class RssFields

class RssFields extends RowPluginBase {
    
    /**
     * Does the row plugin support to add fields to its output.
     *
     * @var bool
     */
    protected $usesFields = TRUE;
    protected function defineOptions() {
        $options = parent::defineOptions();
        $options['title_field'] = [
            'default' => '',
        ];
        $options['link_field'] = [
            'default' => '',
        ];
        $options['description_field'] = [
            'default' => '',
        ];
        $options['creator_field'] = [
            'default' => '',
        ];
        $options['date_field'] = [
            'default' => '',
        ];
        $options['guid_field_options']['contains']['guid_field'] = [
            'default' => '',
        ];
        $options['guid_field_options']['contains']['guid_field_is_permalink'] = [
            'default' => TRUE,
        ];
        return $options;
    }
    public function buildOptionsForm(&$form, FormStateInterface $form_state) {
        parent::buildOptionsForm($form, $form_state);
        $initial_labels = [
            '' => $this->t('- None -'),
        ];
        $view_fields_labels = $this->displayHandler
            ->getFieldLabels();
        $view_fields_labels = array_merge($initial_labels, $view_fields_labels);
        $form['title_field'] = [
            '#type' => 'select',
            '#title' => $this->t('Title field'),
            '#description' => $this->t('The field that is going to be used as the RSS item title for each row.'),
            '#options' => $view_fields_labels,
            '#default_value' => $this->options['title_field'],
            '#required' => TRUE,
        ];
        $form['link_field'] = [
            '#type' => 'select',
            '#title' => $this->t('Link field'),
            '#description' => $this->t('The field that is going to be used as the RSS item link for each row. This must either be an internal unprocessed path like "node/123" or a processed, root-relative URL as produced by fields like "Link to content".'),
            '#options' => $view_fields_labels,
            '#default_value' => $this->options['link_field'],
            '#required' => TRUE,
        ];
        $form['description_field'] = [
            '#type' => 'select',
            '#title' => $this->t('Description field'),
            '#description' => $this->t('The field that is going to be used as the RSS item description for each row.'),
            '#options' => $view_fields_labels,
            '#default_value' => $this->options['description_field'],
            '#required' => TRUE,
        ];
        $form['creator_field'] = [
            '#type' => 'select',
            '#title' => $this->t('Creator field'),
            '#description' => $this->t('The field that is going to be used as the RSS item creator for each row.'),
            '#options' => $view_fields_labels,
            '#default_value' => $this->options['creator_field'],
            '#required' => TRUE,
        ];
        $form['date_field'] = [
            '#type' => 'select',
            '#title' => $this->t('Publication date field'),
            '#description' => $this->t('The field that is going to be used as the RSS item pubDate for each row. It needs to be in RFC 2822 format.'),
            '#options' => $view_fields_labels,
            '#default_value' => $this->options['date_field'],
            '#required' => TRUE,
        ];
        $form['guid_field_options'] = [
            '#type' => 'details',
            '#title' => $this->t('GUID settings'),
            '#open' => TRUE,
        ];
        $form['guid_field_options']['guid_field'] = [
            '#type' => 'select',
            '#title' => $this->t('GUID field'),
            '#description' => $this->t('The globally unique identifier of the RSS item.'),
            '#options' => $view_fields_labels,
            '#default_value' => $this->options['guid_field_options']['guid_field'],
            '#required' => TRUE,
        ];
        $form['guid_field_options']['guid_field_is_permalink'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('GUID is permalink'),
            '#description' => $this->t('The RSS item GUID is a permalink.'),
            '#default_value' => $this->options['guid_field_options']['guid_field_is_permalink'],
        ];
    }
    public function validate() {
        $errors = parent::validate();
        $required_options = [
            'title_field',
            'link_field',
            'description_field',
            'creator_field',
            'date_field',
        ];
        foreach ($required_options as $required_option) {
            if (empty($this->options[$required_option])) {
                $errors[] = $this->t('Row style plugin requires specifying which views fields to use for RSS item.');
                break;
            }
        }
        // Once more for guid.
        if (empty($this->options['guid_field_options']['guid_field'])) {
            $errors[] = $this->t('Row style plugin requires specifying which views fields to use for RSS item.');
        }
        return $errors;
    }
    public function render($row) {
        static $row_index;
        if (!isset($row_index)) {
            $row_index = 0;
        }
        if (function_exists('rdf_get_namespaces')) {
            // Merge RDF namespaces in the XML namespaces in case they are used
            // further in the RSS content.
            $xml_rdf_namespaces = [];
            foreach (rdf_get_namespaces() as $prefix => $uri) {
                $xml_rdf_namespaces['xmlns:' . $prefix] = $uri;
            }
            $this->view->style_plugin->namespaces += $xml_rdf_namespaces;
        }
        // Create the RSS item object.
        $item = new \stdClass();
        $item->title = $this->getField($row_index, $this->options['title_field']);
        $item->link = $this->getAbsoluteUrl($this->getField($row_index, $this->options['link_field']));
        $field = $this->getField($row_index, $this->options['description_field']);
        $item->description = is_array($field) ? $field : [
            '#markup' => $field,
        ];
        $item->elements = [
            [
                'key' => 'pubDate',
                'value' => $this->getField($row_index, $this->options['date_field']),
            ],
            [
                'key' => 'dc:creator',
                'value' => $this->getField($row_index, $this->options['creator_field']),
                'namespace' => [
                    'xmlns:dc' => 'http://purl.org/dc/elements/1.1/',
                ],
            ],
        ];
        $guid_is_permalink_string = 'false';
        $item_guid = $this->getField($row_index, $this->options['guid_field_options']['guid_field']);
        if ($this->options['guid_field_options']['guid_field_is_permalink']) {
            $guid_is_permalink_string = 'true';
            $item_guid = $this->getAbsoluteUrl($item_guid);
        }
        $item->elements[] = [
            'key' => 'guid',
            'value' => $item_guid,
            'attributes' => [
                'isPermaLink' => $guid_is_permalink_string,
            ],
        ];
        $row_index++;
        foreach ($item->elements as $element) {
            if (isset($element['namespace'])) {
                $this->view->style_plugin->namespaces = array_merge($this->view->style_plugin->namespaces, $element['namespace']);
            }
        }
        $build = [
            '#theme' => $this->themeFunctions(),
            '#view' => $this->view,
            '#options' => $this->options,
            '#row' => $item,
            '#field_alias' => isset($this->field_alias) ? $this->field_alias : '',
        ];
        return $build;
    }
    
