class SearchPageForm

class SearchPageForm extends FormBase implements WorkspaceSafeFormInterface {

  /**
   * The search page entity.
   *
   * @var \Drupal\search\SearchPageInterface
   */
  protected $entity;

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'search_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, SearchPageInterface $search_page = NULL) {
    $this->entity = $search_page;
    $plugin = $this->entity
      ->getPlugin();
    $form_state
      ->set('search_page_id', $this->entity
      ->id());
    $form['basic'] = [
      '#type' => 'container',
      '#attributes' => [
        'class' => [
          'container-inline',
        ],
      ],
    ];
    $form['basic']['keys'] = [
      '#type' => 'search',
      '#title' => $this
        ->t('Enter your keywords'),
      '#default_value' => $plugin
        ->getKeywords(),
      '#size' => 30,
      '#maxlength' => 255,
    ];

    // processed_keys is used to coordinate keyword passing between other forms
    // that hook into the basic search form.
    $form['basic']['processed_keys'] = [
      '#type' => 'value',
      '#value' => '',
    ];
    $form['basic']['submit'] = [
      '#type' => 'submit',
      '#value' => $this
        ->t('Search'),
    ];
    $form['help_link'] = [
      '#type' => 'link',
      '#url' => new Url('search.help_' . $this->entity
        ->id()),
      '#title' => $this
        ->t('About searching'),
      '#options' => [
        'attributes' => [
          'class' => 'search-help-link',
        ],
      ],
    ];

    // Allow the plugin to add to or alter the search form.
    $plugin
      ->searchFormAlter($form, $form_state);
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {

    // Redirect to the search page with keywords in the GET parameters.
    // Plugins with additional search parameters will need to provide their
    // own form submit handler to replace this, so they can put their values
    // into the GET as well. If so, make sure to put 'keys' into the GET
    // parameters so that the search results generation is triggered.
    $query = $this->entity
      ->getPlugin()
      ->buildSearchUrlQuery($form_state);
    $route = 'search.view_' . $form_state
      ->get('search_page_id');
    $form_state
      ->setRedirect($route, [], [
      'query' => $query,
    ]);
  }

}