code: public function onUpdate(ResourceEventInterface $event)
      {
          $line = $this->getInvoiceLineFromEvent($event);
  
          $this->preventForbiddenChange($line);
  
          $this->stockUnitAssigner->applyInvoiceLine($line);
  
          // If invoice item quantity has changed
          if ($this->persistenceHelper->isChanged($line, 'quantity')) {
              $this->scheduleInvoiceContentChangeEvent($line->getInvoice());
          }
      }
docs: Update event handler.
  
  @param ResourceEventInterface $event