AutoResetEvent 的异步（可等待）等价物是什么？
如果在经典的线程同步中我们会使用这样的东西：
AutoResetEvent signal = new AutoResetEvent(false);
void Thread1Proc()
{
//do some stuff
//..
//..
signal.WaitOne(); //wait for an outer thread to signal we are good to continue
//do some more stuff
//..
//..
}
void Thread2Proc()
{
//do some stuff
//..
//..
signal.Set(); //signal the other thread it's good to go
//do some more stuff
//..
//..
}
我希望在新的异步处理方式中，会出现这样的情况：
SomeAsyncAutoResetEvent asyncSignal = new SomeAsyncAutoResetEvent();
async void Task1Proc()
{
//do some stuff
//..
//..
await asyncSignal.WaitOne(); //wait for an outer thread to signal we are good to continue
//do some more stuff
//..
//..
}
async void Task2Proc()
{
//do some stuff
//..
//..
asyncSignal.Set(); //signal the other thread it's good to go
//do some more stuff
//..
//..
}
我见过其他定制的解决方案，但在某个时间点我设法得到的解决方案仍然涉及锁定线程。我不希望这只是为了使用新的等待语法。我正在寻找一种真正的可等待信号机制，它不会锁定任何线程。
任务并行库中是否缺少某些内容？
编辑：只是为了澄清：SomeAsyncAutoResetEvent 是一个完全组成的类名，在我的示例中用作占位符。
如果您想构建自己的，Stephen Toub 有关于该主题的权威博客文章。
如果您想使用已经编写的库，我的 AsyncEx 库中有一个。 AFAIK，截至撰写本文时没有其他选择。
这是 Stephen Toub 的 AsyncAutoResetEvent 的
来源，以防他的博客离线。
internal sealed class AsyncAutoResetEvent
{
private static readonly Task s_completed = Task.FromResult(true);
private readonly Queue<TaskCompletionSource<bool>> _waits = new Queue<TaskCompletionSource<bool>>();
private bool _signaled;
public Task WaitAsync()
{
lock (_waits)
{
if (_signaled)
{
_signaled = false;
return s_completed;
}
else
{
var tcs = new TaskCompletionSource<bool>();
_waits.Enqueue(tcs);
return tcs.Task;
}
}
}
public void Set()
{
TaskCompletionSource<bool>? toRelease = null;
lock (_waits)
{
if (_waits.Count > 0)
{
toRelease = _waits.Dequeue();
}
else if (!_signaled)
{
_signaled = true;
}
}
toRelease?.SetResult(true);
}
}
我认为 MSDN 上有一个很好的例子：https://msdn.microsoft.com/en-us/library/hh873178%28v=vs.110%29.aspx#WHToTap
public static Task WaitOneAsync(this WaitHandle waitHandle)
{
if (waitHandle == null)
throw new ArgumentNullException("waitHandle");
var tcs = new TaskCompletionSource<bool>();
var rwh = ThreadPool.RegisterWaitForSingleObject(waitHandle,
delegate { tcs.TrySetResult(true); }, null, -1, true);
var t = tcs.Task;
t.ContinueWith( (antecedent) => rwh.Unregister(null));
return t;
}
这是我编写的一个版本，它允许您指定超时。它源自 Stephen Toub 的解决方案。我们目前在生产工作负载中使用它。
public class AsyncAutoResetEvent
{
readonly LinkedList<TaskCompletionSource<bool>> waiters =
new LinkedList<TaskCompletionSource<bool>>();
bool isSignaled;
public AsyncAutoResetEvent(bool signaled)
{
this.isSignaled = signaled;
}
public Task<bool> WaitAsync(TimeSpan timeout)
{
return this.WaitAsync(timeout, CancellationToken.None);
}
public async Task<bool> WaitAsync(TimeSpan timeout, CancellationToken cancellationToken)
{
TaskCompletionSource<bool> tcs;
lock (this.waiters)
{
if (this.isSignaled)
{
this.isSignaled = false;
return true;
}
else if (timeout == TimeSpan.Zero)
{
return this.isSignaled;
}
else
{
tcs = new TaskCompletionSource<bool>();
this.waiters.AddLast(tcs);
}
}
Task winner = await Task.WhenAny(tcs.Task, Task.Delay(timeout, cancellationToken));
if (winner == tcs.Task)
{
// The task was signaled.
return true;
}
else
{
// We timed-out; remove our reference to the task.
// This is an O(n) operation since waiters is a LinkedList<T>.
lock (this.waiters)
{
bool removed = this.waiters.Remove(tcs);
Debug.Assert(removed);
return false;
}
}
}
public void Set()
{
lock (this.waiters)
{
if (this.waiters.Count > 0)
{
// Signal the first task in the waiters list. This must be done on a new
// thread to avoid stack-dives and situations where we try to complete the
// same result multiple times.
