class BlockPageVariant

class BlockPageVariant extends VariantBase implements PageVariantInterface, ContainerFactoryPluginInterface {
    
    /**
     * The block repository.
     *
     * @var \Drupal\block\BlockRepositoryInterface
     */
    protected $blockRepository;
    
    /**
     * The block view builder.
     *
     * @var \Drupal\Core\Entity\EntityViewBuilderInterface
     */
    protected $blockViewBuilder;
    
    /**
     * The Block entity type list cache tags.
     *
     * @var string[]
     */
    protected $blockListCacheTags;
    
    /**
     * The render array representing the main page content.
     *
     * @var array
     */
    protected $mainContent = [];
    
    /**
     * The page title: a string (plain title) or a render array (formatted title).
     *
     * @var string|array
     */
    protected $title = '';
    
    /**
     * Constructs a new BlockPageVariant.
     *
     * @param array $configuration
     *   A configuration array containing information about the plugin instance.
     * @param string $plugin_id
     *   The plugin ID for the plugin instance.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\block\BlockRepositoryInterface $block_repository
     *   The block repository.
     * @param \Drupal\Core\Entity\EntityViewBuilderInterface $block_view_builder
     *   The block view builder.
     * @param string[] $block_list_cache_tags
     *   The Block entity type list cache tags.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, BlockRepositoryInterface $block_repository, EntityViewBuilderInterface $block_view_builder, array $block_list_cache_tags) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->blockRepository = $block_repository;
        $this->blockViewBuilder = $block_view_builder;
        $this->blockListCacheTags = $block_list_cache_tags;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('block.repository'), $container->get('entity_type.manager')
            ->getViewBuilder('block'), $container->get('entity_type.manager')
            ->getDefinition('block')
            ->getListCacheTags());
    }
    
    /**
     * {@inheritdoc}
     */
    public function setMainContent(array $main_content) {
        $this->mainContent = $main_content;
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setTitle($title) {
        $this->title = $title;
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        // Track whether blocks showing the main content and messages are displayed.
        $main_content_block_displayed = FALSE;
        $messages_block_displayed = FALSE;
        $build = [
            '#cache' => [
                'tags' => $this->blockListCacheTags,
            ],
        ];
        // Load all region content assigned via blocks.
        $cacheable_metadata_list = [];
        foreach ($this->blockRepository
            ->getVisibleBlocksPerRegion($cacheable_metadata_list) as $region => $blocks) {
            
            /** @var \Drupal\block\BlockInterface[] $blocks */
            foreach ($blocks as $key => $block) {
                $block_plugin = $block->getPlugin();
                if ($block_plugin instanceof MainContentBlockPluginInterface) {
                    $block_plugin->setMainContent($this->mainContent);
                    $main_content_block_displayed = TRUE;
                }
                elseif ($block_plugin instanceof TitleBlockPluginInterface) {
                    $block_plugin->setTitle($this->title);
                }
                elseif ($block_plugin instanceof MessagesBlockPluginInterface) {
                    $messages_block_displayed = TRUE;
                }
                $build[$region][$key] = $this->blockViewBuilder
                    ->view($block);
                // The main content block cannot be cached: it is a placeholder for the
                // render array returned by the controller. It should be rendered as-is,
                // with other placed blocks "decorating" it. Analogous reasoning for the
                // title block.
                if ($block_plugin instanceof MainContentBlockPluginInterface || $block_plugin instanceof TitleBlockPluginInterface) {
                    unset($build[$region][$key]['#cache']['keys']);
                }
            }
            if (!empty($build[$region])) {
                // \Drupal\block\BlockRepositoryInterface::getVisibleBlocksPerRegion()
                // returns the blocks in sorted order.
                $build[$region]['#sorted'] = TRUE;
            }
        }
        // If no block that shows the main content is displayed, still show the main
        // content. Otherwise the end user will see all displayed blocks, but not
        // the main content they came for.
        if (!$main_content_block_displayed) {
            $build['content']['system_main'] = $this->mainContent;
        }
        // If no block displays status messages, still render them.
        if (!$messages_block_displayed) {
            $build['content']['messages'] = [
                '#weight' => -1000,
                '#type' => 'status_messages',
                '#include_fallback' => TRUE,
            ];
        }
        // If any render arrays are manually placed, render arrays and blocks must
        // be sorted.
        if (!$main_content_block_displayed || !$messages_block_displayed) {
            unset($build['content']['#sorted']);
        }
        // The access results' cacheability is currently added to the top level of the
        // render array. This is done to prevent issues with empty regions being
        // displayed.
        // This would need to be changed to allow caching of block regions, as each
        // region must then have the relevant cacheable metadata.
        $merged_cacheable_metadata = CacheableMetadata::createFromRenderArray($build);
        foreach ($cacheable_metadata_list as $cacheable_metadata) {
            $merged_cacheable_metadata = $merged_cacheable_metadata->merge($cacheable_metadata);
        }
        $merged_cacheable_metadata->applyTo($build);
        return $build;
    }

}