d8-calendar-ror/src/Controller/CalendarRorController.php

<?php
namespace Drupal\calendar_ror\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Template\TwigEnvironment;
use Drupal\node\NodeInterface;

class CalendarRorController extends ControllerBase {
    protected $state;
    protected $twig;
    public function __construct($state, TwigEnvironment $twig) {
        $this->state = $state;
        $this->twig = $twig;
    }

    public static function create(ContainerInterface $container) {
        return new static(
            $container->get('state'),
            $container->get('twig')
        );
    }

    private function query() {
        $query = \Drupal::entityQuery('node');
        $query
            ->condition('status', '1')
            ->condition('type', 'eventi')
            ->sort('nid', 'DESC')
            ->range(0, 20);
        $nids = $query->execute();
        $data = array();
        $nodes_e = \Drupal\node\Entity\Node::loadMultiple($nids);
        $data['nodes'] =  array();
        foreach($nodes_e as $nid => $node) {
            $na = $node->toArray();
            if(count($na['body']) > 0) { // c'e' gente che pubblica articoli senza body
                $body = $na['body'][0]['value'];
            } else {
                $body = '';
            }
            $data['nodes'][$nid] =  $na;
            $data['nodes'][$nid]['summary'] = htmlspecialchars(substr(
                html_entity_decode(strip_tags($body)),
                0, 3500), ENT_XML1, 'UTF-8');
        }

        return $data;
    }
    public function ics() {
        // TODO: use $path
        $template = $this->twig->loadTemplate(drupal_get_path('module', 'calendar_ror') . '/templates/calendar.ics.twig');
        $tmpl_data = [ 'ror' => array('calendar' => $this->query(),
            'url' => \Drupal::service('path.current')->getPath(),
        )];
        //$variables['ror']['url'] = 
        $xml = $template->render($tmpl_data);
        $resp = new Response($xml, 200, array( 'Content-Type' => 'application/rss+xml'));
        return $resp;
    }
}