)) {
         $outputPdf = $this->labelGenerator->combineLabelsPdf($labelsContent);
         return $this->fileFactory->create('ShippingLabels.pdf', $outputPdf->render(), DirectoryList::VAR_DIR, 'application/pdf');
     }
     $this->messageManager->addError(__('There are no shipping labels related to selected shipments.'));
     return $this->resultRedirectFactory->create()->setPath('sales/shipment/');
 }