SignedUrlResult class sealed

Represents a per-item result from StorageFileApi.createSignedUrlsResult.

Use exhaustive pattern matching to handle both outcomes:

for (final result in results) {
  switch (result) {
    case SignedUrlSuccess(:final signedUrl):
      print('URL: $signedUrl');
    case SignedUrlFailure(:final error):
      print('Missing: $error');
  }
}