namespace Drupalavenue_cartEventSubscriber;
use SymfonyComponentEventDispatcherEventSubscriberInterface;
use Drupalstate_machineEventWorkflowTransitionEvent;
use DrupalCoreEntityEntityTypeManager;


class OrderCompleteSubscriber implements EventSubscriberInterface {

/**
* DrupalCoreEntityEntityTypeManager definition.
*
* @var DrupalCoreEntityEntityTypeManager
*/
protected $entityTypeManager;

/**
 * Constructor.
 */
public function __construct(EntityTypeManager $entity_type_manager) {
 $this->entityTypeManager = $entity_type_manager;
 }

 /**
  * {@inheritdoc}
  */
  static function getSubscribedEvents() {
   $events['commerce_order.fulfill.post_transition'] =
   ['orderCompleteHandler'];

   return $events;
  }

   /**
   * This method is called whenever the
   commerce_order.place.post_transition event is
   * dispatched.
   *
   * @param WorkflowTransitionEvent $event
   */
   public function orderCompleteHandler(WorkflowTransitionEvent $event) {
/** @var Drupalcommerce_orderEntityOrderInterface $order */
$order = $event->getEntity();
// Order items in the cart.
$items = $order->getItems();

// Write your custom logic here.

Drupal::logger('avenue_cart')->notice('order completed');

}

}