trait FlagPermissionsTrait in Flag 8.4

trait FlagPermissionsTrait {

  /**
   * Grants flag and unflag permission to the given flag.
   *
   * @param \Drupal\flag\FlagInterface $flag
   *   The flag on which to grant permissions.
   * @param array|string $role_id
   *   (optional) The ID of the role to grant permissions. If omitted, the
   *   authenticated role is assumed.
   * @param bool $can_flag
   *   (optional) TRUE to grant the role flagging permission, FALSE to not grant
   *   flagging permission to the role. If omitted, TRUE is assumed.
   * @param bool $can_unflag
   *   Optional TRUE to grant the role unflagging permission, FALSE to not grant
   *   unflagging permission to the role. If omitted, TRUE is assumed.
   */
  protected function grantFlagPermissions(FlagInterface $flag, $role_id = RoleInterface::AUTHENTICATED_ID, $can_flag = TRUE, $can_unflag = TRUE) {

    // Grant the flag permissions to the authenticated role, so that both
    // users have the same roles and share the render cache.
    $role = Role::load($role_id);
    if ($can_flag) {
      $role
        ->grantPermission('flag ' . $flag
        ->id());
    }
    if ($can_unflag) {
      $role
        ->grantPermission('unflag ' . $flag
        ->id());
    }
    $role
      ->grantPermission('access contextual links');
    $role
      ->save();
  }

}