ServerEndpointController.php

<?php

declare (strict_types=1);
namespace Drupal\sdc_test\Controller;


/**
 * An endpoint to serve a component during tests.
 *
 * @internal
 */
final class ServerEndpointController {
    
    /**
     * Render an arbitrary render array.
     */
    public function renderArray() : array {
        $render_array = \Drupal::state()->get('sdc_test_component', [
            '#markup' => 'Set your component in state using the sdc_test_component key.',
        ]);
        return [
            '#type' => 'container',
            '#cache' => [
                'max-age' => 0,
            ],
            // Magic wrapper ID to pull the HTML from.
'#attributes' => [
                'id' => 'sdc-wrapper',
            ],
            'component' => $render_array,
        ];
    }

}