/**
* Finds the field name for the field carrying the changed timestamp, if any.
*
* @param \Drupal\Core\Entity\FieldableEntityInterface $entity
* The entity.
*
* @return string|null
* The name of the field found or NULL if not found.
*/
protected function getChangedFieldName(FieldableEntityInterface $entity)
{
foreach ($entity->getFieldDefinitions() as $field) {
if ($field->getType() == 'changed') {
return $field->getName();
}
}
}