class SystemMenuBlock

class SystemMenuBlock extends BlockBase implements ContainerFactoryPluginInterface {
  
  /**
   * The menu link tree service.
   *
   * @var \Drupal\Core\Menu\MenuLinkTreeInterface
   */
  protected $menuTree;
  
  /**
   * The active menu trail service.
   *
   * @var \Drupal\Core\Menu\MenuActiveTrailInterface
   */
  protected $menuActiveTrail;
  
  /**
   * Constructs a new SystemMenuBlock.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin ID for the plugin instance.
   * @param array $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Menu\MenuLinkTreeInterface $menu_tree
   *   The menu tree service.
   * @param \Drupal\Core\Menu\MenuActiveTrailInterface $menu_active_trail
   *   The active menu trail service.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, MenuLinkTreeInterface $menu_tree, MenuActiveTrailInterface $menu_active_trail) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->menuTree = $menu_tree;
    $this->menuActiveTrail = $menu_active_trail;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($configuration, $plugin_id, $plugin_definition, $container->get('menu.link_tree'), $container->get('menu.active_trail'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $config = $this->configuration;
    $defaults = $this->defaultConfiguration();
    $form['menu_levels'] = [
      '#type' => 'details',
      '#title' => $this->t('Menu levels'),
      // Open if not set to defaults.
'#open' => $defaults['level'] !== $config['level'] || $defaults['depth'] !== $config['depth'],
      '#process' => [
        [
          self::class,
          'processMenuLevelParents',
        ],
      ],
    ];
    $options = range(0, $this->menuTree
      ->maxDepth());
    unset($options[0]);
    $form['menu_levels']['level'] = [
      '#type' => 'select',
      '#title' => $this->t('Initial visibility level'),
      '#default_value' => $config['level'],
      '#options' => $options,
      '#description' => $this->t('The menu is only visible if the menu link for the current page is at this level or below it. Use level 1 to always display this menu.'),
      '#required' => TRUE,
    ];
    $options[0] = $this->t('Unlimited');
    $form['menu_levels']['depth'] = [
      '#type' => 'select',
      '#title' => $this->t('Number of levels to display'),
      '#default_value' => $config['depth'],
      '#options' => $options,
      '#description' => $this->t('This maximum number includes the initial level.'),
      '#required' => TRUE,
    ];
    $form['menu_levels']['expand_all_items'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Expand all menu links'),
      '#default_value' => !empty($config['expand_all_items']),
      '#description' => $this->t('Override the option found on each menu link used for expanding children and instead display the whole menu tree as expanded.'),
    ];
    return $form;
  }
  
  /**
   * Form API callback: Processes the menu_levels field element.
   *
   * Adjusts the #parents of menu_levels to save its children at the top level.
   */
  public static function processMenuLevelParents(&$element, FormStateInterface $form_state, &$complete_form) {
    array_pop($element['#parents']);
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    $this->configuration['level'] = $form_state->getValue('level');
    $this->configuration['depth'] = $form_state->getValue('depth');
    $this->configuration['expand_all_items'] = $form_state->getValue('expand_all_items');
  }
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    $menu_name = $this->getDerivativeId();
    if ($this->configuration['expand_all_items']) {
      $parameters = new MenuTreeParameters();
      $active_trail = $this->menuActiveTrail
        ->getActiveTrailIds($menu_name);
      $parameters->setActiveTrail($active_trail);
    }
    else {
      $parameters = $this->menuTree
        ->getCurrentRouteMenuTreeParameters($menu_name);
    }
    // Adjust the menu tree parameters based on the block's configuration.
    $level = $this->configuration['level'];
    $depth = $this->configuration['depth'];
    $parameters->setMinDepth($level);
    // When the depth is configured to zero, there is no depth limit. When depth
    // is non-zero, it indicates the number of levels that must be displayed.
    // Hence this is a relative depth that we must convert to an actual
    // (absolute) depth, that may never exceed the maximum depth.
    if ($depth > 0) {
      $parameters->setMaxDepth(min($level + $depth - 1, $this->menuTree
        ->maxDepth()));
    }
    // For menu blocks with start level greater than 1, only show menu items
    // from the current active trail. Adjust the root according to the current
    // position in the menu in order to determine if we can show the subtree.
    if ($level > 1) {
      if (count($parameters->activeTrail) >= $level) {
        // Active trail array is child-first. Reverse it, and pull the new menu
        // root based on the parent of the configured start level.
        $menu_trail_ids = array_reverse(array_values($parameters->activeTrail));
        $menu_root = $menu_trail_ids[$level - 1];
        $parameters->setRoot($menu_root)
          ->setMinDepth(1);
        if ($depth > 0) {
          $parameters->setMaxDepth(min($level - 1 + $depth - 1, $this->menuTree
            ->maxDepth()));
        }
      }
      else {
        return [];
      }
    }
    $tree = $this->menuTree
      ->load($menu_name, $parameters);
    $manipulators = [
      [
        'callable' => 'menu.default_tree_manipulators:checkAccess',
      ],
      [
        'callable' => 'menu.default_tree_manipulators:generateIndexAndSort',
      ],
    ];
    $tree = $this->menuTree
      ->transform($tree, $manipulators);
    return $this->menuTree
      ->build($tree);
  }
  
  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'level' => 1,
      'depth' => 0,
      'expand_all_items' => FALSE,
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getCacheTags() {
    // Even when the menu block renders to the empty string for a user, we want
    // the cache tag for this menu to be set: whenever the menu is changed, this
    // menu block must also be re-rendered for that user, because maybe a menu
    // link that is accessible for that user has been added.
    $cache_tags = parent::getCacheTags();
    $cache_tags[] = 'config:system.menu.' . $this->getDerivativeId();
    return $cache_tags;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getCacheContexts() {
    // ::build() uses MenuLinkTreeInterface::getCurrentRouteMenuTreeParameters()
    // to generate menu tree parameters, and those take the active menu trail
    // into account. Therefore, we must vary the rendered menu by the active
    // trail of the rendered menu.
    // Additional cache contexts, e.g. those that determine link text or
    // accessibility of a menu, will be bubbled automatically.
    $menu_name = $this->getDerivativeId();
    return Cache::mergeContexts(parent::getCacheContexts(), [
      'route.menu_active_trails:' . $menu_name,
    ]);
  }

}