class BlockViewBuilder

class BlockViewBuilder extends EntityViewBuilder implements TrustedCallbackInterface {
  
  /**
   * {@inheritdoc}
   */
  public function buildComponents(array &$build, array $entities, array $displays, $view_mode) {
  }
  
  /**
   * {@inheritdoc}
   */
  public function view(EntityInterface $entity, $view_mode = 'full', $langcode = NULL) {
    $build = $this->viewMultiple([
      $entity,
    ], $view_mode, $langcode);
    return reset($build);
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewMultiple(array $entities = [], $view_mode = 'full', $langcode = NULL) {
    /** @var \Drupal\block\BlockInterface[] $entities */
    $build = [];
    foreach ($entities as $entity) {
      $entity_id = $entity->id();
      $plugin = $entity->getPlugin();
      $cache_tags = Cache::mergeTags($this->getCacheTags(), $entity->getCacheTags());
      $cache_tags = Cache::mergeTags($cache_tags, $plugin->getCacheTags());
      // Create the render array for the block as a whole.
      // @see template_preprocess_block().
      $build[$entity_id] = [
        '#cache' => [
          'keys' => [
            'entity_view',
            'block',
            $entity->id(),
          ],
          'contexts' => Cache::mergeContexts($entity->getCacheContexts(), $plugin->getCacheContexts()),
          'tags' => $cache_tags,
          'max-age' => $plugin->getCacheMaxAge(),
        ],
        '#weight' => $entity->getWeight(),
      ];
      // Allow altering of cacheability metadata or setting #create_placeholder.
      $this->moduleHandler
        ->alter([
        'block_build',
        "block_build_" . $plugin->getBaseId(),
      ], $build[$entity_id], $plugin);
      if ($plugin instanceof MainContentBlockPluginInterface || $plugin instanceof TitleBlockPluginInterface) {
        // Immediately build a #pre_render-able block, since this block cannot
        // be built lazily.
        $build[$entity_id] += static::buildPreRenderableBlock($entity, $this->moduleHandler());
      }
      else {
        // Assign a #lazy_builder callback, which will generate a #pre_render-
        // able block lazily (when necessary).
        $build[$entity_id] += [
          '#lazy_builder' => [
            static::class . '::lazyBuilder',
            [
              $entity_id,
              $view_mode,
              $langcode,
            ],
          ],
        ];
      }
    }
    return $build;
  }
  
  /**
   * Builds a #pre_render-able block render array.
   *
   * @param \Drupal\block\BlockInterface $entity
   *   A block config entity.
   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
   *   The module handler service.
   *
   * @return array
   *   A render array with a #pre_render callback to render the block.
   */
  protected static function buildPreRenderableBlock($entity, ModuleHandlerInterface $module_handler) {
    $plugin = $entity->getPlugin();
    $plugin_id = $plugin->getPluginId();
    $base_id = $plugin->getBaseId();
    $derivative_id = $plugin->getDerivativeId();
    $configuration = $plugin->getConfiguration();
    // Inject runtime contexts.
    if ($plugin instanceof ContextAwarePluginInterface) {
      $contexts = \Drupal::service('context.repository')->getRuntimeContexts($plugin->getContextMapping());
      \Drupal::service('context.handler')->applyContextMapping($plugin, $contexts);
    }
    // Create the render array for the block as a whole.
    // @see template_preprocess_block().
    $build = [
      '#theme' => 'block',
      '#attributes' => [],
      // All blocks get a "Configure block" contextual link.
'#contextual_links' => [
        'block' => [
          'route_parameters' => [
            'block' => $entity->id(),
          ],
        ],
      ],
      '#weight' => $entity->getWeight(),
      '#configuration' => $configuration,
      '#plugin_id' => $plugin_id,
      '#base_plugin_id' => $base_id,
      '#derivative_plugin_id' => $derivative_id,
      '#id' => $entity->id(),
      '#pre_render' => [
        static::class . '::preRender',
      ],
      // Add the entity so that it can be used in the #pre_render method.
'#block' => $entity,
    ];
    // If an alter hook wants to modify the block contents, it can append
    // another #pre_render hook.
    $module_handler->alter([
      'block_view',
      "block_view_{$base_id}",
    ], $build, $plugin);
    return $build;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function trustedCallbacks() {
    return [
      'preRender',
      'lazyBuilder',
    ];
  }
  
  /**
   * #lazy_builder callback; builds a #pre_render-able block.
   *
   * @param $entity_id
   *   A block config entity ID.
   * @param $view_mode
   *   The view mode the block is being viewed in.
   *
   * @return array
   *   A render array with a #pre_render callback to render the block.
   */
  public static function lazyBuilder($entity_id, $view_mode) {
    return static::buildPreRenderableBlock(Block::load($entity_id), \Drupal::service('module_handler'));
  }
  
  /**
   * #pre_render callback for building a block.
   *
   * Renders the content using the provided block plugin, and then:
   * - if there is no content, aborts rendering, and makes sure the block won't
   *   be rendered.
   * - if there is content, moves the contextual links from the block content to
   *   the block itself.
   */
  public static function preRender($build) {
    $content = $build['#block']->getPlugin()
      ->build();
    // Remove the block entity from the render array, to ensure that blocks
    // can be rendered without the block config entity.
    unset($build['#block']);
    if ($content !== NULL && !Element::isEmpty($content)) {
      // Place the $content returned by the block plugin into a 'content' child
      // element, as a way to allow the plugin to have complete control of its
      // properties and rendering (for instance, its own #theme) without
      // conflicting with the properties used above, or alternate ones used by
      // alternate block rendering approaches in contrib (for instance, Panels).
      // However, the use of a child element is an implementation detail of this
      // particular block rendering approach. Semantically, the content returned
      // by the plugin "is the" block, and in particular, #attributes and
      // #contextual_links is information about the *entire* block. Therefore,
      // we must move these properties from $content and merge them into the
      // top-level element.
      foreach ([
        '#attributes',
        '#contextual_links',
      ] as $property) {
        if (isset($content[$property])) {
          $build[$property] += $content[$property];
          unset($content[$property]);
        }
      }
      $build['content'] = $content;
    }
    else {
      // Abort rendering: render as the empty string and ensure this block is
      // render cached, so we can avoid the work of having to repeatedly
      // determine whether the block is empty. For instance, modifying or adding
      // entities could cause the block to no longer be empty.
      $build = [
        '#markup' => '',
        '#cache' => $build['#cache'],
      ];
      // If $content is not empty, then it contains cacheability metadata, and
      // we must merge it with the existing cacheability metadata. This allows
      // blocks to be empty, yet still bubble cacheability metadata, to indicate
      // why they are empty.
      if (!empty($content)) {
        CacheableMetadata::createFromRenderArray($build)->merge(CacheableMetadata::createFromRenderArray($content))
          ->applyTo($build);
      }
    }
    return $build;
  }

}