class SearchBlockForm

class SearchBlockForm extends FormBase {
    
    /**
     * The search page repository.
     *
     * @var \Drupal\search\SearchPageRepositoryInterface
     */
    protected $searchPageRepository;
    
    /**
     * The config factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $configFactory;
    
    /**
     * The renderer.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $renderer;
    
    /**
     * Constructs a new SearchBlockForm.
     *
     * @param \Drupal\search\SearchPageRepositoryInterface $search_page_repository
     *   The search page repository.
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer.
     */
    public function __construct(SearchPageRepositoryInterface $search_page_repository, ConfigFactoryInterface $config_factory, RendererInterface $renderer) {
        $this->searchPageRepository = $search_page_repository;
        $this->configFactory = $config_factory;
        $this->renderer = $renderer;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('search.search_page_repository'), $container->get('config.factory'), $container->get('renderer'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'search_block_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, $entity_id = NULL) {
        // Set up the form to submit using GET to the correct search page.
        if (!$entity_id) {
            $entity_id = $this->searchPageRepository
                ->getDefaultSearchPage();
            // SearchPageRepository::getDefaultSearchPage() depends on
            // search.settings.  The dependency needs to be added before the
            // conditional return, otherwise the block would get cached without the
            // necessary cacheability metadata in case there is no default search page
            // and would not be invalidated if that changes.
            $this->renderer
                ->addCacheableDependency($form, $this->configFactory
                ->get('search.settings'));
        }
        if (!$entity_id) {
            $form['message'] = [
                '#markup' => $this->t('Search is currently disabled'),
            ];
            return $form;
        }
        $route = 'search.view_' . $entity_id;
        $form['#action'] = Url::fromRoute($route)->toString();
        $form['#method'] = 'get';
        $form['keys'] = [
            '#type' => 'search',
            '#title' => $this->t('Search'),
            '#title_display' => 'invisible',
            '#size' => 15,
            '#default_value' => '',
            '#attributes' => [
                'title' => $this->t('Enter the terms you wish to search for.'),
            ],
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Search'),
            // Prevent op from showing up in the query string.
'#name' => '',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // This form submits to the search page, so processing happens there.
    }

}