Flutter 如何将请求字段正确添加到DART多部分请求

我有一个Flutter 应用程序，在其中我通常使用http请求从模型到Go API的请求发布，如下所示.

if (dateRequired != null) {
  data['date_required'] = dateRequired?.formatDbDate();
} else {
  data['date_required'] = null;
}

然而，我想同时上传文件和解码json数据.上传文件如下所示，但我从下面的代码中得到的结果是 或者(null)，它会在我的API中抛出一个解码错误.我原以为是{"date_required": null}或{"date_required": 2023-11-12}.

我怎么才能到那里呢？我注意到DART http使用Map<String, dynamic>，而DART MultipartRequest使用Map<String, String>，所以我可以使用NULL.这将需要在我的Go API中进一步解析.

Future<dynamic> uploadFile(
      BuildContext context, User user, String dateRequiredString) async {
    final navigator = Navigator.of(context);

    const storage = FlutterSecureStorage();

    String? token = await storage.read(key: 'jwt');

    Map<String, String> headers = {
      "Authorization": "Bearer $token",
    };

    final bytes = file?.bytes;

    if (file == null || bytes == null) return;

    final multipartFile =
        http.MultipartFile.fromBytes('file', bytes, filename: file?.name);

    final request = http.MultipartRequest('POST', Uri.parse(user.fileUrl));

    request.files.add(multipartFile);

    request.headers.addAll(headers);

    request.fields['date_required'] = dateRequiredString;

生成字段的DART代码，根据@mandy8055的答案进行编辑.

                                if (_formKey.currentState!.validate()) {
                                  if (_formKey
                                          .currentState
                                          ?.fields['date_required']
                                          ?.value !=
                                      null) {
                                    dateRequired = _formKey.currentState
                                        ?.fields['date_required']!.value;
                                  }

                                  Map<String, dynamic> data = {
                                    'date_required': dateRequired != null
                                        ? dateRequired!.formatDbDate()
                                        : null
                                  };

                                  print(data);

                                  Map<String, String> dateRequiredString =
                                      jsonEncode(data)
                                          as Map<String, String>;

                                  print(dateRequiredString);

这将产生以下控制台输出

{date_required: null}
Error: Expected a value of type 'Map<String, String>', but got one of type 'String'

进一步编辑:

                            _formKey.currentState?.save();

                            if (_formKey.currentState!.validate()) {
                              if (_formKey
                                      .currentState
                                      ?.fields['date_required']
                                      ?.value !=
                                  null) {
                                dateRequired = _formKey.currentState
                                    ?.fields['date_required']!.value;
                              }

                              Map<String, dynamic> data = {
                                'date_required': dateRequired != null
                                    ? dateRequired!.formatDbDate()
                                    : null
                              };

                              String jsonString = jsonEncode(data);

                              Map<String, dynamic> decodedMap =
                                  jsonDecode(jsonString);

                              Map<String, String> dateRequiredString =
                                  decodedMap.cast<String, String>();

                              print(data);

                              print(dateRequiredString);

                              uploadFile(
                                  context, user, dateRequiredString);
                            }

这将抛出下面的错误(这里的日期应该为空).我看到 keys 没有被引用，但我认为它应该被引用.

{date_required: null}
Error: Expected a value of type 'String', but got one of type 'Null'

以下是演示该问题的一些工作代码.我需要将Map<String, dynamic>转换为Map<String, String>，并保留"DATE_REQUIRED"键周围的引号.

import 'dart:convert';

void main() {
  Map<String, dynamic> data = {'date_required': null};

  print(data);

  String jsonString = jsonEncode(data);

  print(jsonString);

  Map<String, dynamic> decodedMap = jsonDecode(jsonString);

  print(decodedMap);

  Map<String, String> dateRequiredString = decodedMap.cast<String, String>();

  print(dateRequiredString);
}

控制台输出:

{date_required: null}
{"date_required":null}
{date_required: null}
: TypeError: null: type 'JSNull' is not a subtype of type 'String'Error: TypeError: null: type 'JSNull' is not a subtype of type 'String'