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(BlockInterface $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;
    }

}