Flutter await does not await until return of function - flutter

Question

I tried to find an answer but my problem is still there.
In my asynchronous upload function I return at the and the generated name of the image, which I want to use to make my database request.
This is my upload function:
Future<String> upload(File imageFile) async {
var stream =
new http.ByteStream(DelegatingStream.typed(imageFile.openRead()));
// get file length
var length = await imageFile.length();
var uri = Uri.parse("http://localhost:8080/upload");
var request = new http.MultipartRequest("POST", uri);
var multipartFile = new http.MultipartFile('file', stream, length,
filename: basename(imageFile.path));
request.files.add(multipartFile);
var response = await request.send();
print(response.statusCode);
var createdFileName = "";
response.stream.transform(utf8.decoder).listen((value) {
createdFileName = value;
print(createdFileName);
});
return createdFileName;
}
I call it like this:
List createdFileNames = [];
for (var e in imagefiles) {
createdFileNames.add(await upload(File(e)));
}
I don't know why, but the createdFileNames are ["",""], but the upload gives as result the right name. In debug mode I can see, that the loop does not wait until the upload has finished.
Do you have any suggestions?
Thank you very much!

Answer 1

response.stream.transform(utf8.decoder).listen((value) {
createdFileName = value;
print(createdFileName);
});
This part in your function is asynchronous, it uses a callback.
But you don't wait for it to finish in any form. You just continue to return the createdFileName, that by that time most likely has not been filled.
I don't know what your stream looks like, if you only need the first value, you could await that instead of listening:
createdFileName = await response.stream.transform(utf8.decoder).first;

Answer 2

Replace
response.stream.transform(utf8.decoder).listen((value) {
createdFileName = value;
print(createdFileName);
});
with
createdFileName=await response.stream.bytesToString();

Answer 3

change code
for (var e in imagefiles) {
upload(File(e)).then((value) => createdFileNames.add(value));
}