class SearchPageForm

class SearchPageForm extends FormBase {
    
    /**
     * 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,
        ]);
    }

}