class SearchBlock

class SearchBlock extends BlockBase implements ContainerFactoryPluginInterface {
    
    /**
     * The form builder.
     *
     * @var \Drupal\Core\Form\FormBuilderInterface
     */
    protected $formBuilder;
    
    /**
     * The search page repository.
     *
     * @var \Drupal\search\SearchPageRepositoryInterface
     */
    protected $searchPageRepository;
    
    /**
     * Constructs a new SearchLocalTask.
     *
     * @param array $configuration
     *   A configuration array containing information about the plugin instance.
     * @param string $plugin_id
     *   The plugin ID for the plugin instance.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
     *   The form builder.
     * @param \Drupal\search\SearchPageRepositoryInterface $search_page_repository
     *   The search page repository.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, FormBuilderInterface $form_builder, SearchPageRepositoryInterface $search_page_repository) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->formBuilder = $form_builder;
        $this->searchPageRepository = $search_page_repository;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('form_builder'), $container->get('search.search_page_repository'));
    }
    
    /**
     * {@inheritdoc}
     */
    protected function blockAccess(AccountInterface $account) {
        return AccessResult::allowedIfHasPermission($account, 'search content');
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        $page = $this->configuration['page_id'] ?? NULL;
        return $this->formBuilder
            ->getForm(SearchBlockForm::class, $page);
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'page_id' => NULL,
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockForm($form, FormStateInterface $form_state) {
        // The configuration for this block is which search page to connect the
        // form to. Options are all configured/active search pages.
        $options = [];
        $active_search_pages = $this->searchPageRepository
            ->getActiveSearchPages();
        foreach ($this->searchPageRepository
            ->sortSearchPages($active_search_pages) as $entity_id => $entity) {
            $options[$entity_id] = $entity->label();
        }
        $form['page_id'] = [
            '#type' => 'select',
            '#title' => $this->t('Search page'),
            '#description' => $this->t('The search page that the form submits to, or Default for the default search page.'),
            '#default_value' => $this->configuration['page_id'],
            '#options' => $options,
            '#empty_option' => $this->t('Default'),
            '#empty_value' => '',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockSubmit($form, FormStateInterface $form_state) {
        // Handle the #empty_value: using the default requires specifying `null` in
        // the config.
        // @see search.schema.yml
        // @see \Drupal\search\Form\SearchBlockForm::buildForm()
        $this->configuration['page_id'] = $form_state->getValue('page_id') ?: NULL;
    }

}