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();
        $displayHandlersCount = count($this->view->displayHandlers);
        for ($i = 0; $i < $displayHandlersCount; $i++) {
            $errors[] = 'error';
        }
        return $errors;
    }

}