View source  
  <?php
namespace Drupal\form_test\Form;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
class FormTestRebuildPreserveValuesForm extends FormBase {
  
  public function getFormId() {
    return 'form_test_form_rebuild_preserve_values_form';
  }
  
  public function buildForm(array $form, FormStateInterface $form_state) {
    
    $form = array(
      'checkbox_1_default_off' => array(
        '#type' => 'checkbox',
        '#title' => t('This checkbox defaults to unchecked'),
        '#default_value' => FALSE,
      ),
      'checkbox_1_default_on' => array(
        '#type' => 'checkbox',
        '#title' => t('This checkbox defaults to checked'),
        '#default_value' => TRUE,
      ),
      'text_1' => array(
        '#type' => 'textfield',
        '#title' => t('This textfield has a non-empty default value.'),
        '#default_value' => 'DEFAULT 1',
      ),
    );
    
    if (!$form_state
      ->has('add_more')) {
      $form['add_more'] = array(
        '#type' => 'submit',
        '#value' => 'Add more',
        '#submit' => array(
          '::addMoreSubmitForm',
        ),
      );
    }
    else {
      $form += array(
        'checkbox_2_default_off' => array(
          '#type' => 'checkbox',
          '#title' => t('This checkbox defaults to unchecked'),
          '#default_value' => FALSE,
        ),
        'checkbox_2_default_on' => array(
          '#type' => 'checkbox',
          '#title' => t('This checkbox defaults to checked'),
          '#default_value' => TRUE,
        ),
        'text_2' => array(
          '#type' => 'textfield',
          '#title' => t('This textfield has a non-empty default value.'),
          '#default_value' => 'DEFAULT 2',
        ),
      );
    }
    
    $form['submit'] = array(
      '#type' => 'submit',
      '#value' => 'Submit',
    );
    return $form;
  }
  
  public function addMoreSubmitForm(array &$form, FormStateInterface $form_state) {
    
    $form_state
      ->set('add_more', TRUE);
    $form_state
      ->setRebuild();
  }
  
  public function submitForm(array &$form, FormStateInterface $form_state) {
    
    drupal_set_message(t('Form values: %values', array(
      '%values' => var_export($form_state
        ->getValues(), TRUE),
    )));
  }
}