Redirect canonical node page

ExternalRedirect.php

<?php

namespace Drupal\mymodule\EventSubscriber;

use Drupal\mymodule\ExternalRedirectHelper;
use Drupal\Core\Routing\TrustedRedirectResponse;
use Drupal\node\NodeInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Event\RequestEvent;
use Symfony\Component\HttpKernel\KernelEvents;

/**
 * Event subscriber to redirect canonical pages to external links.
 */
class ExternalRedirect implements EventSubscriberInterface {

  /**
   * The external redirect helper service.
   *
   * @var \Drupal\mymodule\ExternalRedirectHelper
   */
  protected $redirectHelper;

  /**
   * Construct the event subscriber.
   *
   * @param \Drupal\mymodule\ExternalRedirectHelper $helper
   *   The external redirect helper service.
   */
  public function __construct(ExternalRedirectHelper $helper) {
    $this->redirectHelper = $helper;
  }

  /**
   * Redirect canonical pages if the node has an external URL set.
   *
   * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event
   *   The request event.
   */
  public function redirectExternalLinks(RequestEvent $event) {
    $request = $event->getRequest();

    // Don't redirect on node edit pages and such.
    if ($request->attributes->get('_route') !== 'entity.node.canonical') {
      return;
    }
    // Get node object.
    $node = $request->attributes->get('node');
    if ($node instanceof NodeInterface && $this->redirectHelper->shouldRedirect($node)) {
      $uri = $this->redirectHelper->getRedirectUrl($node);
      $response = new TrustedRedirectResponse($uri, Response::HTTP_MOVED_PERMANENTLY);
      $event->setResponse($response);
    }

  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    return [
      KernelEvents::REQUEST => ['redirectExternalLinks', 10],
    ];
  }

}

ExternalRedirectHelper.php

<?php

namespace Drupal\mymodule;

use Drupal\Core\Session\AccountProxyInterface;
use Drupal\node\NodeInterface;

/**
 * Utility methods for handling external content redirects.
 */
class ExternalRedirectHelper {

  /**
   * Node types that can have external URLs defined.
   *
   * @var string[]
   */
  const EXTERNAL_NODE_TYPES = [
    'external_link',
  ];

  /**
   * The name of the field defining the external URL.
   *
   * @var string
   */
  const EXTERNAL_URL_FIELD_NAME = 'field_external_link';

  /**
   * The current user.
   *
   * @var \Drupal\Core\Session\AccountProxyInterface
   *   The current user.
   */
  protected $currentUser;

  /**
   * Construct the utility service.
   *
   * @param \Drupal\Core\Session\AccountProxyInterface $account
   *   The current user.
   */
  public function __construct(AccountProxyInterface $account) {
    $this->currentUser = $account;
  }

  /**
   * Determine if the given node contains external content to redirect to.
   *
   * Will not redirect if the current user has access to edit the node.
   *
   * @param \Drupal\node\NodeInterface $node
   *   The content node.
   *
   * @return bool
   *   Returns TRUE if the given content should be redirected.
   */
  public function shouldRedirect(NodeInterface $node):bool {
    return !$node->access('update', $this->currentUser)
      && $this->isExternalContent($node);
  }

  /**
   * Determine if the given content is external.
   *
   * @param \Drupal\node\NodeInterface $node
   *   The content node.
   *
   * @return bool
   *   Returns TRUE if the content is external.
   */
  public function isExternalContent(NodeInterface $node):bool {
    return in_array($node->getType(), static::EXTERNAL_NODE_TYPES)
      && $node->hasField(static::EXTERNAL_URL_FIELD_NAME)
      && !$node->get(static::EXTERNAL_URL_FIELD_NAME)->isEmpty()
      && !empty($node->get(static::EXTERNAL_URL_FIELD_NAME)->uri);
  }

  /**
   * Get the redirect URL.
   *
   * @param \Drupal\node\NodeInterface $node
   *
   * @return string
   *   The external URI.
   */
  public function getRedirectUrl(NodeInterface $node):string {
    return $node->get(static::EXTERNAL_URL_FIELD_NAME)->uri;
  }

}

mymodule.module

<?php

/**
 * Implements hook_ENTITY_TYPE_view_alter().
 */
function mymodule_node_view_alter(array &$build, NodeInterface $node, EntityViewDisplayInterface $display) {
  $helper = \Drupal::service('mymodule.redirect_helper');
  if ($build['#view_mode'] === 'full' && $helper->isExternalContent($node)) {
    $url = $helper->getRedirectUrl($node);
    \Drupal::messenger()->addWarning(t('This content will redirect to <a href="@url">@url</a> for anonymous users.', ['@url' => $url]));
  }
}

mymodule.services.yml

services:
  mymodule.redirect_helper:
    class: Drupal\mymodule\ExternalRedirectHelper
    arguments: ['@current_user']
  mymodule.event_subscriber:
    class: Drupal\mymodule\EventSubscriber\ExternalRedirect
    arguments: ['@mymodule.redirect_helper']
    tags:
      - { name: event_subscriber }