cached method
Check the given url is cached or not.
Implementation
Future<bool> cached(String url) async {
temporaryDirectory ??= await getTemporaryDirectory();
final hash = md5.convert(utf8.encode(url)).toString();
String filePath;
if (prefix != null) {
filePath = path.join(temporaryDirectory!.path, identifier, prefix, hash);
} else {
filePath = path.join(temporaryDirectory!.path, identifier, hash);
}
final file = File(filePath);
return file.exists();
}