Flag action link plugin which returns login link.

AjaxActionLogin.php

<?php

namespace Drupal\forkdin_core\Plugin\ActionLink;

use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Url;
use Drupal\flag\FlagInterface;
use Drupal\flag\Plugin\ActionLink\AJAXactionLink;

/**
 * Provides the AJAX link type.
 *
 * This class is an extension of the Reload link type, but modified to
 * provide AJAX links.
 *
 * @ActionLinkType(
 *   id = "forkdin_core_ajax_link_login",
 *   label = @Translation("AJAX link (login)"),
 *   description = "An AJAX JavaScript request will be made without reloading the page."
 * )
 */
class AjaxActionLogin extends AJAXactionLink {

  /**
   * {@inheritdoc}
   */
  public function getAsFlagLink(FlagInterface $flag, EntityInterface $entity) {
    $render = parent::getAsFlagLink($flag, $entity);
    $action = $this->getAction($flag, $entity);
    $access = $flag->actionAccess($action, $this->currentUser, $entity);
    if (!$access->isAllowed()) {
      $render = [
        '#type' => 'link',
        '#title' => $flag->getFlagShortText(),
        '#url' => Url::fromRoute('user.login', [], [
          'query' => [
            'destination' => $this->getUrl($action, $flag, $entity)->toString(),
          ]
        ])
      ];
      CacheableMetadata::createFromRenderArray($render)
        ->addCacheableDependency($access)
        ->applyTo($render);

      $render = [
        '#markup' => \Drupal::getContainer()->get('renderer')->renderPlain($render),
      ];
    }

    return $render;
  }

}