如何为async-await调用创建包装器？

Question

据我所知，没有对ConnectAsync/AcceptAsync/SendAsync/ReceiveAsync，等的内置(或框架扩展)支持。我如何编写自己的包装器来支持异步等待机制。例如，我当前处理内联和回调(在SocketAsyncEventArgs中指定)的ReceiveAsync的代码：

private void PostReceive(SocketAsyncEventArgs e)
{       
    e.SetBuffer(ReceiveBuffer.DataBuffer, ReceiveBuffer.Count, ReceiveBuffer.Remaining);            
    e.Completed += Receive_Completed;

            // if ReceiveAsync returns false, then completion happened inline
    if (m_RemoteSocket.ReceiveAsync(e) == false)
    {
        Receive_Completed(this, e);
    }                          
}

。

private void Receive_Completed(object sender, SocketAsyncEventArgs e)
{   
    e.Completed -= Receive_Completed;       

    if (e.BytesTransferred == 0 || e.SocketError != SocketError.Success)
    {
        if (e.BytesTransferred > 0)
        {                   
            OnDataReceived(e);
        }

        Disconnect(e);                
        return;
    }

    OnDataReceived(e);

    //
    // we do not push the SocketAsyncEventArgs back onto the pool, instead
    // we reuse it in the next receive call
    //
    PostReceive(e);
}

Answer 1

诀窍是使用TaskCompletionSource来处理这种情况。

我在博客上提到了这一点。详情请参见Preparing Existing code For Await。

Answer 2

你也可以写一个自定义的awaitable，在这种情况下我更喜欢它。这是微软的Stephen Toub的一项技术。你可以在这里阅读更多关于这项技术的内容。http://blogs.msdn.com/b/pfxteam/archive/2011/12/15/10248293.aspx

以下是可等待的自定义：

public sealed class SocketAwaitable : INotifyCompletion
{
    private readonly static Action SENTINEL = () => { };
    internal bool m_wasCompleted;
    internal Action m_continuation;
    internal SocketAsyncEventArgs m_eventArgs;
    public SocketAwaitable(SocketAsyncEventArgs eventArgs)
    {
        if (eventArgs == null) throw new ArgumentNullException("eventArgs");
        m_eventArgs = eventArgs;
        eventArgs.Completed += delegate
        {
            var prev = m_continuation ?? Interlocked.CompareExchange(
                ref m_continuation, SENTINEL, null);
            if (prev != null) prev();
        };
    }
    internal void Reset()
    {
        m_wasCompleted = false;
        m_continuation = null;
    }
    public SocketAwaitable GetAwaiter() { return this; }
    public bool IsCompleted { get { return m_wasCompleted; } }
    public void OnCompleted(Action continuation)
    {
        if (m_continuation == SENTINEL ||
            Interlocked.CompareExchange(
                ref m_continuation, continuation, null) == SENTINEL)
        {
            Task.Run(continuation);
        }
    }
    public void GetResult()
    {
        if (m_eventArgs.SocketError != SocketError.Success)
            throw new SocketException((int)m_eventArgs.SocketError);
    }
}

将一些扩展方法添加到socket类中，使其更加方便：

public static class SocketExtensions
{
    public static SocketAwaitable ReceiveAsync(this Socket socket,
        SocketAwaitable awaitable)
    {
        awaitable.Reset();
        if (!socket.ReceiveAsync(awaitable.m_eventArgs))
            awaitable.m_wasCompleted = true;
        return awaitable;
    }
    public static SocketAwaitable SendAsync(this Socket socket,
        SocketAwaitable awaitable)
    {
        awaitable.Reset();
        if (!socket.SendAsync(awaitable.m_eventArgs))
            awaitable.m_wasCompleted = true;
        return awaitable;
    }
    // ... 
}

使用中：

    static async Task ReadAsync(Socket s)
    {
        // Reusable SocketAsyncEventArgs and awaitable wrapper 
        var args = new SocketAsyncEventArgs();
        args.SetBuffer(new byte[0x1000], 0, 0x1000);
        var awaitable = new SocketAwaitable(args);

        // Do processing, continually receiving from the socket 
        while (true)
        {
            await s.ReceiveAsync(awaitable);
            int bytesRead = args.BytesTransferred;
            if (bytesRead <= 0) break;

            Console.WriteLine(bytesRead);
        }
    }

Answer 3

对于套接字的东西，.NET 4.5中有一个包装器。如果您使用的是APM4，我建议您使用.NET，而不是基于事件的异步模式。它转换成Task要容易得多。