ConfigSingleImportForm::validateForm

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

public function validateForm(array &$form, FormStateInterface $form_state) {
  // The confirmation step needs no additional validation.
  if ($this->data) {
    return;
  }

  try {
    // Decode the submitted import.
    $data = Yaml::decode($form_state->getValue('import'));
  }
  catch (InvalidDataTypeException $e) {
    $form_state->setErrorByName('import', $this->t('The import failed with the following message: %message', ['%message' => $e->getMessage()]));
  }

  // Validate for config entities.
  if ($form_state->getValue('config_type') !== 'system.simple') {
    $definition = $this->entityManager->getDefinition($form_state->getValue('config_type'));
    $id_key = $definition->getKey('id');

    // If a custom entity ID is specified, override the value in the
    // configuration data being imported.
    if (!$form_state->isValueEmpty('custom_entity_id')) {
      $data[$id_key] = $form_state->getValue('custom_entity_id');
    }

    $entity_storage = $this->entityManager->getStorage($form_state->getValue('config_type'));
    // If an entity ID was not specified, set an error.
    if (!isset($data[$id_key])) {
      $form_state->setErrorByName('import', $this->t('Missing ID key "@id_key" for this @entity_type import.', array('@id_key' => $id_key, '@entity_type' => $definition->getLabel())));
      return;
    }

    $config_name = $definition->getConfigPrefix() . '.' . $data[$id_key];
    // If there is an existing entity, ensure matching ID and UUID.
    if ($entity = $entity_storage->load($data[$id_key])) {
      $this->configExists = $entity;
      if (!isset($data['uuid'])) {
        $form_state->setErrorByName('import', $this->t('An entity with this machine name already exists but the import did not specify a UUID.'));
        return;
      }
      if ($data['uuid'] !== $entity->uuid()) {
        $form_state->setErrorByName('import', $this->t('An entity with this machine name already exists but the UUID does not match.'));
        return;
      }
    }
    // If there is no entity with a matching ID, check for a UUID match.
    elseif (isset($data['uuid']) && $entity_storage->loadByProperties(array('uuid' => $data['uuid']))) {
      $form_state->setErrorByName('import', $this->t('An entity with this UUID already exists but the machine name does not match.'));
    }
  }
  else {
    $config_name = $form_state->getValue('config_name');
    $config = $this->config($config_name);
    $this->configExists = !$config->isNew() ? $config : FALSE;
  }

  // Use ConfigImporter validation.
  if (!$form_state->getErrors()) {
    $source_storage = new StorageReplaceDataWrapper($this->configStorage);
    $source_storage->replaceData($config_name, $data);
    $storage_comparer = new StorageComparer(
    $source_storage, 
    $this->configStorage, 
    $this->configManager
    );

    if (!$storage_comparer->createChangelist()->hasChanges()) {
      $form_state->setErrorByName('import', $this->t('There are no changes to import.'));
    }
    else {
      $config_importer = new ConfigImporter(
      $storage_comparer, 
      $this->eventDispatcher, 
      $this->configManager, 
      $this->lock, 
      $this->typedConfigManager, 
      $this->moduleHandler, 
      $this->moduleInstaller, 
      $this->themeHandler, 
      $this->getStringTranslation()
      );

      try {
        $config_importer->validate();
        $form_state->set('config_importer', $config_importer);
      }
      catch (ConfigImporterException $e) {
        // There are validation errors.
        $item_list = [
          '#theme' => 'item_list',
          '#items' => $config_importer->getErrors(),
          '#title' => $this->t('The configuration cannot be imported because it failed validation for the following reasons:'),
        ];
        $form_state->setErrorByName('import', $this->renderer->render($item_list));
      }
    }
  }

  // Store the decoded version of the submitted import.
  $form_state->setValueForElement($form['import'], $data);
}