class BatchTestMultiStepForm

class BatchTestMultiStepForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'batch_test_multistep_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $step = $form_state->get('step');
    if (empty($step)) {
      $step = 1;
      $form_state->set('step', $step);
    }
    $form['step_display'] = [
      '#markup' => 'step ' . $step . '<br/>',
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => 'Submit',
    ];
    // This is a POST form with multiple steps that does not transition from one
    // step to the next via POST requests, but via GET requests, because it uses
    // Batch API to advance through the steps.
    $form['#cache']['max-age'] = 0;
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    batch_test_stack(NULL, TRUE);
    $step = $form_state->get('step');
    switch ($step) {
      case 1:
        batch_set(_batch_test_batch_1());
        break;

      case 2:
        batch_set(_batch_test_batch_2());
        break;

    }
    if ($step < 2) {
      $form_state->set('step', ++$step);
      $form_state->setRebuild();
    }
    $form_state->setRedirect('batch_test.redirect');
  }

}