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,
        ]);
    }

}