ConfigSingleImportForm::submitForm

public ConfigSingleImportForm::submitForm(array &$form, FormStateInterface $form_state)

public function submitForm(array &$form, FormStateInterface $form_state) {
  // If this form has not yet been confirmed, store the values and rebuild.
  if (!$this->data) {
    $form_state->setRebuild();
    $this->data = $form_state->getValues();
    return;
  }

  /** @var \Drupal\Core\Config\ConfigImporter $config_importer */
  $config_importer = $form_state->get('config_importer');
  if ($config_importer->alreadyImporting()) {
    drupal_set_message($this->t('Another request may be importing configuration already.'), 'error');
  }
  else {
    try {
      $sync_steps = $config_importer->initialize();
      $batch = [
        'operations' => [],
        'finished' => [ConfigSync::class, 'finishBatch'],
        'title' => $this->t('Importing configuration'),
        'init_message' => $this->t('Starting configuration import.'),
        'progress_message' => $this->t('Completed @current step of @total.'),
        'error_message' => $this->t('Configuration import has encountered an error.'),
      ];
      foreach ($sync_steps as $sync_step) {
        $batch['operations'][] = [[ConfigSync::class, 'processBatch'], [$config_importer, $sync_step]];
      }

      batch_set($batch);
    }
    catch (ConfigImporterException $e) {
      // There are validation errors.
      drupal_set_message($this->t('The configuration import failed for the following reasons:'), 'error');
      foreach ($config_importer->getErrors() as $message) {
        drupal_set_message($message, 'error');
      }
    }
  }
}