Uploading with FormData throws a TypeError: type 'List<int>' is not a subtype of type 'Uint8List' of 'value'

[fromlabs]

Package

dio

Version

5.4.3+1

Operating-System

Android, iOS

Adapter

Default Dio

Output of flutter doctor -v

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[✓] Connected device (5 available)
    • iPhone di Roberto Tassi (mobile) • 00008101-00111C3A0190001E            • ios            • iOS 17.5.1 21F90
    • iPhone 15 Pro Max (mobile)       • 27013218-0307-4598-9E9D-1ADA200AC39C • ios            • com.apple.CoreSimulator.SimRuntime.iOS-17-5 (simulator)
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[✓] Network resources
    • All expected network resources are available.

• No issues found!

Dart Version

No response

Steps to Reproduce

final formData = dio.FormData();
// fileData is not a Uint8List!
List<int> fileData = ...;
formData.files.add(MapEntry(
  'file',
  dio.MultipartFile.fromBytes(
    fileData,
    filename: fileName,
    contentType: contentType,
  ),
));

Expected Result

No error

Actual Result

flutter: Uncaught Error: type 'List<int>' is not a subtype of type 'Uint8List' of 'value'
flutter: Stacktrace: #0      _StreamController.add (dart:async/stream_controller.dart:604:14)
flutter: #1      _rootRunUnary (dart:async/zone.dart:1407:47)
flutter: #2      _CustomZone.runUnary (dart:async/zone.dart:1308:19)
flutter: #3      _CustomZone.runUnaryGuarded (dart:async/zone.dart:1217:7)
flutter: #4      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:365:11)
flutter: #5      _BufferingStreamSubscription._add (dart:async/stream_impl.dart:297:7)
flutter: #6      _MultiStreamController.addSync (dart:async/stream_impl.dart:1127:36)
flutter: #7      new Stream.fromIterable.<anonymous closure>.next (dart:async/stream.dart:375:26)
flutter: #8      _rootRun (dart:async/zone.dart:1399:13)
flutter: #9      _CustomZone.run (dart:async/zone.dart:1301:19)
flutter: #10     _CustomZone.runGuarded (dart:async/zone.dart:1209:7)
flutter: #11     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1249:23)
flutter: #12     _microtaskLoop (dart:async/schedule_microtask.dart:40:21)
flutter: #13     _startMicrotaskLoop (dart:async/schedule_microtask.dart:49:5)

[fromlabs]

I think is related to:

• error: ../../../.pub-cache/hosted/pub.dev/dio-5.4.3/lib/src/form_data.dart:175:54: Error: The argument type 'List<int>' can't be assigned to the parameter type 'Uint8List'. #2184
• Running Gradle task 'assembleProfile'.. ../../../.pub-cache/hosted/pub.dev/dio-5.4.3/lib/src/ form_data.dart:175:54: Error: The argument type 'List <int>' can't be assigned to the parameter type 'Uint8list'. #2186

I suppose the problem is in form_data.dart:

final controller = StreamController<Uint8List>(sync: false);

...

Future<void>(() async {
  for (final file in files) {
    writeUtf8('--$boundary$_rn');
    writeUtf8(_headerForFile(file));
    await writeStreamToSink(file.value.finalize(), controller);
    writeLine();
  }
}).then((_) {
  writeUtf8('--$boundary--$_rn');
}).whenComplete(() {
  controller.close();
});

multipart_file.dart
Stream<List<int>> finalize();

Calling writeStreamToSink(Stream<List<int>>, StreamController<Uint8List>) throws a TypeError because is trying to add List<int> to a Uint8List sink.

[AlexV525]

@fromlabs Could you try updating the implementation to below solve the problem?

Future<void>(() async {
  for (final file in files) {
    writeUtf8('--$boundary$_rn');
    writeUtf8(_headerForFile(file));
    await writeStreamToSink<Uint8List>(
      file.value
          .finalize()
          .map((e) => e is Uint8List ? e : Uint8List.fromList(e)),
      controller,
    );
    writeLine();
  }
}).then((_) {
  writeUtf8('--$boundary--$_rn');
}).whenComplete(() {
  controller.close();
});

[fromlabs]

Yes I've already tried your solution and it works.

Meanwhile I found a workaround on my code:

dio.MultipartFile.fromBytes(
  Uint8List.fromList(fileData),
  filename: fileName,
  contentType: contentType,
),

Thanks @AlexV525!

[AlexV525]

@fromlabs Are you able to extract a minimal reproducible example? I'm wondering why the issue (if it exists) escapes from our tests.

[fromlabs]

I think because formdata_test.dart doesn't test directly MultipartFile.fromBytes with real List<int>, but tests just MultipartFile.fromString, MultipartFile.fromFile and MultipartFile.fromFileSync which work under the hood with Uint8List.

[AlexV525]

Huge thanks for your inputs!