Question

在下面的代码中，Connect()似乎工作正常。但是webSocket.ReceiveAsync()中的ReceiveMessage()似乎在服务器没有提供任何内容的时候设置cancelToken.IsCancellationRequested = true和webSocket.State = Aborted。与此同时，相同的ReceiveAsync()调用会引发一个（无用的）OperationCanceledException，没有内部异常。

我使用了我认为是相同的WebSocket交互而没有这个问题的同一服务器，但使用更简单的异步代码。直到我尝试将它转换成一个可观察到我有这个问题。

它被Rx取消了，还是取消了？我发现在.NET WebSockets中通过未观察到的异常（http://www.salmanq.com/blog/5-things-you-probably-didnt-know-about-net-websockets/2013/04/）引起了这样的事情，但在添加TaskScheduler.UnobservedTaskException += ExceptionLogger之后我找不到任何证据。

有什么想法吗？

public static IObservable<string> GetWebSocketStream(Uri uri)
{
    var buffer = new byte[2048];
    return Observable
        .Using(cancelToken => Connect(uri, cancelToken),
            (webSocket, cancelToken) => Task.FromResult(
                Observable.Defer(
                    () => ReceiveMessage(webSocket, buffer, cancelToken).ToObservable()
                ).Repeat()
            )
        ).Publish().RefCount();
}

private static async Task<ClientWebSocket> Connect(Uri uri, CancellationToken token)
{
    var webSocket = new ClientWebSocket();
    await webSocket.ConnectAsync(uri, token).ConfigureAwait(false);
    return webSocket;
}

private static async Task<string> ReceiveMessage(ClientWebSocket webSocket, byte[] buffer, CancellationToken cancelToken)
{
    var segment = new ArraySegment<byte>(buffer);
    var message = new StringBuilder();
    WebSocketReceiveResult result;
    do
    {
        result = await webSocket.ReceiveAsync(segment, cancelToken).ConfigureAwait(false);
        message.Append(Encoding.UTF8.GetString(segment.Array, 0, result.Count));
    } while (!cancelToken.IsCancellationRequested && !result.EndOfMessage);
    return message.ToString();
}

编辑：为了澄清，我的问题/问题可能与BOTH WebSockets和Rx有关。以下类似的代码在同一设备上保持打开状态并接收数据：

public static void Test() {
    var ts = new CancellationTokenSource();
    AsyncTest(new Uri("ws://server/test/"), ts.Token).Wait();
}

public static async Task AsyncTest(Uri uri, CancellationToken token) {
    var socket = await GetConnectedWebSocket(uri, token);
    var buffer = new byte[4000];
    while (!token.IsCancellationRequested)
    {
        try
        {
            var msg = await ReceiveMessage(socket, buffer, token);
            Debug.WriteLine(msg);
        }
        catch (Exception e)
        {
            Debug.WriteLine(e.Message);
            throw;
        }
    }
}

Answer 1

The problem is with cancelToken passed to ReceiveMessage function. In Rx implementation before Task finish (in your case Task.FromResult) cleanup code is executed and as a port if it cancelToken is disposed. I had the same problem when I tried to use such code:

return Observable.Using(cancelToken => Connect(uri, authToken, cancelToken), (webSocket, token) =>
            Task.FromResult(Observable.Create<string>((observer) => WebSocketReceiver.WebSocketReceiverSubscribe(observer, webSocket, cancelToken))))
            .Publish().RefCount();

but this code works:

return Observable.Using(cancelToken => Connect(uri, authToken, cancelToken), (webSocket, token) =>
            Task.FromResult(Observable.Create<string>((observer, ct) => WebSocketReceiver.WebSocketReceiverSubscribe(observer, webSocket, ct))))
            .Publish().RefCount();

As I can see Observable.Defer doesn't pass CancellationToken object so you will have to create external one and pass it to ReceiveMessage. Alternatively you can play with Observable.Create as in my example.

Observable.Using {WebSocket.ReceiveAsync（）;早点中止并取消

1 个答案: