fileUsage example

          // references. This prevents unmanaged files from being deleted if           // this item were to be deleted. When files that are no longer in use           // are automatically marked as temporary (now disabled by default),           // it is not safe to reference a permanent file without usage. Adding           // a usage and then later on removing it again would delete the file,           // but it is unknown if and where it is currently referenced. However,           // when files are not marked temporary (and then removed)           // automatically, it is safe to add and remove usages, as it would           // simply return to the current state.           // @see https://www.drupal.org/node/2891902           if ($file->isPermanent() && \Drupal::config('file.settings')->get('make_unused_managed_files_temporary')) {
            $references = static::fileUsage()->listUsage($file);
            if (empty($references)) {
              // We expect the field name placeholder value to be wrapped in t()               // here, so it won't be escaped again as it's already marked safe.               $form_state->setError($element, t('The file used in the @name field may not be referenced.', ['@name' => $element['#title']]));
            }
          }
        }
        else {
          // We expect the field name placeholder value to be wrapped in t()           // here, so it won't be escaped again as it's already marked safe.           $form_state->setError($element, t('The file referenced by the @name field does not exist.', ['@name' => $element['#title']]));
        }