TaskCompletionSource<bool> tcs = this.waiters.First.Value;
Task.Run(() => tcs.SetResult(true));
this.waiters.RemoveFirst();
}
else if (!this.isSignaled)
{
// No tasks are pending
this.isSignaled = true;
}
}
}
public override string ToString()
{
return $"Signaled: {this.isSignaled.ToString()}, Waiters: {this.waiters.Count.ToString()}";
}
}
我也在寻找 AsyncAutoResetEvent 类，现在似乎在命名空间 Microsoft.VisualStudio.Threading 中有一个可用
// Summary:
// An asynchronous implementation of an AutoResetEvent.
[DebuggerDisplay("Signaled: {signaled}")]
public class AsyncAutoResetEvent
也可以，但是这样可能会淡化使用
asyncawaitAutoResetEvent asyncSignal = new AutoResetEvent();
async void Task1Proc()
{
//do some stuff
//..
//..
await Task.Run(() => asyncSignal.WaitOne()); //wait for an outer thread to signal we are good to continue
//do some more stuff
//..
//..
}
使用 SemaphoreSlim 实现可能要简单得多，我还没有在生产中测试过它，但它应该可以正常工作。
public class AsyncAutoResetEvent : IDisposable
{
private readonly SemaphoreSlim _waiters;
public AsyncAutoResetEvent(bool initialState)
=> _waiters = new SemaphoreSlim(initialState ? 1 : 0, 1);
public Task<bool> WaitOneAsync(TimeSpan timeout, CancellationToken cancellationToken = default)
=> _waiters.WaitAsync(timeout, cancellationToken);
public Task WaitOneAsync(CancellationToken cancellationToken = default)
=> _waiters.WaitAsync(cancellationToken);
public void Set()
{
lock (_waiters)
{
if (_waiters.CurrentCount == 0)
_waiters.Release();
}
}
public override string ToString()
=> $"Signaled: {_waiters.CurrentCount != 0}";
private bool _disposed;
protected virtual void Dispose(bool disposing)
{
if (!_disposed)
{
if (disposing)
{
_waiters.Dispose();
}
_disposed = true;
}
}
public void Dispose()
{
// Do not change this code. Put cleanup code in 'Dispose(bool disposing)' method
Dispose(disposing: true);
GC.SuppressFinalize(this);
}
}
安装
Microsoft.VisualStudio.ThreadingAsyncAutoResetEventAutoResetEvent 的异步实现。
文档：https://learn.microsoft.com/en-us/dotnet/api/microsoft.visualstudio.threading.asyncautoresetevent
我使用可选的超时（毫秒）扩展了 Oleg Gordeev 提供的 MSDN 示例：
public static Task WaitOneAsync(this WaitHandle waitHandle, double timeout = 0)
{
if (waitHandle == null) throw new ArgumentNullException("waitHandle");
var tcs = new TaskCompletionSource<bool>();
if (timeout > 0)
{
var timer = new System.Timers.Timer(timeout)
{ Enabled = true, AutoReset = false };
ElapsedEventHandler del = default;
del = delegate (object x, System.Timers.ElapsedEventArgs y)
{
tcs.TrySetResult(true);
timer.Elapsed -= del;
timer.Dispose();
};
timer.Elapsed += del;
}
var rwh = ThreadPool.RegisterWaitForSingleObject(waitHandle,
delegate { tcs.TrySetResult(true); },
null, -1, true);
var t = tcs.Task;
t.ContinueWith((antecedent) => rwh.Unregister(null));
return t;
}
这是我的一次性事件版本可以由多个线程等待。它内部依赖于
BoundedChannelpublic class AsyncOneTimeEvent<T>
{
private T Result { get; set; }
private readonly Channel<bool> _channel = Channel.CreateBounded<bool>(new BoundedChannelOptions(1)
{
SingleReader = false,
SingleWriter = true,
FullMode = BoundedChannelFullMode.DropWrite,
});
public async Task<T> GetResult()
{
await _channel.Reader.WaitToReadAsync().ConfigureAwait(false);
return this.Result;
}
public void SetResult(T result)
{
this.Result = result;
_channel.Writer.Complete();
}
public void SetError(Exception ex)
{
_channel.Writer.Complete(ex);
}
}
这是我使用
SemaphoreSlimSemaphoreSlim.WaitAsyncusing System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
/// <summary>
/// Represents an event that, when signaled, resets automatically after releasing a single waiting task.
/// </summary>
public sealed class AutoResetEventAsync : IDisposable {
/// <summary>
/// Waits asynchronously until a signal is received.
/// </summary>
/// <returns>Task completed when the event is signaled.</returns>
public async ValueTask WaitAsync() {
if (CheckSignaled()) return;
SemaphoreSlim s;
lock (Q) Q.Enqueue(s = new(0, 1));
await s.WaitAsync();
lock (Q) if (Q.Count > 0 && Q.Peek() == s) Q.Dequeue().Dispose();
}
/// <summary>
/// Waits asynchronously until a signal is received or the time runs out.
