Order.php

<?php

declare (strict_types=1);
namespace Drupal\Core\Hook\Order;

use Drupal\Core\Hook\OrderOperation\FirstOrLast;
use Drupal\Core\Hook\OrderOperation\OrderOperation;

/**
 * Set this implementation to be first or last.
 */
enum Order : int implements OrderInterface
{
  // This implementation should execute first.
  case First = 1;
  // This implementation should execute last.
  case Last = 0;
  
  /**
   * {@inheritdoc}
   */
  public function getOperation(string $identifier) : OrderOperation {
    return new FirstOrLast($identifier, $this === self::Last);
  }
}