resume downloading messes up the file

Question

i'm trying to download a large file and be able to resume the downloading where it left off,

i'm currently using this code to achieve this

//downloadUntil is a large number (~10GB) so that if we don't specify
//our own number the http will automatically trim it to the file size.

downloadFile(String url, {int downloadFrom=0, int downloadUntil=99999999999 }) async {
  var httpClient = http.Client();
  var request = new http.Request('GET', Uri.parse(url));
  
  //we will add Range to the header, downloadFrom will be zero
  //if the file is being downloaded for the first time,
  //and it will be a different number if we're resuming the download.
  request.headers.addAll({'Range': 'bytes=$downloadFrom-$downloadUntil'});
  var response = httpClient.send(request);

  //create a RandomAccessFile object so that we can write the chunks to the file.
  RandomAccessFile raf = await File(downloadPath).open(mode: FileMode.write);

  //this variable will keep track of the downloaded bytes,
  //we use it to offset the file when writing to it.
  int downloaded = downloadFrom;

  response.asStream().listen((http.StreamedResponse r) {
    r.stream.listen((List<int> chunk) async {
      //print the percentage
      debugPrint('downloadPercentage: ${downloaded / r.contentLength * 100}');

      //offsetting the file
      raf.setPositionSync(downloaded);
      //then write the downloaded chunk to the file
      raf.writeFromSync(chunk);
 
      //update the downloaded variable
      downloaded += chunk.length;
    }, onDone: () async {
      debugPrint('download completed');
      return;
    });
  });
}

the file works fine when for example i have a 150MB video file and download only 50MB by executing this code:-

downloadFile("https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4", downloadUntil: 50000000//~50MB);

the video file works fine, i can watch the downloaded part, everything works as intended.

then i execute this code to download the remaining bytes:-

downloadFile("https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4", downloadFrom: 50000000//~50MB);

then the video file doesn't work anymore, it gets unseekable stream,

i also tried to download the file by downloadFrom:49999900 and replace some of the downloaded bytes to avoid skipping any bytes, also tried downloadFrom: 50000001 to avoid replacing any downloaded bytes, none of them worked.

what am i doing wrong here?

Answer 1

You are opening the file with FileMode.write which overwrites all the existing bytes saved by the previous download. Change it to FileMode.append and you are good to go.