CartOnlySingleItem.php

class CartOnlySingleItem implements EventSubscriberInterface {

  use StringTranslationTrait;

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    return [
      CartEvents::CART_ENTITY_ADD => ['limitNumberEntriesAdd'],
      CartEvents::CART_ORDER_ITEM_UPDATE => ['limitNumberEntriesUpdate'],
    ];
  }

  /**
   * Limit the number of items added to the cart.
   *
   * @param \Drupal\commerce_cart\Event\CartEntityAddEvent $event
   *   The cart event.
   */
  public function limitNumberEntriesAdd(CartEntityAddEvent $event) {
    $this->limitItemQuantity($event->getOrderItem());
  }

  /**
   * Limit the number of items added to the cart.
   *
   * @param \Drupal\commerce_cart\Event\CartOrderItemUpdateEvent $event
   *   The cart event.
   */
  public function limitNumberEntriesUpdate(CartOrderItemUpdateEvent $event) {
    $this->limitItemQuantity($event->getOrderItem());
  }

  /**
   * Limit the number of items added to the cart.
   *
   * @param \Drupal\commerce_order\Entity\OrderItemInterface $orderItem
   *   The order item.
   */
  protected function limitItemQuantity(OrderItemInterface $orderItem) {
    if ($orderItem->getQuantity() > 1) {
      $orderItem->setQuantity(1)->save();
      // Clear out existing status messages related to addition to the cart.
      drupal_get_messages('status');
      drupal_set_message($this->t('Only a single instance of an event is allowed at a time in your cart.'), 'error');
    }
  }
  
}