class IncrementalForm

class IncrementalForm extends MigrateUpgradeFormBase {

  /**
   * The date formatter service.
   *
   * @var \Drupal\Core\Datetime\DateFormatterInterface
   */
  protected $dateFormatter;

  /**
   * IncrementalForm constructor.
   *
   * @param \Drupal\Core\State\StateInterface $state
   *   The state service.
   * @param \Drupal\Core\Datetime\DateFormatterInterface $date_formatter
   *   The date formatter service.
   * @param \Drupal\Core\TempStore\PrivateTempStoreFactory $tempstore_private
   *   The private tempstore factory service.
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The config factory service.
   * @param \Drupal\migrate\Plugin\MigrationPluginManagerInterface $migration_plugin_manager
   *   The migration plugin manager service.
   */
  public function __construct(StateInterface $state, DateFormatterInterface $date_formatter, PrivateTempStoreFactory $tempstore_private, ConfigFactoryInterface $config_factory, MigrationPluginManagerInterface $migration_plugin_manager) {
    parent::__construct($config_factory, $migration_plugin_manager, $state, $tempstore_private);
    $this->dateFormatter = $date_formatter;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container
      ->get('state'), $container
      ->get('date.formatter'), $container
      ->get('tempstore.private'), $container
      ->get('config.factory'), $container
      ->get('plugin.manager.migration'));
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'migrate_drupal_ui_incremental_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

    // Get all the data needed for this form.
    $date_performed = $this->state
      ->get('migrate_drupal_ui.performed');

    // If data is missing or this is the wrong step, start over.
    if (!$date_performed || $this->store
      ->get('step') != 'incremental') {
      return $this
        ->restartUpgradeForm();
    }
    $form = parent::buildForm($form, $form_state);
    $form['#title'] = $this
      ->t('Upgrade');

    // @todo Add back support for rollbacks.
    //   https://www.drupal.org/node/2687849
    $form['upgrade_option_item'] = [
      '#type' => 'item',
      '#prefix' => $this
        ->t('An upgrade has already been performed on this site. To perform a new migration, create a clean and empty new install of Drupal @version. Rollbacks are not yet supported through the user interface. For more information, see the <a href=":url">upgrading handbook</a>.', [
        '@version' => $this->destinationSiteVersion,
        ':url' => 'https://www.drupal.org/upgrade/migrate',
      ]),
      '#description' => $this
        ->t('Last upgrade: @date', [
        '@date' => $this->dateFormatter
          ->format($date_performed),
      ]),
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->store
      ->set('step', 'credential');
    $form_state
      ->setRedirect('migrate_drupal_ui.upgrade_credential');
  }

  /**
   * {@inheritdoc}
   */
  public function getConfirmText() {
    return $this
      ->t('Import new configuration and content from old site');
  }

}