/// </summary>
/// <param name="millisecondsTimeout">The number of milliseconds to wait, <see cref="System.Threading.Timeout.Infinite"/>
/// (-1) to wait indefinitely, or zero to return immediately.</param>
/// <returns>Task completed when the event is signaled or the time runs out.</returns>
public async ValueTask WaitAsync(int millisecondsTimeout) {
if (CheckSignaled()) return;
SemaphoreSlim s;
lock (Q) Q.Enqueue(s = new(0, 1));
await s.WaitAsync(millisecondsTimeout);
lock (Q) if (Q.Count > 0 && Q.Peek() == s) Q.Dequeue().Dispose();
}
/// <summary>
/// Waits asynchronously until a signal is received, the time runs out or the token is cancelled.
/// </summary>
/// <param name="millisecondsTimeout">The number of milliseconds to wait, <see cref="System.Threading.Timeout.Infinite"/>
/// (-1) to wait indefinitely, or zero to return immediately.</param>
/// <param name="cancellationToken">The <see cref="System.Threading.CancellationToken"/> to observe.</param>
/// <returns>Task completed when the event is signaled, the time runs out or the token is cancelled.</returns>
public async ValueTask WaitAsync(int millisecondsTimeout, CancellationToken cancellationToken) {
if (CheckSignaled()) return;
SemaphoreSlim s;
lock (Q) Q.Enqueue(s = new(0, 1));
try {
await s.WaitAsync(millisecondsTimeout, cancellationToken);
}
finally {
lock (Q) if (Q.Count > 0 && Q.Peek() == s) Q.Dequeue().Dispose();
}
}
/// <summary>
/// Waits asynchronously until a signal is received or the token is cancelled.
/// </summary>
/// <param name="cancellationToken">The <see cref="System.Threading.CancellationToken"/> to observe.</param>
/// <returns>Task completed when the event is signaled or the token is cancelled.</returns>
public async ValueTask WaitAsync(CancellationToken cancellationToken) {
if (CheckSignaled()) return;
SemaphoreSlim s;
lock (Q) Q.Enqueue(s = new(0, 1));
try {
await s.WaitAsync(cancellationToken);
}
finally {
lock (Q) if (Q.Count > 0 && Q.Peek() == s) Q.Dequeue().Dispose();
}
}
/// <summary>
/// Waits asynchronously until a signal is received or the time runs out.
/// </summary>
/// <param name="timeout">A <see cref="System.TimeSpan"/> that represents the number of milliseconds to wait,
/// a <see cref="System.TimeSpan"/> that represents -1 milliseconds to wait indefinitely, or a System.TimeSpan
/// that represents 0 milliseconds to return immediately.</param>
/// <returns>Task completed when the event is signaled or the time runs out.</returns>
public async ValueTask WaitAsync(TimeSpan timeout) {
if (CheckSignaled()) return;
SemaphoreSlim s;
lock (Q) Q.Enqueue(s = new(0, 1));
await s.WaitAsync(timeout);
lock (Q) if (Q.Count > 0 && Q.Peek() == s) Q.Dequeue().Dispose();
}
/// <summary>
/// Waits asynchronously until a signal is received, the time runs out or the token is cancelled.
/// </summary>
/// <param name="timeout">A <see cref="System.TimeSpan"/> that represents the number of milliseconds to wait,
/// a <see cref="System.TimeSpan"/> that represents -1 milliseconds to wait indefinitely, or a System.TimeSpan
/// that represents 0 milliseconds to return immediately.</param>
/// <param name="cancellationToken">The <see cref="System.Threading.CancellationToken"/> to observe.</param>
/// <returns>Task completed when the event is signaled, the time runs out or the token is cancelled.</returns>
public async ValueTask WaitAsync(TimeSpan timeout, CancellationToken cancellationToken) {
if (CheckSignaled()) return;
SemaphoreSlim s;
lock (Q) Q.Enqueue(s = new(0, 1));
try {
await s.WaitAsync(timeout, cancellationToken);
}
finally {
lock (Q) if (Q.Count > 0 && Q.Peek() == s) Q.Dequeue().Dispose();
}
}
/// <summary>
/// Sets the state of the event to signaled, allowing one or more waiting tasks to proceed.
/// </summary>
public void Set() {
SemaphoreSlim? toRelease = null;
lock (Q) {
if (Q.Count > 0) toRelease = Q.Dequeue();
else if (!IsSignaled) IsSignaled = true;
}
toRelease?.Release();
}
/// <summary>
/// Sets the state of the event to non nonsignaled, making the waiting tasks to wait.
/// </summary>
public void Reset() => IsSignaled = false;
/// <summary>
/// Disposes any semaphores left in the queue.
/// </summary>
public void Dispose() {
lock (Q) {
while (Q.Count > 0) Q.Dequeue().Dispose();
}
}
/// <summary>
/// Checks the <see cref="IsSignaled"/> state and resets it when it's signaled.
/// </summary>
/// <returns>True if the event was in signaled state.</returns>
private bool CheckSignaled() {
lock (Q) {
if (IsSignaled) {
IsSignaled = false;
return true;
}
return false;
}
}
private readonly Queue<SemaphoreSlim> Q = new();
private volatile bool IsSignaled;
}
我使用
SemaphoreSlimSemaphoreSlimAutoResetEvent