c# - 在 C# 中使用异步方法进行消息循环

Question

我正在制作一个在线通信应用程序，我想异步处理消息。我发现异步等待模式在实现消息循环方面很有用。

下面是我目前得到的:

CancellationTokenSource cts=new CancellationTokenSource(); //This is used to disconnect the client.

public Action<Member> OnNewMember; //Callback field

async void NewMemberCallback(ConnectionController c, Member m, Stream stream){
    //This is called when a connection with a new member is established.
    //The class ConnectionController is used to absorb the difference in protocol between TCP and UDP.

    MessageLoop(c, m,stream,cts.Token);
    if(OnNewMember!=null)OnNewMember(m);
}

async Task MessageLoop(ConnectionController c, Member m, Stream stream, CancellationToken ct){
    MemoryStream msgbuffer=new MemoryStream();
    MemoryStream buffer2=new MemoryStream();

    while(true){
        try{
            await ReceiveandSplitMessage(stream, msgbuffer,buffer2,ct); //This stops until data is received.
            DecodeandProcessMessage(msgbuffer);
        catch( ...Exception ex){
            //When the client disconnects
            c.ClientDisconnected(m);
            return;
        }

    }
}

然后我收到一些警告，说在 NewMemberCallback 中，没有等待对 MessageLoop 的调用。我实际上不需要等待 MessageLoop 方法，因为该方法在连接断开之前不会返回。像这样使用异步被认为是一种好习惯吗？我听说不等待异步方法不好，但我也听说我应该消除不必要的等待。还是将异步模式用于消息循环甚至被认为是不好的？

Answer 1

通常您希望跟踪已开始的任务，以避免重入并处理异常。示例:

Task _messageLoopTask = null;

async void NewMemberCallback(ConnectionController c, Member m, Stream stream)
{
    if (_messageLoopTask != null)
    {
        // handle re-entrancy
        MessageBox.Show("Already started!");
        return;
    }

    _messageLoopTask = MessageLoop(c, m,stream,cts.Token);

    if (OnNewMember!=null) OnNewMember(m);

    try
    {
        await _messageLoopTask;
    }
    catch (OperationCanceledException ex)
    {
        // swallow cancelation
    }
    catch (AggregateException ex) 
    { 
        // swallow cancelation
        ex.Handle(ex => ex is OperationCanceledException);
    }
    finally
    {
        _messageLoopTask = null;
    }
}

查看 Lucian Wischik 的 "Async re-entrancy, and the patterns to deal with it" .

如果您可以有多个 MessageLoop 实例，那么您就不必担心重入问题，但您仍然希望观察异常。