是否可以等待一个事件而不是另一个异步方法？

Question

在我的 C#/XAML Metro 应用程序中，有一个按钮可以启动一个长时间运行的进程。 因此，按照建议，我使用 async/await 来确保 UI 线程不会被阻塞：

private async void Button_Click_1(object sender, RoutedEventArgs e) 
{
     await GetResults();
}

private async Task GetResults()
{ 
     // Do lot of complex stuff that takes a long time
     // (e.g. contact some web services)
  ...
}

有时，GetResults 中发生的事情需要额外的用户输入才能继续。 为简单起见，假设用户只需单击“继续”按钮。

我的问题是：如何暂停执行 GetResults 以等待诸如单击另一个按钮之类的事件？

这是实现我正在寻找的一种丑陋的方式：“继续”按钮的事件处理程序设置了一个标志......

private bool _continue = false;
private void buttonContinue_Click(object sender, RoutedEventArgs e)
{
    _continue = true;
}

...并且 GetResults 定期对其进行轮询：

 buttonContinue.Visibility = Visibility.Visible;
 while (!_continue) await Task.Delay(100);  // poll _continue every 100ms
 buttonContinue.Visibility = Visibility.Collapsed;

轮询显然很糟糕（忙于等待/浪费周期），我正在寻找基于事件的东西。

有任何想法吗？

顺便说一句，在这个简化的例子中，一个解决方案当然是将 GetResults() 分成两部分，从开始按钮调用第一部分，从继续按钮调用第二部分。 实际上，GetResults 中发生的事情更复杂，执行过程中的不同点可能需要不同类型的用户输入。 因此，将逻辑分解为多种方法并非易事。

Answer 1

您可以使用SemaphoreSlim 类的实例作为信号：

private SemaphoreSlim signal = new SemaphoreSlim(0, 1);

// set signal in event
signal.Release();

// wait for signal somewhere else
await signal.WaitAsync();

---

或者，您可以使用TaskCompletionSource<T> 类的实例来创建一个Task<T>来表示按钮单击的结果：

private TaskCompletionSource<bool> tcs = new TaskCompletionSource<bool>();

// complete task in event
tcs.SetResult(true);

// wait for task somewhere else
await tcs.Task;

Answer 2

当您有不寻常的事情需要await时，最简单的答案通常是TaskCompletionSource （或一些基于TaskCompletionSource启用async原语）。

在这种情况下，您的需求非常简单，因此您可以直接使用TaskCompletionSource ：

private TaskCompletionSource<object> continueClicked;

private async void Button_Click_1(object sender, RoutedEventArgs e) 
{
  // Note: You probably want to disable this button while "in progress" so the
  //  user can't click it twice.
  await GetResults();
  // And re-enable the button here, possibly in a finally block.
}

private async Task GetResults()
{ 
  // Do lot of complex stuff that takes a long time
  // (e.g. contact some web services)

  // Wait for the user to click Continue.
  continueClicked = new TaskCompletionSource<object>();
  buttonContinue.Visibility = Visibility.Visible;
  await continueClicked.Task;
  buttonContinue.Visibility = Visibility.Collapsed;

  // More work...
}

private void buttonContinue_Click(object sender, RoutedEventArgs e)
{
  if (continueClicked != null)
    continueClicked.TrySetResult(null);
}

从逻辑上讲， TaskCompletionSource就像一个async ManualResetEvent ，除了您只能“设置”一次事件并且事件可以有“结果”（在这种情况下，我们没有使用它，所以我们只是将结果设置为null ） .

