Creating default pages in Drupal 8

lookup-page-example.php

<?php
use Drupal\node\Entity\Node;

$current_language = \Drupal::languageManager()->getCurrentLanguage();
$default_pages = \Drupal::state()->get('my_module.default_pages', array());
$node_id = (isset($default_pages['front_' . $current_language->getId()]) ? $default_pages['front_' . $current_language->getId()] : NULL);

if (!$node_id) {
  return NULL;
}

$node = Node::load($node_id);

my_front_module__my_front_module.module

<?php

/**
 * Return node of current language so metatag attaches metatags.
 *
 * @param \Drupal\Core\Routing\CurrentRouteMatch $route
 * @return \Drupal\Core\Entity\EntityInterface|null
 */
function my_front_module_metatag_route_entity($route) {
  if ($route && $route->getRouteName() == 'my_front_module.front') {
    return \Drupal\my_front_module\Controller\FrontController::getFrontPageForCurrentLanguage();
  }
}

my_front_module__my_front_module.routing.yml

 my_front_module.front:
  path: '/front'
  defaults:
    _controller: '\Drupal\my_front_module\Controller\FrontController::content'
    _title_callback: '\Drupal\my_front_module\Controller\FrontController::title'
  requirements:
    _permission: 'access content'

my_front_module__my_front_module.services.yml

services:
  my_front_module.default_pages_subscriber:
    class: '\Drupal\my_front_module\EventSubscriber\DefaultPagesSubscriber'
    tags:
      - { name: 'event_subscriber' }

my_front_module__src__Controller__FrontController.php

<?php

namespace Drupal\my_front_module\Controller;

use Drupal\Core\Controller\ControllerBase;
use Drupal\node\Entity\Node;

class FrontController extends ControllerBase {

  /**
   * @return \Drupal\Core\Language\LanguageInterface
   */
  private static function currentLanguage() {
    $language_manger = \Drupal::languageManager();

    return $language_manger->getCurrentLanguage();
  }

  /**
   * @return \Drupal\Core\Entity\EntityInterface
   */
  public static function getFrontPageForCurrentLanguage() {
    $current_language = self::currentLanguage();

    $default_pages = \Drupal::state()->get('my_front_module.default_pages', array());
    $node_id = (isset($default_pages['front_' . $current_language->getId()]) ? $default_pages['front_' . $current_language->getId()] : NULL);

    if (!$node_id) {
      return NULL;
    }

    $node = Node::load($node_id);

    if (!$node) {
      return NULL;
    }

    return $node;
  }

  public function title() {
    $node = $this->getFrontPageForCurrentLanguage();
    if (!$node) {
      return '';
    }

    return $node->label();
  }

  public function content() {
    $node = $this->getFrontPageForCurrentLanguage();
    if (!$node) {
      return array(
        '#type' => 'markup',
        '#markup' => t('Frontpage not found!'),
      );
    }

    $view_builder = \Drupal::entityTypeManager()->getViewBuilder('node');
    $build = $view_builder->view($node, 'default');

    return $build;
  }
}

my_front_module__src__EventSubscriber__DefaultPagesSubscriber.php

<?php
/**
 * @file
 * Contains \Drupal\my_front_module\EventSubscriber\DefaultPagesSubscriber.
 */

namespace Drupal\my_front_module\EventSubscriber;

use Drupal\my_module\EventSubscriber\DefaultPagesBase;

/**
 * Event Subscriber DefaultPageSubscriber.
 */
class DefaultPagesSubscriber extends DefaultPagesBase {

  /**
   * {@inheritdoc}
   */
  protected function getDefaultPages() {
    $frontpage_base = [
      'type' => 'page',
      'path' => 'home',
      'title' => 'Home',
    ];

    return [
      'frontpage_nl' => $frontpage_base + [
        'field_description' => 'Dit is de voorpagina!',
      ],
      'frontpage_en' => $frontpage_base + [
        'field_description' => 'This is the frontpage!',
      ],
      'frontpage_de' => $frontpage_base + [
        'field_description' => 'Dies ist die Titelseite!',
      ],
    ];
  }
}

my_module__my_module.drush.inc

<?php

/**
 * Implements hook_drush_command().
 */
function my_module_drush_command() {
  $items = array();
  $items['default-pages'] = [
    'description' => 'Installs the default pages and registers them in configuration.',
    'arguments' => [],
    'drupal dependencies' => ['my_module'],
    'aliases' => [],
  ];
  return $items;
}

