DioError [DioErrorType.DEFAULT]: Converting object to an encodable object failed: Instance of 'FormData' in Flutter

Question

I want to upload my image to the server using the Dio package. I see on the Dio docs that to upload file using form data is like below:

var formData = FormData.fromMap({
  'name': 'wendux',
  'age': 25,
  'file': await MultipartFile.fromFile('./text.txt',filename: 'upload.txt')
});
response = await dio.post('/info', data: formData);

After that, I have a function to send the image to the API. And this is my function:

Future<CommonResponse> submitDocumentSpaj(SubmitDocumentRequest data) async {
    final fileName = data.file.path.split('/').last;
    final formData = FormData.fromMap({
      'multipartFile':
          await MultipartFile.fromFile(data.file.path, filename: fileName),
      'path_id': 9,
      'subFolder': '${data.subFolder}',
      'lus_id': 1,
      'username': '${data.username}',
      'type': data.loginType,
      'filename': '$fileName',
      'no_temp': data.noTemp,
      'last_index': data.lastFile,
    });
    print('multipartFile --> ${data.file.path}');
    print('path_id --> 9');
    print('subFolder --> ${data.subFolder}');
    print('lus_id --> 1');
    print('username --> ${data.username}');
    print('type --> ${data.loginType}');
    print('filename --> $fileName');
    print('no_temp --> ${data.noTemp}');
    print('last_index --> ${data.lastFile}');
    final response = await dio.post(
      'my_url',
      data: formData,
      options: Options(
        contentType:
            'multipart/form-data; boundary=<calculated when request is sent>',
        receiveTimeout: 60000,
        sendTimeout: 60000,
      ),
    );
    return CommonResponse.fromJson(response.data);
  }

I check the data is not null, but I get the error like below:

flutter: Dio Response Error --> DioError [DioErrorType.DEFAULT]: Converting object to an encodable object failed: Instance of 'FormData'
flutter: #0      _JsonStringifier.writeObject (dart:convert/json.dart:687:7)
flutter: #1      _JsonStringStringifier.printOn (dart:convert/json.dart:876:17)
flutter: #2      _JsonStringStringifier.stringify (dart:convert/json.dart:861:5)
flutter: #3      JsonEncoder.convert (dart:convert/json.dart:261:30)
flutter: #4      JsonCodec.encode (dart:convert/json.dart:171:45)
flutter: #5      jsonEncode (dart:convert/json.dart:81:10)
flutter: #6      LoggingInterceptors.onResponse (package:core/network/logging_interceptors.dart:36:12)
flutter: #7      DioMixin._request._interceptorWrapper.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:dio/src/dio.dart:849:37)
flutter: #8      DioMixin.checkIfNeedEnqueue (package:dio/src/dio.dart:1121:22)
flutter: #9      DioMixin._request._interceptorWrapper.<anonymous closure>.<anonymous closure> (package:dio/src/dio.dart:846:22)
flutter: #10     new Future.<anonymous closure> (dart:async/future.dart:175:37)
flutter: #11     _rootRun (dart:async/zone.dart:1182:47)
flutter: #12     _CustomZone.run (dart:async/zone.dart:1093:19)
flutter: #13     _CustomZone.runGuarded (dart:async/zone.dart:997:7)
flutter: #14     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1037:23)
flutter: #15     _rootRun (dart:async/zone.dart:1190:13)
flutter: #16     _CustomZone.run (dart:async/zone.dart:1093:19)
flutter: #17     _CustomZone.bindCallback.<anonymous closure> (dart:async/zone.dart:1021:23)
flutter: #18     Timer._createTimer.<anonymous closure> (dart:async-patch/timer_patch.dart:18:15)
flutter: #19     _Timer._runTimers (dart:isolate-patch/timer_impl.dart:397:19)
flutter: #20     _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:428:5)
flutter: #21     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

I already search for the error in google and StackOverflow, but still not fixed. Can anyone know why I must do to fix my error? Because I believe my code is the same as in the docs in dio, but the difference is I add custom headers like my API and also add custom RTO.

Headers in API:

And I already check via Postman with the same data and file as from the Flutter, it's success upload to the server, but from Flutter getting an error.

Answer 1

For loading the image I use image_picker package. This is my code:

TextButton(
  onPressed: () async {
    Dio dio = Dio();
    var response;

    var multiPartFile = List<MultipartFile>();
    multiPartFile.add(MultipartFile.fromFileSync(_image.path,
        filename: "image.jpg"));
    var formData = FormData.fromMap({
      "files": multiPartFile,
    });
    response = await dio.post(URI, data: formData);
    print('RESPONSE: $response');
  },
  child: Text("POST"),
),

You may also try sending image as base64 :

final pickedFile = await picker.getImage(source: ImageSource.camera,maxHeight: 480,maxWidth: 640);
List<int> imageBytes = await pickedFile.readAsBytes();
String base64Image = base64Encode(imageBytes);

Dio dio = Dio();
var response;

var formData = FormData.fromMap({
  "image": '$base64Image',
});
response = await dio.post(URI, data: formData);