TestMultipleFormController.php

<?php

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

use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Form\FormState;

/**
 * Controller for block_test module.
 */
class TestMultipleFormController extends ControllerBase {
  public function testMultipleForms() {
    $form_state = new FormState();
    $build = [
      'form1' => $this->formBuilder()
        ->buildForm('\\Drupal\\block_test\\Form\\TestForm', $form_state),
      'form2' => $this->formBuilder()
        ->buildForm('\\Drupal\\block_test\\Form\\FavoriteAnimalTestForm', $form_state),
    ];
    // Output all attached placeholders trough
    // \Drupal\Core\Messenger\MessengerInterface::addMessage(), so we can
    // see if there's only one in the tests.
    $post_render_callable = function ($elements) {
      $matches = [];
      preg_match_all('<form\\s(.*?)action="(.*?)"(.*)>', (string) $elements, $matches);
      $action_values = $matches[2];
      foreach ($action_values as $action_value) {
        $this->messenger()
          ->addStatus('Form action: ' . $action_value);
      }
      return $elements;
    };
    $build['#post_render'] = [
      $post_render_callable,
    ];
    return $build;
  }

}