/**
 * Callback function for default pages.
 */
function drush_my_module_default_pages() {
  drush_print('Ensuring default pages:');

  /** @var Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher $dispatcher */
  $dispatcher = \Drupal::service('event_dispatcher');
  $dispatcher->dispatch('my_module.default_pages');
}

my_module__my_module.module

<?php

use Drupal\Core\Access\AccessResult;

/**
 * Implements hook_node_access().
 *
 * Makes sure default pages nodes aren't deleted.
 */
function my_module_node_access(\Drupal\node\NodeInterface $node, $op, \Drupal\Core\Session\AccountInterface $account) {
  if ($op == 'delete') {
    $default_pages = \Drupal::state()->get('my_module.default_pages', array());
    $default_pages_nids = array_flip($default_pages);
    if (isset($default_pages_nids[$node->id()])) {
      // Return forbidden when this is a default page.
      return AccessResult::forbidden();
    }
  }

  // No opinion by default.
  return AccessResult::neutral();
}

my_module__src__EventSubscriber__DefaultPagesBase.php

<?php
/**
 * @file
 * Contains \Drupal\my_module\EventSubscriber\DefaultPagesBase.
 */

namespace Drupal\my_module\EventSubscriber;

use Symfony\Component\EventDispatcher\Event;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Drupal\node\Entity\Node;
use Drupal\Core\Language\LanguageInterface;

/**
 * Event Subscriber DefaultPageSubscriber.
 */
abstract class DefaultPagesBase implements EventSubscriberInterface {

  public static $stateKey = 'my_module.default_pages';

  public function onCreateDefaultPages(Event $event) {
    $default_pages = $this->getDefaultPages();

    if (count($default_pages)) {
      foreach ($default_pages as $id => $default_page) {
        if ($this->defaultPageExists($id)) {
          drush_print($id . ': Page already exists!');
          continue;
        }

        if ($this->createDefaultPage($id, $default_page)) {
          drush_print($id . ': Page created!');
        }
        else {
          drush_set_error($id . ': Error while creating page!');
        }
      }
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events['my_module.default_pages'][] = ['onCreateDefaultPages'];
    return $events;
  }

  /**
   * @return array
   */
  abstract protected function getDefaultPages();

  /**
   * @param $id
   * @return bool
   */
  private function defaultPageExists($id) {
    $default_pages = \Drupal::state()->get(self::$stateKey, array());
    $entity_id = (isset($default_pages[$id]) ? $default_pages[$id] : NULL);

    if ($entity_id && $node = Node::load($entity_id)) {
      return TRUE;
    }

    return FALSE;
  }

  /**
   * @param $id
   * @param $page
   * @return bool
   */
  private function createDefaultPage($id, $page) {

    if (!isset($page['langcode'])) {
      $page['langcode'] = LanguageInterface::LANGCODE_NOT_SPECIFIED;
    }

    $path = FALSE;
    if (isset($page['path'])) {
      $path = $page['path'];
      unset($page['path']);
    }

    try {
      $node = Node::create($page);
      $node->save();

      if ($path) {
        /* @var \Drupal\Core\Path\AliasStorageInterface $path_storage */
        $path_storage = \Drupal::service('path.alias_storage');
        $saved_path = $path_storage->save("/node/" . $node->id(), '/' . $path, $page['langcode']);
      }

      /* @var \Drupal\Core\Config\ConfigFactoryInterface $config */
      $default_pages = \Drupal::state()->get(self::$stateKey, array());
      $default_pages[$id] = $node->id();
      \Drupal::state()->set(self::$stateKey, $default_pages);

      return TRUE;
    }
    catch (\Exception $e) {
      drush_set_error('Error while creating default page:');
      drush_set_error($e->getMessage());
    }

    return FALSE;
  }
}