未来方法调用 api 有时 returns null 有时不

Question

在该方法中，输出如下：

I/flutter ( 2928): 200

I/flutter ( 2928): 空

I/flutter ( 2928): ['Images' 实例]

这导致 snapshot.data 在我的 FutureBuilder 中为空，因为 well.Any 为什么会这样？

Future<List<Images>> getData( File f ) async {

List<Images> list;

// String link = "https://clothest.herokuapp.com/";
String link ="https://us-central1-velvety-rookery-274308.cloudfunctions.net/function-1";
var stream = new http.ByteStream(DelegatingStream.typed(f.openRead()));
var length = await f.length();
var postUri = Uri.parse(link);
var request = new http.MultipartRequest("POST", postUri);
var multipartFileSign = new http.MultipartFile('File', stream, length,
    filename: basename(f.path));
request.files.add(multipartFileSign);
request.headers.addAll({"content-type": "application/json"});
var response = await request.send();

print(response.statusCode); //200 OK

if (response.statusCode == 200){
  response.stream.transform(utf8.decoder).listen((value) {

          var data = json.decode(value);
          var rest = data["Items"] as List;

          list = rest.map<Images>((json) => Images.fromJson(json)).toList();

          print(list.toString());


  });

}
print(list.toString());
return list;

}

Answer 1

所以我设法通过使用另一个包而不是 HTTP 包来解决这个问题。 Dios 包对于 HTTP 请求是最好的。更新代码：

Future<List<Images>> getData( File f ) async {
  List<Images> lst=[];
  String link ="https://us-central1-velvety-rookery-274308.cloudfunctions.net/function-1";
  Dio dio = new Dio();
  FormData formdata = new FormData(); // just like JS
  formdata.add('File', new UploadFileInfo(f, basename(f.path)));
  await dio.post(link, data: formdata, options: Options(
    method: 'POST',
    responseType: ResponseType.JSON // or ResponseType.JSON
)).then((response) {
  print(response);
  if(response.statusCode==200){ //OK
     var responseBody = response.data;  //Map
     var rest = responseBody["Items"] as List;
     //print(rest);
     lst = rest.map<Images>((json) => Images.fromJson(json)).toList();   //instances of 'Images' object
     //print(lst);
  }
}



).catchError((error) => print(error));

return lst;

未来方法调用 api 有时 returns null 有时不

Future method calling api sometimes returns null sometimes not

api

future

flutter