    /**
     * Retrieves a views field value from the style plugin.
     *
     * @param $index
     *   The index count of the row as expected by views_plugin_style::getField().
     * @param $field_id
     *   The ID assigned to the required field in the display.
     *
     * @return string|null|\Drupal\Component\Render\MarkupInterface
     *   An empty string if there is no style plugin, or the field ID is empty.
     *   NULL if the field value is empty. If neither of these conditions apply,
     *   a MarkupInterface object containing the rendered field value.
     */
    public function getField($index, $field_id) {
        if (empty($this->view->style_plugin) || !is_object($this->view->style_plugin) || empty($field_id)) {
            return '';
        }
        return $this->view->style_plugin
            ->getField($index, $field_id);
    }
    
    /**
     * Convert a rendered URL string to an absolute URL.
     *
     * @param string $url_string
     *   The rendered field value ready for display in a normal view.
     *
     * @return string
     *   A string with an absolute URL.
     */
    protected function getAbsoluteUrl($url_string) {
        // If the given URL already starts with a leading slash, it's been processed
        // and we need to simply make it an absolute path by prepending the host.
        if (strpos($url_string, '/') === 0) {
            $host = \Drupal::request()->getSchemeAndHttpHost();
            // @todo Views should expect and store a leading /.
            // @see https://www.drupal.org/node/2423913
            return $host . $url_string;
        }
        else {
            return Url::fromUserInput('/' . $url_string)->setAbsolute()
                ->toString();
        }
    }

}