class RelationshipRouteAccessCheck

final class RelationshipRouteAccessCheck implements AccessInterface {
  
  /**
   * The route requirement key for this access check.
   *
   * @var string
   */
  const ROUTE_REQUIREMENT_KEY = '_jsonapi_relationship_route_access';
  
  /**
   * The JSON:API entity access checker.
   *
   * @var \Drupal\jsonapi\Access\EntityAccessChecker
   */
  protected $entityAccessChecker;
  
  /**
   * RelationshipRouteAccessCheck constructor.
   *
   * @param \Drupal\jsonapi\Access\EntityAccessChecker $entity_access_checker
   *   The JSON:API entity access checker.
   */
  public function __construct(EntityAccessChecker $entity_access_checker) {
    $this->entityAccessChecker = $entity_access_checker;
  }
  
  /**
   * Checks access to the relationship field on the given route.
   *
   * @param \Symfony\Component\Routing\Route $route
   *   The route to check against.
   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
   *   The route match.
   * @param \Drupal\Core\Session\AccountInterface $account
   *   The currently logged in account.
   *
   * @return \Drupal\Core\Access\AccessResultInterface
   *   The access result.
   */
  public function access(Route $route, RouteMatchInterface $route_match, AccountInterface $account = NULL) {
    [$relationship_field_name, $field_operation] = explode('.', $route->getRequirement(static::ROUTE_REQUIREMENT_KEY));
    assert(in_array($field_operation, [
      'view',
      'edit',
    ], TRUE));
    $entity_operation = $field_operation === 'view' ? 'view' : 'update';
    if ($resource_type = $route_match->getParameter(Routes::RESOURCE_TYPE_KEY)) {
      assert($resource_type instanceof ResourceType);
      $entity = $route_match->getParameter('entity');
      $internal_name = $resource_type->getInternalName($relationship_field_name);
      if ($entity instanceof FieldableEntityInterface && $entity->hasField($internal_name)) {
        $entity_access = $this->entityAccessChecker
          ->checkEntityAccess($entity, $entity_operation, $account);
        $field_access = $entity->get($internal_name)
          ->access($field_operation, $account, TRUE);
        // Ensure that access is respected for different entity revisions.
        $access_result = $entity_access->andIf($field_access);
        if (!$access_result->isAllowed()) {
          $reason = "The current user is not allowed to {$field_operation} this relationship.";
          $access_reason = $access_result instanceof AccessResultReasonInterface ? $access_result->getReason() : NULL;
          $detailed_reason = empty($access_reason) ? $reason : $reason . " {$access_reason}";
          $access_result->setReason($detailed_reason);
        }
        return $access_result;
      }
    }
    return AccessResult::neutral();
  }

}