asp.net-core - flutter websocket连接问题

Question

我正在尝试开发一个 flutter 应用程序，它连接到服务器并使用 websocket 交换数据。服务器位于.Net Core中，并使用Asp.Net Core Websockets来实现此功能。

我面临的问题是，我的 flutter 应用程序无法连接到服务器并引发以下错误。

E/flutter (31498): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter (31498): WebSocketChannelException: WebSocketChannelException: WebSocketException: Connection to 'http://127.0.0.1/client#' was not upgraded to websocket
E/flutter (31498): #0      new IOWebSocketChannel._withoutSocket.<anonymous closure> (package:web_socket_channel/io.dart:83:24)
E/flutter (31498): #1      _invokeErrorHandler (dart:async/async_error.dart:13:29)
E/flutter (31498): #2      _HandleErrorStream._handleError (dart:async/stream_pipe.dart:286:9)
E/flutter (31498): #3      _ForwardingStreamSubscription._handleError (dart:async/stream_pipe.dart:168:13)
E/flutter (31498): #4      _RootZone.runBinaryGuarded (dart:async/zone.dart:1328:10)
E/flutter (31498): #5      _BufferingStreamSubscription._sendError.sendError (dart:async/stream_impl.dart:350:15)
E/flutter (31498): #6      _BufferingStreamSubscription._sendError (dart:async/stream_impl.dart:367:16)
E/flutter (31498): #7      _BufferingStreamSubscription._addError (dart:async/stream_impl.dart:266:7)
E/flutter (31498): #8      _SyncStreamController._sendError (dart:async/stream_controller.dart:767:19)
E/flutter (31498): #9      _StreamController._addError (dart:async/stream_controller.dart:647:7)
E/flutter (31498): #10     _RootZone.runBinaryGuarded (dart:async/zone.dart:1328:10)
E/flutter (31498): #11     _BufferingStreamSubscription._sendError.sendError (dart:async/stream_impl.dart:350:15)
E/flutter (31498): #12     _BufferingStreamSubscription._sendError (dart:async/stream_impl.dart:367:16)
E/flutter (31498): #13     _BufferingStreamSubscription._addError (dart:async/stream_impl.dart:266:7)
E/flutter (31498): #14     _SyncStreamController._sendError (dart:async/stream_controller.dart:767:19)
E/flutter (31498): #15     _StreamController._addError (dart:async/stream_controller.dart:647:7)
E/flutter (31498): #16     new Stream.fromFuture.<anonymous closure> (dart:async/stream.dart:110:18)
E/flutter (31498): #17     _RootZone.runBinary (dart:async/zone.dart:1386:54)
E/flutter (31498): #18     _FutureListener.handleError (dart:async/future_impl.dart:141:20)
E/flutter (31498): #19     Future._propagateToListeners.handleError (dart:async/future_impl.dart:649:47)
E/flutter (31498): #20     Future._propagateToListeners (dart:async/future_impl.dart:670:24)
E/flutter (31498): #21     Future._completeError (dart:async/future_impl.dart:489:5)
E/flutter (31498): #22     Future._asyncCompleteError.<anonymous closure> (dart:async/future_impl.dart:537:7)
E/flutter (31498): #23     _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
E/flutter (31498): #24     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)

重要的是，该应用程序能够连接到其他 websocket 服务器。它还可以连接到我开发的其他原型(prototype)服务器，以便在同一台机器上进行测试。

.Net core 服务器也非常实用，我可以轻松地将浏览器连接到服务器并与其通信。

仅当我尝试将 flutter 应用程序连接到 .Net core websocket 服务器时，问题才会出现。

我正在使用默认模板应用程序进行测试。这是我的 MyHomePage 类的代码

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final channel = new IOWebSocketChannel.connect("ws://192.168.1.139/client");

这是 _MyHomePageState 实现

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

initState() {
        super.initState();
        widget.channel.stream.listen(this.onData, onError: onError, onDone: onDone);

    (() async {
      setState(() {

      });
    });
  }

  onDone(){
    debugPrint("Socket is closed");
  }

  onError(err){
    debugPrint(err.runtimeType.toString());
    WebSocketChannelException ex = err;
    debugPrint(ex.message);
  }

  onData(event){
    debugPrint(event);
  }

总是会导致 onError 并输出以下内容

I/flutter ( 4608): WebSocketChannelException
I/flutter ( 4608): WebSocketChannelException: WebSocketException: Connection to 'http://18.217.117.92/client#' was not upgraded to websocket
I/flutter ( 4608): Socket is closed

我知道它与 dart websocket 包如何与服务器执行握手有关，但无法查明问题。

任何人都可以弄清楚这里到底出了什么问题吗？

Answer 1

我希望这会对很多人有所帮助，尽管这个问题不再相关。问题出在 .Net core websocket 实现上。

通过一些研究并分析 Wireshark 日志后，我可以意识到 .Net core 期望以特定格式的 websocket 协议(protocol)更新 header (即实现区分大小写)，这对于 .Net core Framework 2.1 来说是正确的。不过，这个问题在下一个版本中立即得到了解决。我切换到测试版，问题得到解决。我就此咨询了 Flutter 开发人员以及 .Net core 团队，他们也证实了这一点。

.Net core的当前版本(在回答这个问题时是3.0，没有这样的问题。