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');
    }

}