Opml.php

<?php

namespace Drupal\views\Plugin\views\style;

use Drupal\Core\Url;

/**
 * Default style plugin to render an OPML feed.
 *
 * @ingroup views_style_plugins
 *
 * @ViewsStyle(
 *   id = "opml",
 *   title = @Translation("OPML Feed"),
 *   help = @Translation("Generates an OPML feed from a view."),
 *   theme = "views_view_opml",
 *   display_types = {"feed"}
 * )
 */
class Opml extends StylePluginBase {
    
    /**
     * {@inheritdoc}
     */
    protected $usesRowPlugin = TRUE;
    
    /**
     * {@inheritdoc}
     */
    public function attachTo(array &$build, $display_id, Url $feed_url, $title) {
        $display = $this->view->displayHandlers
            ->get($display_id);
        $url_options = [];
        $input = $this->view
            ->getExposedInput();
        if ($input) {
            $url_options['query'] = $input;
        }
        $url_options['absolute'] = TRUE;
        $url = $feed_url->setOptions($url_options)
            ->toString();
        if ($display->hasPath()) {
            if (empty($this->preview)) {
                $build['#attached']['feed'][] = [
                    $url,
                    $title,
                ];
            }
        }
        else {
            $this->view->feedIcons[] = [
                '#theme' => 'feed_icon',
                '#url' => $url,
                '#title' => $title,
            ];
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function render() {
        $rows = [];
        foreach ($this->view->result as $row_index => $row) {
            $this->view->row_index = $row_index;
            $rows[] = $this->view->rowPlugin
                ->render($row);
        }
        $build = [
            '#theme' => $this->themeFunctions(),
            '#view' => $this->view,
            '#options' => $this->options,
            '#rows' => $rows,
            '#attached' => [
                'http_header' => [
                    [
                        'Content-Type',
                        'text/xml; charset=utf-8',
                    ],
                ],
            ],
        ];
        unset($this->view->row_index);
        return $build;
    }

}