getSubPathname example

$file = $fileinfo->openFile('r');
        while (!$type && !$file->eof()) {
          preg_match('@^type:\s*(\'|")?(\w+)\1?\s*(?:\#.*)?$@', $file->fgets(), $matches);
          if (isset($matches[2])) {
            $type = $matches[2];
          }
        }
        if (empty($type)) {
          continue;
        }
        $name = $fileinfo->getBasename('.info.yml');
        $pathname = $dir_prefix . $fileinfo->getSubPathname();

        // Determine whether the extension has a main extension file.         // For theme engines, the file extension is .engine.         if ($type == 'theme_engine') {
          $filename = $name . '.engine';
        }
        // For profiles/modules/themes, it is the extension type.         else {
          $filename = $name . '.' . $type;
        }
        if (!file_exists($this->root . '/' . dirname($pathname) . '/' . $filename)) {