Answer 3

这是我使用的一个实用程序类：

public class AsyncEventListener
{
    private readonly Func<bool> _predicate;

    public AsyncEventListener() : this(() => true)
    {

    }

    public AsyncEventListener(Func<bool> predicate)
    {
        _predicate = predicate;
        Successfully = new Task(() => { });
    }

    public void Listen(object sender, EventArgs eventArgs)
    {
        if (!Successfully.IsCompleted && _predicate.Invoke())
        {
            Successfully.RunSynchronously();
        }
    }

    public Task Successfully { get; }
}

这是我如何使用它：

var itChanged = new AsyncEventListener();
someObject.PropertyChanged += itChanged.Listen;

// ... make it change ...

await itChanged.Successfully;
someObject.PropertyChanged -= itChanged.Listen;

Answer 4

简单的助手类：

public class EventAwaiter<TEventArgs>
{
    private readonly TaskCompletionSource<TEventArgs> _eventArrived = new TaskCompletionSource<TEventArgs>();

    private readonly Action<EventHandler<TEventArgs>> _unsubscribe;

    public EventAwaiter(Action<EventHandler<TEventArgs>> subscribe, Action<EventHandler<TEventArgs>> unsubscribe)
    {
        subscribe(Subscription);
        _unsubscribe = unsubscribe;
    }

    public Task<TEventArgs> Task => _eventArrived.Task;

    private EventHandler<TEventArgs> Subscription => (s, e) =>
        {
            _eventArrived.TrySetResult(e);
            _unsubscribe(Subscription);
        };
}

用法：

var valueChangedEventAwaiter = new EventAwaiter<YourEventArgs>(
                            h => example.YourEvent += h,
                            h => example.YourEvent -= h);
await valueChangedEventAwaiter.Task;

Answer 5

理想情况下，您不需要。 虽然您当然可以阻塞异步线程，但这是一种资源浪费，而且并不理想。

考虑一个典型的例子，当按钮等待被点击时，用户去吃午饭。

如果您在等待用户输入时暂停了异步代码，那么在该线程暂停时它只是在浪费资源。

也就是说，最好在异步操作中，将需要维护的状态设置为启用按钮并“等待”单击的点。 此时，您的GetResults方法将停止。

然后，单击该按钮时，根据您所储存的状态，启动另一个异步任务，继续工作。

因为SynchronizationContext将在调用GetResults的事件处理程序中捕获（编译器将这样做，因为使用了await关键字，并且SynchronizationContext.Current应该是非空的，因为您在 UI 应用程序中)，您可以像这样使用async / await ：

private async void Button_Click_1(object sender, RoutedEventArgs e) 
{
     await GetResults();

     // Show dialog/UI element.  This code has been marshaled
     // back to the UI thread because the SynchronizationContext
     // was captured behind the scenes when
     // await was called on the previous line.
     ...

     // Check continue, if true, then continue with another async task.
     if (_continue) await ContinueToGetResultsAsync();
}

private bool _continue = false;
private void buttonContinue_Click(object sender, RoutedEventArgs e)
{
    _continue = true;
}

private async Task GetResults()
{ 
     // Do lot of complex stuff that takes a long time
     // (e.g. contact some web services)
  ...
}

ContinueToGetResultsAsync是在您的按钮被按下时继续获取结果的方法。 如果您的按钮没有被按下，那么您的事件处理程序什么都不做。

Answer 6

Stephen Toub 在他的博客上发布了这个AsyncManualResetEvent类。

public class AsyncManualResetEvent 
{ 
    private volatile TaskCompletionSource<bool> m_tcs = new TaskCompletionSource<bool>();

    public Task WaitAsync() { return m_tcs.Task; } 

    public void Set() 
    { 
        var tcs = m_tcs; 
        Task.Factory.StartNew(s => ((TaskCompletionSource<bool>)s).TrySetResult(true), 
            tcs, CancellationToken.None, TaskCreationOptions.PreferFairness, TaskScheduler.Default); 
        tcs.Task.Wait(); 
    }

    public void Reset() 
    { 
        while (true) 
        { 
            var tcs = m_tcs; 
            if (!tcs.Task.IsCompleted || 
                Interlocked.CompareExchange(ref m_tcs, new TaskCompletionSource<bool>(), tcs) == tcs) 
                return; 
        } 
    } 
}

Answer 7

使用反应式扩展 (Rx.Net)

var eventObservable = Observable
            .FromEventPattern<EventArgs>(
                h => example.YourEvent += h,
                h => example.YourEvent -= h);

var res = await eventObservable.FirstAsync();

