Flutter：未處理的異常：無效的參數

Question

我正在嘗試發送兩個圖像文件列表，但我有一個錯誤，我不知道為什么會發生這種情況，這是我使用的 function：

Future<dynamic> upload(List<File> imageFile, List<File> backID) async {

    var headers =
        Provider.of<LoginUserProvider>(context, listen: false).httpHeader;
    var uri = Uri.parse(
        "xyz");

    var request = new http.MultipartRequest("POST", uri);
    var requestCopy = new http.MultipartRequest("POST", uri);
    request.headers.addAll(headers);
    //request.headers.addAll(Environment.requestHeaderMedia);
    for (int i = 0; i < uploadedFilesList.length; i++) {
      var length = await imageFile[i].length();
      var stream =
          // ignore: deprecated_member_use
          new http.ByteStream(DelegatingStream.typed(imageFile[i].openRead()));
      var multipartFile = http.MultipartFile(
        'back_id_photos[$i]',
        stream,
        length,
        contentType: MediaType('application', 'x-tar'),
      );
      var multipartFilePhotos = new http.MultipartFile(
        'photos[$i]',
        stream,
        length,
        contentType: MediaType('application', 'x-tar'),
      );
      request.fields['section_id'] = VillaID.toString();
      request.fields['start_date'] = requeststartDate;
      request.fields['end_date'] = requestendDate;
      request.fields['names'] = _nameController.toString();
      request.fields['phones'] = _phoneController.toString();
      request.fields['photos'] = multipartFilePhotos.toString();
      request.fields['back_id_photos'] = multipartFile.toString();
      //contentType: new MediaType('image', 'png'));

      request.files.add(multipartFile);
      request.files.add(multipartFilePhotos);
      var response = await request.send();
      print(response.statusCode);
      response.stream.transform(utf8.decoder).listen((value) {
        print(value);
      });
      var streamedResponse = await requestCopy.send();
      var responses = await http.Response.fromStream(streamedResponse);
      // ignore: unused_local_variable
      final responseData = json.decode(responses.body) as Map<String, dynamic>;
      print(json.decode(responses.body));
      if (response.statusCode == 200 || response.statusCode == 201) {
        return true;
      }
    }
  }

並且在

var response = await request.send();

我正好在點（。）之前有錯誤，這是顯示的錯誤

未處理的異常：無效參數（輸入）：不能是 null 誰能告訴我我該怎么做才能解決這個問題！

Answer 1

如下發送您的多部分文件並使用await http.Multipart因為訪問文件是異步 function並且需要 promise 才能運行。

嘗試這個：

await http.MultipartFile(
    'back_id_photos[$i]',
    stream,
    length,
    contentType: MediaType('application', 'x-tar'),
  );

或者

request.files.add(new http.MultipartFile.fromBytes('file', await File.fromUri("<path/to/file>").readAsBytes(), contentType: new MediaType('image', 'jpeg')))

按照這個鏈接