class ChangedItem

class ChangedItem extends CreatedItem {

  /**
   * {@inheritdoc}
   */
  public function preSave() {
    parent::preSave();

    // Set the timestamp to request time if it is not set.
    if (!$this->value) {
      $this->value = \Drupal::time()
        ->getRequestTime();
    }
    else {

      // On an existing entity translation, the changed timestamp will only be
      // set to the request time automatically if at least one other field value
      // of the entity has changed. This detection does not run on new entities
      // and will be turned off if the changed timestamp is set manually before
      // save, for example during migrations or by using
      // \Drupal\content_translation\ContentTranslationMetadataWrapperInterface::setChangedTime().

      /** @var \Drupal\Core\Entity\ContentEntityInterface $entity */
      $entity = $this
        ->getEntity();

      /** @var \Drupal\Core\Entity\ContentEntityInterface $original */
      $original = $entity->original;
      $langcode = $entity
        ->language()
        ->getId();
      if (!$entity
        ->isNew() && $original && $original
        ->hasTranslation($langcode)) {
        $original_value = $original
          ->getTranslation($langcode)
          ->get($this
          ->getFieldDefinition()
          ->getName())->value;
        if ($this->value == $original_value && $entity
          ->hasTranslationChanges()) {
          $this->value = \Drupal::time()
            ->getRequestTime();
        }
      }
    }
  }

}