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.
  }

}