class SiteMaintenanceModeForm

class SiteMaintenanceModeForm extends ConfigFormBase {
    
    /**
     * The state keyvalue collection.
     *
     * @var \Drupal\Core\State\StateInterface
     */
    protected $state;
    
    /**
     * The permission handler.
     *
     * @var \Drupal\user\PermissionHandlerInterface
     */
    protected $permissionHandler;
    
    /**
     * Constructs a new SiteMaintenanceModeForm.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The factory for configuration objects.
     * @param \Drupal\Core\State\StateInterface $state
     *   The state keyvalue collection to use.
     * @param \Drupal\user\PermissionHandlerInterface $permission_handler
     *   The permission handler.
     */
    public function __construct(ConfigFactoryInterface $config_factory, StateInterface $state, PermissionHandlerInterface $permission_handler) {
        parent::__construct($config_factory);
        $this->state = $state;
        $this->permissionHandler = $permission_handler;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('config.factory'), $container->get('state'), $container->get('user.permissions'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'system_site_maintenance_mode';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'system.maintenance',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $config = $this->config('system.maintenance');
        $permissions = $this->permissionHandler
            ->getPermissions();
        $permission_label = $permissions['access site in maintenance mode']['title'];
        $form['maintenance_mode'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Put site into maintenance mode'),
            '#default_value' => $this->state
                ->get('system.maintenance_mode'),
            '#description' => $this->t('Visitors will only see the maintenance mode message. Only users with the "@permission-label" <a href=":permissions-url">permission</a> will be able to access the site. Authorized users can log in directly via the <a href=":user-login">user login</a> page.', [
                '@permission-label' => $permission_label,
                ':permissions-url' => Url::fromRoute('user.admin_permissions')->toString(),
                ':user-login' => Url::fromRoute('user.login')->toString(),
            ]),
        ];
        $form['maintenance_mode_message'] = [
            '#type' => 'textarea',
            '#title' => $this->t('Message to display when in maintenance mode'),
            '#default_value' => $config->get('message'),
        ];
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->config('system.maintenance')
            ->set('message', $form_state->getValue('maintenance_mode_message'))
            ->save();
        $this->state
            ->set('system.maintenance_mode', $form_state->getValue('maintenance_mode'));
        parent::submitForm($form, $form_state);
    }

}