flutter - Flutter/Dart-为什么我的Future返回Null？

Question

我想返回从Http Post的response.body中提取的字符串。但是我的代码返回null而不是字符串。如果将字符串oldname放在request.send()的范围内，则可以正常打印，但是在调用uploadAudio方法时需要将其返回。我究竟做错了什么？

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

我认为您的混淆来自您混合使用await和then()的事实。我建议您总体上保持一个概念。
我已经重写了您的代码，因此它现在到处都使用await(也将其清理了一下):

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;
}

如您所见，现在不需要所有嵌套的 then()方法，现在更容易阅读代码。