您可以使用 Nuget 包 System.Reactive 添加 Rx

测试样品：

    private static event EventHandler<EventArgs> _testEvent;

    private static async Task Main()
    {
        var eventObservable = Observable
            .FromEventPattern<EventArgs>(
                h => _testEvent += h,
                h => _testEvent -= h);

        Task.Delay(5000).ContinueWith(_ => _testEvent?.Invoke(null, new EventArgs()));

        var res = await eventObservable.FirstAsync();

        Console.WriteLine("Event got fired");
    }

Answer 8

我正在使用我自己的 AsyncEvent 类来处理可等待的事件。

public delegate Task AsyncEventHandler<T>(object sender, T args) where T : EventArgs;

public class AsyncEvent : AsyncEvent<EventArgs>
{
    public AsyncEvent() : base()
    {
    }
}

public class AsyncEvent<T> where T : EventArgs
{
    private readonly HashSet<AsyncEventHandler<T>> _handlers;

    public AsyncEvent()
    {
        _handlers = new HashSet<AsyncEventHandler<T>>();
    }

    public void Add(AsyncEventHandler<T> handler)
    {
        _handlers.Add(handler);
    }

    public void Remove(AsyncEventHandler<T> handler)
    {
        _handlers.Remove(handler);
    }

    public async Task InvokeAsync(object sender, T args)
    {
        foreach (var handler in _handlers)
        {
            await handler(sender, args);
        }
    }

    public static AsyncEvent<T> operator+(AsyncEvent<T> left, AsyncEventHandler<T> right)
    {
        var result = left ?? new AsyncEvent<T>();
        result.Add(right);
        return result;
    }

    public static AsyncEvent<T> operator-(AsyncEvent<T> left, AsyncEventHandler<T> right)
    {
        left.Remove(right);
        return left;
    }
}

要在引发事件的类中声明事件：

public AsyncEvent MyNormalEvent;
public AsyncEvent<ProgressEventArgs> MyCustomEvent;

要引发事件：

if (MyNormalEvent != null) await MyNormalEvent.InvokeAsync(this, new EventArgs());
if (MyCustomEvent != null) await MyCustomEvent.InvokeAsync(this, new ProgressEventArgs());

要订阅事件：

MyControl.Click += async (sender, args) => {
    // await...
}

MyControl.Click += (sender, args) => {
    // synchronous code
    return Task.CompletedTask;
}

Answer 9

这是一个包含六种方法的小工具箱，可用于将事件转换为任务：

/// <summary>Converts a .NET event, conforming to the standard .NET event pattern
/// based on <see cref="EventHandler"/>, to a Task.</summary>
public static Task EventToAsync(
    Action<EventHandler> addHandler,
    Action<EventHandler> removeHandler)
{
    var tcs = new TaskCompletionSource<object>();
    addHandler(Handler);
    return tcs.Task;

    void Handler(object sender, EventArgs e)
    {
        removeHandler(Handler);
        tcs.SetResult(null);
    }
}

/// <summary>Converts a .NET event, conforming to the standard .NET event pattern
/// based on <see cref="EventHandler{TEventArgs}"/>, to a Task.</summary>
public static Task<TEventArgs> EventToAsync<TEventArgs>(
    Action<EventHandler<TEventArgs>> addHandler,
    Action<EventHandler<TEventArgs>> removeHandler)
{
    var tcs = new TaskCompletionSource<TEventArgs>();
    addHandler(Handler);
    return tcs.Task;

    void Handler(object sender, TEventArgs e)
    {
        removeHandler(Handler);
        tcs.SetResult(e);
    }
}

/// <summary>Converts a .NET event, conforming to the standard .NET event pattern
/// based on a supplied event delegate type, to a Task.</summary>
public static Task<TEventArgs> EventToAsync<TDelegate, TEventArgs>(
    Action<TDelegate> addHandler, Action<TDelegate> removeHandler)
{
    var tcs = new TaskCompletionSource<TEventArgs>();
    TDelegate handler = default;
    Action<object, TEventArgs> genericHandler = (sender, e) =>
    {
        removeHandler(handler);
        tcs.SetResult(e);
    };
    handler = (TDelegate)(object)genericHandler.GetType().GetMethod("Invoke")
        .CreateDelegate(typeof(TDelegate), genericHandler);
    addHandler(handler);
    return tcs.Task;
}

