c# - 如何制作非阻塞等待句柄？

Question

本质上，我正在做的是创建一个 Web 服务器来处理 API 调用，然后当它完成后继续执行方法，所以本质上是：

new WebServer(myAutoResetEvent);
myAutoResetEvent.WaitOne();

但是，这会阻塞线程直到那时。有什么办法可以使这个异步？将它包装在一个await Task.Run()电话中是否可以，即await Task.Run(() => myAutoResetEvent.WaitOne())？

谢谢！

Answer 1

通常，WebServerctor 不应该做任何有趣的事情。应该有一个Task WebServer.RunAsync运行服务器的函数。然后，您可以使用生成的任务进行同步和协调。

如果您不希望这样，您可以使用 aTaskCompletionSource<object>作为一次性异步就绪事件。

我相信ThreadPool班级有办法有效地等待 aWaitHandle被设置，但这是一个更糟糕的解决方案。

Answer 2

您不应该阻塞ThreadPool线程，这是导致ThreadPool饥饿的快速方法，而是提供了一种异步等待WaitHandle实例的方法，称为ThreadPool.RegisterWaitForSingleObject.

通过使用ThreadPool.RegisterWaitForSingleObject回调注册以在WaitHandle可用时调用，不幸的是，这不是开箱即用的异步/等待兼容，使此异步/等待兼容的完整实现如下：

public static class WaitHandleExtensions
{
    public static Task WaitOneAsync(this WaitHandle waitHandle, CancellationToken cancellationToken)
    {    
        return WaitOneAsync(waitHandle, Timeout.Infinite, cancellationToken);
    }

    public static async Task<bool> WaitOneAsync(this WaitHandle waitHandle, int timeout, CancellationToken cancellationToken)
    {    
        // A Mutex can't use RegisterWaitForSingleObject as a Mutex requires the wait and release to be on the same thread
        // but RegisterWaitForSingleObject acquires the Mutex on a ThreadPool thread.
        if (waitHandle is Mutex)
            throw new ArgumentException(StringResources.MutexMayNotBeUsedWithWaitOneAsyncAsThreadIdentityIsEnforced, nameof(waitHandle));

        cancellationToken.ThrowIfCancellationRequested();

        var tcs = new TaskCompletionSource<bool>();
        var rwh = ThreadPool.RegisterWaitForSingleObject(waitHandle, OnWaitOrTimerCallback, tcs, timeout, true);

        var cancellationCallback = BuildCancellationCallback(rwh, tcs);

        using (cancellationToken.Register(cancellationCallback))
        {
            try
            {
                return await tcs.Task.ConfigureAwait(false);
            }
            finally
            {
                rwh.Unregister(null);
            }
        }
    }

    private static Action BuildCancellationCallback(RegisteredWaitHandle rwh, TaskCompletionSource<bool> tcs)
    {    
        return () =>
        {
            if (rwh.Unregister(null))
            {
                tcs.SetCanceled();
            }
        };
    }

    private static void OnWaitOrTimerCallback(object state, bool timedOut)
    {    
        var taskCompletionSource = (TaskCompletionSource<bool>)state;

        taskCompletionSource.SetResult(!timedOut);
    }
}

唯一的限制是它不能与Mutex.

这可以像这样使用：

await myAutoResetEvent.WaitOneAsync(cancellationToken).ConfigureAwait(false);

Answer 3

要考虑的另一种方法是使用HttpSelfHostServer(System.Web.Http.SelfHost.dll) 并将所有线程细节留给其实现。

var config = new HttpSelfHostConfiguration("http://localhost:9999");
var tcs = new TaskCompletionSource<Uri>();

using (var server = new HttpSelfHostServer(config, new MessageHandler(tcs)))
{
    await server.OpenAsync();

    await tcs.Task;

    await server.CloseAsync();
}

return tcs.Task.Result;

class MessageHandler : HttpMessageHandler
{
    private readonly TaskCompletionSource<Uri> _task;

    public MessageHandler(TaskCompletionSource<Uri> task)
    {
        _task = task;
    }

    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        _task.SetResult(request.RequestUri);
        return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK));
    }
}