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);
    }
}