resume downloading messes up the file - flutter

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.