/// <summary>Converts a named .NET event, conforming to the standard .NET event
/// pattern based on <see cref="EventHandler"/>, to a Task.</summary>
public static Task EventToAsync(object target, string eventName)
{
    var type = target.GetType();
    var eventInfo = type.GetEvent(eventName);
    if (eventInfo == null) throw new InvalidOperationException("Event not found.");
    var tcs = new TaskCompletionSource<object>();
    EventHandler handler = default;
    handler = new EventHandler((sender, e) =>
    {
        eventInfo.RemoveEventHandler(target, handler);
        tcs.SetResult(null);
    });
    eventInfo.AddEventHandler(target, handler);
    return tcs.Task;
}

/// <summary>Converts a named .NET event, conforming to the standard .NET event
/// pattern based on <see cref="EventHandler{TEventArgs}"/>, to a Task.</summary>
public static Task<TEventArgs> EventToAsync<TEventArgs>(
    object target, string eventName)
{
    var type = target.GetType();
    var eventInfo = type.GetEvent(eventName);
    if (eventInfo == null) throw new InvalidOperationException("Event not found.");
    var tcs = new TaskCompletionSource<TEventArgs>();
    EventHandler<TEventArgs> handler = default;
    handler = new EventHandler<TEventArgs>((sender, e) =>
    {
        eventInfo.RemoveEventHandler(target, handler);
        tcs.SetResult(e);
    });
    eventInfo.AddEventHandler(target, handler);
    return tcs.Task;
}

/// <summary>Converts a generic Action-based .NET event to a Task.</summary>
public static Task<TArgument> EventActionToAsync<TArgument>(
    Action<Action<TArgument>> addHandler,
    Action<Action<TArgument>> removeHandler)
{
    var tcs = new TaskCompletionSource<TArgument>();
    addHandler(Handler);
    return tcs.Task;

    void Handler(TArgument arg)
    {
        removeHandler(Handler);
        tcs.SetResult(arg);
    }
}

所有这些方法都在创建一个Task ，该Task将在下一次调用相关事件时完成。 此任务永远不会出现故障或取消，它可能只会成功完成。

标准事件（ Progress<T>.ProgressChanged ）的使用示例：

var p = new Progress<int>();

//...

int result = await EventToAsync<int>(
    h => p.ProgressChanged += h, h => p.ProgressChanged -= h);

// ...or...

int result = await EventToAsync<EventHandler<int>, int>(
    h => p.ProgressChanged += h, h => p.ProgressChanged -= h);

// ...or...

int result = await EventToAsync<int>(p, "ProgressChanged");

非标准事件的使用示例：

public static event Action<int> MyEvent;

//...

int result = await EventActionToAsync<int>(h => MyEvent += h, h => MyEvent -= h);

任务完成后取消订阅该事件。 没有提供更早的退订机制。

Answer 10

这是我用于测试的一个类，它支持 CancellationToken。

此 Test 方法向我们展示了等待引发ClassWithEvent的MyEvent的实例。 ：

    public async Task TestEventAwaiter()
    {
        var cls = new ClassWithEvent();

        Task<bool> isRaisedTask = EventAwaiter<ClassWithEvent>.RunAsync(
            cls, 
            nameof(ClassWithEvent.MyMethodEvent), 
            TimeSpan.FromSeconds(3));

        cls.Raise();
        Assert.IsTrue(await isRaisedTask);
        isRaisedTask = EventAwaiter<ClassWithEvent>.RunAsync(
            cls, 
            nameof(ClassWithEvent.MyMethodEvent), 
            TimeSpan.FromSeconds(1));

        System.Threading.Thread.Sleep(2000);

