class SystemManager

class SystemManager {
    
    /**
     * Module handler service.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * The request stack.
     *
     * @var \Symfony\Component\HttpFoundation\RequestStack
     */
    protected $requestStack;
    
    /**
     * The menu link tree manager.
     *
     * @var \Drupal\Core\Menu\MenuLinkTreeInterface
     */
    protected $menuTree;
    
    /**
     * The active menu trail service.
     *
     * @var \Drupal\Core\Menu\MenuActiveTrailInterface
     */
    protected $menuActiveTrail;
    
    /**
     * A static cache of menu items.
     *
     * @var array
     */
    protected $menuItems;
    
    /**
     * Requirement severity -- Requirement successfully met.
     */
    const REQUIREMENT_OK = 0;
    
    /**
     * Requirement severity -- Warning condition; proceed but flag warning.
     */
    const REQUIREMENT_WARNING = 1;
    
    /**
     * Requirement severity -- Error condition; abort installation.
     */
    const REQUIREMENT_ERROR = 2;
    
    /**
     * Constructs a SystemManager object.
     *
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler.
     * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
     *   The request stack.
     * @param \Drupal\Core\Menu\MenuLinkTreeInterface $menu_tree
     *   The menu tree manager.
     * @param \Drupal\Core\Menu\MenuActiveTrailInterface $menu_active_trail
     *   The active menu trail service.
     */
    public function __construct(ModuleHandlerInterface $module_handler, RequestStack $request_stack, MenuLinkTreeInterface $menu_tree, MenuActiveTrailInterface $menu_active_trail) {
        $this->moduleHandler = $module_handler;
        $this->requestStack = $request_stack;
        $this->menuTree = $menu_tree;
        $this->menuActiveTrail = $menu_active_trail;
    }
    
    /**
     * Checks for requirement severity.
     *
     * @return bool
     *   Returns the status of the system.
     */
    public function checkRequirements() {
        $requirements = $this->listRequirements();
        return $this->getMaxSeverity($requirements) == static::REQUIREMENT_ERROR;
    }
    
    /**
     * Displays the site status report. Can also be used as a pure check.
     *
     * @return array
     *   An array of system requirements.
     */
    public function listRequirements() {
        // Load .install files
        include_once DRUPAL_ROOT . '/core/includes/install.inc';
        drupal_load_updates();
        // Check run-time requirements and status information.
        $requirements = $this->moduleHandler
            ->invokeAll('requirements', [
            'runtime',
        ]);
        $this->moduleHandler
            ->alter('requirements', $requirements);
        uasort($requirements, function ($a, $b) {
            if (!isset($a['weight'])) {
                if (!isset($b['weight'])) {
                    return strcasecmp($a['title'], $b['title']);
                }
                return -$b['weight'];
            }
            return isset($b['weight']) ? $a['weight'] - $b['weight'] : $a['weight'];
        });
        return $requirements;
    }
    
    /**
     * Extracts the highest severity from the requirements array.
     *
     * @param $requirements
     *   An array of requirements, in the same format as is returned by
     *   hook_requirements().
     *
     * @return int
     *   The highest severity in the array.
     */
    public function getMaxSeverity(&$requirements) {
        $severity = static::REQUIREMENT_OK;
        foreach ($requirements as $requirement) {
            if (isset($requirement['severity'])) {
                $severity = max($severity, $requirement['severity']);
            }
        }
        return $severity;
    }
    
    /**
     * Loads the contents of a menu block.
     *
     * This function is often a destination for these blocks.
     * For example, 'admin/structure/types' needs to have a destination to be
     * valid in the Drupal menu system, but too much information there might be
     * hidden, so we supply the contents of the block.
     *
     * @return array
     *   A render array suitable for
     *   \Drupal\Core\Render\RendererInterface::render().
     */
    public function getBlockContents() {
        // We hard-code the menu name here since otherwise a link in the tools menu
        // or elsewhere could give us a blank block.
        $link = $this->menuActiveTrail
            ->getActiveLink('admin');
        if ($link && ($content = $this->getAdminBlock($link))) {
            $output = [
                '#theme' => 'admin_block_content',
                '#content' => $content,
            ];
        }
        else {
            $output = [
                '#markup' => t('You do not have any administrative items.'),
            ];
        }
        return $output;
    }
    
    /**
     * Provide a single block on the administration overview page.
     *
     * @param \Drupal\Core\Menu\MenuLinkInterface $instance
     *   The menu item to be displayed.
     *
     * @return array
     *   An array of menu items, as expected by admin-block-content.html.twig.
     */
    public function getAdminBlock(MenuLinkInterface $instance) {
        $content = [];
        // Only find the children of this link.
        $link_id = $instance->getPluginId();
        $parameters = new MenuTreeParameters();
        $parameters->setRoot($link_id)
            ->excludeRoot()
            ->setTopLevelOnly()
            ->onlyEnabledLinks();
        $tree = $this->menuTree
            ->load(NULL, $parameters);
        $manipulators = [
            [
                'callable' => 'menu.default_tree_manipulators:checkAccess',
            ],
            [
                'callable' => 'menu.default_tree_manipulators:generateIndexAndSort',
            ],
        ];
        $tree = $this->menuTree
            ->transform($tree, $manipulators);
        foreach ($tree as $key => $element) {
            // Only render accessible links.
            if (!$element->access
                ->isAllowed()) {
                // @todo Bubble cacheability metadata of both accessible and
                //   inaccessible links. Currently made impossible by the way admin
                //   blocks are rendered.
                continue;
            }
            
            /** @var \Drupal\Core\Menu\MenuLinkInterface $link */
            $link = $element->link;
            $content[$key]['title'] = $link->getTitle();
            $content[$key]['options'] = $link->getOptions();
            $content[$key]['description'] = $link->getDescription();
            $content[$key]['url'] = $link->getUrlObject();
        }
        ksort($content);
        return $content;
    }

}