code: public function onInsert(ResourceEventInterface $event)
      {
          $item = $this->getSupplierDeliveryItemFromEvent($event);
  
          // Credit stock unit received quantity
          if (null === $orderItem = $item->getOrderItem()) {
              throw new RuntimeException("OrderItem must be set.");
          }
  
          if (null !== $stockUnit = $orderItem->getStockUnit()) {
              $stockUnit->addGeocode($item->getGeocode());
  
              $this->stockUnitUpdater->updateReceived($stockUnit, $item->getQuantity(), true);
          } elseif ($orderItem->hasSubjectIdentity()) {
              throw new RuntimeException("Failed to retrieve stock unit.");
          }
  
          // Dispatch supplier order content change event
          if (null === $order = $orderItem->getOrder()) {
              throw new RuntimeException("Order must be set.");
          }
  
          $this->scheduleSupplierOrderContentChangeEvent($order);
      }
docs: Insert event handler.
  
  @param ResourceEventInterface $event