File uploading in Flutter/Dart

fileupload.dart

import 'dart:async';
import 'dart:convert';
import 'dart:core';
import 'dart:io';
import 'package:http/http.dart' as http;

typedef OnUploadProgressCallback = void Function(int sentBytes, int totalBytes);

Future<String> fileUpload({@required File file, OnUploadProgressCallback onUploadProgress}) async {
    assert(file != null);
    final HttpClient httpClient = getHttpClient();
    final Uri uri = Uri.https('api.4d.customers.cloud54.de', '/users/avatar');
    final HttpClientRequest request = await httpClient.postUrl(uri);
    int byteCount = 0;
    final http.MultipartFile multipart = await http.MultipartFile.fromPath('file', file.path);
    final http.MultipartRequest requestMultipart = http.MultipartRequest('', Uri.parse('uri'));
    requestMultipart.files.add(multipart);
    final http.ByteStream msStream = requestMultipart.finalize();
    final int totalByteLength = requestMultipart.contentLength;
    request.contentLength = totalByteLength;
    request.headers.set(HttpHeaders.contentTypeHeader, requestMultipart.headers[HttpHeaders.contentTypeHeader]);
    request.headers.set('Authorization', 'Bearer ${my_client.HttpClient.token}');
    final Stream<List<int>> streamUpload = msStream.transform(
      StreamTransformer<List<int>, List<int>>.fromHandlers(
        handleData: (List<int> data, EventSink<List<int>> sink) {
          sink.add(data);
          byteCount += data.length;
          if (onUploadProgress != null) {
            onUploadProgress(byteCount, totalByteLength);
          }
        },
        handleError: (Object error, StackTrace stack, EventSink<List<int>> sink) {
          throw error;
        },
        handleDone: (EventSink<List<int>> sink) {
          sink.close();
        }
      )
    );
    await request.addStream(streamUpload);
    final HttpClientResponse httpResponse = await request.close();
    final int statusCode = httpResponse.statusCode;
    if (statusCode ~/ 100 != 2) {
      throw Exception('Error uploading file, Status code: ${httpResponse.statusCode}');
    } else {
      return await readResponseAsString(httpResponse);
    }
  }

 static Future<String> readResponseAsString(HttpClientResponse response) {
    final Completer<String> completer = Completer<String>();
    final StringBuffer contents = StringBuffer();
    response.transform(utf8.decoder).listen((String data) {
      contents.write(data);
    }, onDone: () => completer.complete(contents.toString()));
    return completer.future;
  }