class BookNavigationBlock

class BookNavigationBlock extends BlockBase implements ContainerFactoryPluginInterface {
    
    /**
     * The current route match.
     *
     * @var \Drupal\Core\Routing\RouteMatchInterface
     */
    protected $routeMatch;
    
    /**
     * The book manager.
     *
     * @var \Drupal\book\BookManagerInterface
     */
    protected $bookManager;
    
    /**
     * The node storage.
     *
     * @var \Drupal\Core\Entity\EntityStorageInterface
     */
    protected $nodeStorage;
    
    /**
     * Constructs a new BookNavigationBlock instance.
     *
     * @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\Core\Routing\RouteMatchInterface $route_match
     *   The current route match.
     * @param \Drupal\book\BookManagerInterface $book_manager
     *   The book manager.
     * @param \Drupal\Core\Entity\EntityStorageInterface $node_storage
     *   The node storage.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, RouteMatchInterface $route_match, BookManagerInterface $book_manager, EntityStorageInterface $node_storage) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->routeMatch = $route_match;
        $this->bookManager = $book_manager;
        $this->nodeStorage = $node_storage;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('current_route_match'), $container->get('book.manager'), $container->get('entity_type.manager')
            ->getStorage('node'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'block_mode' => "all pages",
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockForm($form, FormStateInterface $form_state) {
        $options = [
            'all pages' => $this->t('Show block on all pages'),
            'book pages' => $this->t('Show block only on book pages'),
        ];
        $form['book_block_mode'] = [
            '#type' => 'radios',
            '#title' => $this->t('Book navigation block display'),
            '#options' => $options,
            '#default_value' => $this->configuration['block_mode'],
            '#description' => $this->t("If <em>Show block on all pages</em> is selected, the block will contain the automatically generated menus for all of the site's books. If <em>Show block only on book pages</em> is selected, the block will contain only the one menu corresponding to the current page's book. In this case, if the current page is not in a book, no block will be displayed. The <em>Page specific visibility settings</em> or other visibility settings can be used in addition to selectively display this block."),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockSubmit($form, FormStateInterface $form_state) {
        $this->configuration['block_mode'] = $form_state->getValue('book_block_mode');
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        $current_bid = 0;
        $node = $this->routeMatch
            ->getParameter('node');
        if ($node instanceof NodeInterface && !empty($node->book['bid'])) {
            $current_bid = $node->book['bid'];
        }
        if ($this->configuration['block_mode'] == 'all pages') {
            $book_menus = [];
            $pseudo_tree = [
                0 => [
                    'below' => FALSE,
                ],
            ];
            foreach ($this->bookManager
                ->getAllBooks() as $book_id => $book) {
                if ($book['bid'] == $current_bid) {
                    // If the current page is a node associated with a book, the menu
                    // needs to be retrieved.
                    $data = $this->bookManager
                        ->bookTreeAllData($node->book['bid'], $node->book);
                    $book_menus[$book_id] = $this->bookManager
                        ->bookTreeOutput($data);
                }
                else {
                    // Since we know we will only display a link to the top node, there
                    // is no reason to run an additional menu tree query for each book.
                    $book['in_active_trail'] = FALSE;
                    // Check whether user can access the book link.
                    $book_node = $this->nodeStorage
                        ->load($book['nid']);
                    $book['access'] = $book_node->access('view');
                    $pseudo_tree[0]['link'] = $book;
                    $book_menus[$book_id] = $this->bookManager
                        ->bookTreeOutput($pseudo_tree);
                }
                $book_menus[$book_id] += [
                    '#book_title' => $book['title'],
                ];
            }
            if ($book_menus) {
                return [
                    '#theme' => 'book_all_books_block',
                ] + $book_menus;
            }
        }
        elseif ($current_bid) {
            // Only display this block when the user is browsing a book and do
            // not show unpublished books.
            $nid = \Drupal::entityQuery('node')->accessCheck(TRUE)
                ->condition('nid', $node->book['bid'], '=')
                ->condition('status', NodeInterface::PUBLISHED)
                ->execute();
            // Only show the block if the user has view access for the top-level node.
            if ($nid) {
                $tree = $this->bookManager
                    ->bookTreeAllData($node->book['bid'], $node->book);
                // There should only be one element at the top level.
                $data = array_shift($tree);
                $below = $this->bookManager
                    ->bookTreeOutput($data['below']);
                if (!empty($below)) {
                    return $below;
                }
            }
        }
        return [];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheContexts() {
        return Cache::mergeContexts(parent::getCacheContexts(), [
            'route.book_navigation',
        ]);
    }
    
    /**
     * {@inheritdoc}
     *
     * @todo Make cacheable in https://www.drupal.org/node/2483181
     */
    public function getCacheMaxAge() {
        return 0;
    }

}