Flutter/Dart - Why does my Future return Null?

Question

I want to return a string extracted from the response.body of an Http Post. But my code returns null instead of the string. The string oldname will print just fine if I put it in the scope of request.send() but I need it to be returned when calling the uploadAudio method. What am I doing wrong?

Future<String> uploadAudio({String currentuserid, String filepath}) async {

  String oldname; 
  final serverurl = "http://example.com/audiofile.php";

  var request = http.MultipartRequest('POST', Uri.parse(serverurl));
  request.fields['userid'] = currentuserid;    

  var multiPartFile = await http.MultipartFile.fromPath("audio", filepath,
      contentType: MediaType("audio", "mp4"));
        request.files.add(multiPartFile);
        request.send().then((result) async {
          http.Response.fromStream(result).then((response) {
                if (response.statusCode == 200) {
                  String serverResponse = response.body;
                  const start = "gxz";
                  const end = "zxg";
                  final startIndex = serverResponse.indexOf(start);
                  final endIndex = serverResponse.indexOf(end, startIndex + start.length);
                  oldname = serverResponse.substring(startIndex + start.length, endIndex);
                }
          });
   });
    print oldname;
    return oldname;
  }

Answer 1

I think your confusion comes from the fact you are mixing the use of await and then(). I will recommend you are staying with one concept in general.

I have rewritten your code so it is now using await everywhere (also cleaned it a little bit up):

Future<String> uploadAudio({String currentuserid, String filepath}) async {
  const serverurl = "http://example.com/audiofile.php";

  final request = http.MultipartRequest('POST', Uri.parse(serverurl))
    ..fields['userid'] = currentuserid;

  final multiPartFile = await http.MultipartFile.fromPath("audio", filepath,
      contentType: MediaType("audio", "mp4"));

  request.files.add(multiPartFile);

  final response = await http.Response.fromStream(await request.send());
  String oldname;

  if (response.statusCode == 200) {
    final serverResponse = response.body;
    const start = "gxz";
    const end = "zxg";
    final startIndex = serverResponse.indexOf(start);
    final endIndex = serverResponse.indexOf(end, startIndex + start.length);
    oldname = serverResponse.substring(startIndex + start.length, endIndex);
  }

  print(oldname);
  return oldname;
}

As you can see, the code are much easier to read now without all that nested then() methods.