TestPageDisplayVariantSubscriber.php

<?php

namespace Drupal\display_variant_test\EventSubscriber;

use Drupal\Core\Plugin\Context\Context;
use Drupal\Core\Plugin\Context\ContextDefinition;
use Drupal\Core\Render\PageDisplayVariantSelectionEvent;
use Drupal\Core\Render\RenderEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Selects the test page display variant.
 */
class TestPageDisplayVariantSubscriber implements EventSubscriberInterface {
    
    /**
     * Selects the page display variant.
     *
     * @param \Drupal\Core\Render\PageDisplayVariantSelectionEvent $event
     *   The event to process.
     */
    public function onSelectPageDisplayVariant(PageDisplayVariantSelectionEvent $event) {
        $event->setPluginId('display_variant_test');
        $event->setPluginConfiguration([
            'required_configuration' => 'A very important, required value.',
        ]);
        $event->addCacheTags([
            'custom_cache_tag',
        ]);
        $context = new Context(new ContextDefinition('string', NULL, TRUE), 'Explicitly passed in context.');
        $event->setContexts([
            'context' => $context,
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() {
        $events[RenderEvents::SELECT_PAGE_DISPLAY_VARIANT][] = [
            'onSelectPageDisplayVariant',
        ];
        return $events;
    }

}