class DisplayTest

class DisplayTest extends DisplayPluginBase {
  
  /**
   * Whether the display allows attachments.
   *
   * @var bool
   */
  protected $usesAttachments = TRUE;
  
  /**
   * {@inheritdoc}
   */
  public function getType() {
    return 'test';
  }
  
  /**
   * {@inheritdoc}
   */
  protected function defineOptions() {
    $options = parent::defineOptions();
    $options['test_option'] = [
      'default' => '',
    ];
    return $options;
  }
  
  /**
   * {@inheritdoc}
   */
  public function optionsSummary(&$categories, &$options) {
    parent::optionsSummary($categories, $options);
    $categories['display_test'] = [
      'title' => $this->t('Display test settings'),
      'column' => 'second',
      'build' => [
        '#weight' => -100,
      ],
    ];
    $test_option = $this->getOption('test_option') ?: $this->t('Empty');
    $options['test_option'] = [
      'category' => 'display_test',
      'title' => $this->t('Test option'),
      'value' => Unicode::truncate($test_option, 24, FALSE, TRUE),
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildOptionsForm(&$form, FormStateInterface $form_state) {
    parent::buildOptionsForm($form, $form_state);
    switch ($form_state->get('section')) {
      case 'test_option':
        $form['#title'] .= $this->t('Test option');
        $form['test_option'] = [
          '#title' => $this->t('Test option'),
          '#type' => 'textfield',
          '#description' => $this->t('This is a textfield for test_option.'),
          '#default_value' => $this->getOption('test_option'),
        ];
        break;

    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateOptionsForm(&$form, FormStateInterface $form_state) {
    parent::validateOptionsForm($form, $form_state);
    \Drupal::logger('views')->notice($form_state->getValue('test_option'));
    switch ($form_state->get('section')) {
      case 'test_option':
        if (!trim($form_state->getValue('test_option'))) {
          $form_state->setError($form['test_option'], $this->t('You cannot have an empty option.'));
        }
        break;

    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitOptionsForm(&$form, FormStateInterface $form_state) {
    parent::submitOptionsForm($form, $form_state);
    switch ($form_state->get('section')) {
      case 'test_option':
        $this->setOption('test_option', $form_state->getValue('test_option'));
        break;

    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function execute() {
    $this->view
      ->build();
    $render = $this->view
      ->render();
    // Render the test option as the title before the view output.
    $render['#prefix'] = '<h1>' . Xss::filterAdmin($this->options['test_option']) . '</h1>';
    return $render;
  }
  
  /**
   * {@inheritdoc}
   */
  public function preview() {
    return $this->execute();
  }
  
  /**
   * {@inheritdoc}
   */
  public function calculateDependencies() {
    return parent::calculateDependencies() + [
      'content' => [
        'DisplayTest',
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function validate() {
    $errors = parent::validate();
    foreach ($this->view->displayHandlers as $display_handler) {
      $errors[] = 'error';
    }
    return $errors;
  }

}