        Assert.IsFalse(await isRaisedTask);
    }

这是事件等待类。

public class EventAwaiter<TOwner>
{
    private readonly TOwner_owner;
    private readonly string _eventName;
    private readonly TaskCompletionSource<bool> _taskCompletionSource;
    private readonly CancellationTokenSource _elapsedCancellationTokenSource;
    private readonly CancellationTokenSource _linkedCancellationTokenSource;
    private readonly CancellationToken _activeCancellationToken;
    private Delegate _localHookDelegate;
    private EventInfo _eventInfo;

    public static Task<bool> RunAsync(
        TOwner owner,
        string eventName,
        TimeSpan timeout,
        CancellationToken? cancellationToken = null)
    {
        return (new EventAwaiter<TOwner>(owner, eventName, timeout, cancellationToken)).RunAsync(timeout);
    }
    private EventAwaiter(
        TOwner owner,
        string eventName,
        TimeSpan timeout,
        CancellationToken? cancellationToken = null)
    {
        if (owner == null) throw new TypeInitializationException(this.GetType().FullName, new ArgumentNullException(nameof(owner)));
        if (eventName == null) throw new TypeInitializationException(this.GetType().FullName, new ArgumentNullException(nameof(eventName)));

        _owner = owner;
        _eventName = eventName;
        _taskCompletionSource = new TaskCompletionSource<bool>();
        _elapsedCancellationTokenSource = new CancellationTokenSource();
        _linkedCancellationTokenSource =
            cancellationToken == null
                ? null
                : CancellationTokenSource.CreateLinkedTokenSource(_elapsedCancellationTokenSource.Token, cancellationToken.Value);
        _activeCancellationToken = (_linkedCancellationTokenSource ?? _elapsedCancellationTokenSource).Token;

        _eventInfo = typeof(TOwner).GetEvent(_eventName);
        Type eventHandlerType = _eventInfo.EventHandlerType;
        MethodInfo invokeMethodInfo = eventHandlerType.GetMethod("Invoke");
        var parameterTypes = Enumerable.Repeat(this.GetType(),1).Concat(invokeMethodInfo.GetParameters().Select(p => p.ParameterType)).ToArray();
        DynamicMethod eventRedirectorMethod = new DynamicMethod("EventRedirect", typeof(void), parameterTypes);
        ILGenerator generator = eventRedirectorMethod.GetILGenerator();
        generator.Emit(OpCodes.Nop);
        generator.Emit(OpCodes.Ldarg_0);
        generator.EmitCall(OpCodes.Call, this.GetType().GetMethod(nameof(OnEventRaised),BindingFlags.Public | BindingFlags.Instance), null);
        generator.Emit(OpCodes.Ret);
        _localHookDelegate = eventRedirectorMethod.CreateDelegate(eventHandlerType,this);
    }
    private void AddHandler()
    {
        _eventInfo.AddEventHandler(_owner, _localHookDelegate);
    }
    private void RemoveHandler()
    {
        _eventInfo.RemoveEventHandler(_owner, _localHookDelegate);
    }
    private Task<bool> RunAsync(TimeSpan timeout)
    {
        AddHandler();
        Task.Delay(timeout, _activeCancellationToken).
            ContinueWith(TimeOutTaskCompleted);

        return _taskCompletionSource.Task;
    }

    private void TimeOutTaskCompleted(Task tsk)
    {
        RemoveHandler();
        if (_elapsedCancellationTokenSource.IsCancellationRequested) return;

        if (_linkedCancellationTokenSource?.IsCancellationRequested == true)
            SetResult(TaskResult.Cancelled);
        else if (!_taskCompletionSource.Task.IsCompleted)
            SetResult(TaskResult.Failed);

    }

    public void OnEventRaised()
    {
        RemoveHandler();
        if (_taskCompletionSource.Task.IsCompleted)
        {
            if (!_elapsedCancellationTokenSource.IsCancellationRequested)
                _elapsedCancellationTokenSource?.Cancel(false);
        }
        else
        {
            if (!_elapsedCancellationTokenSource.IsCancellationRequested)
                _elapsedCancellationTokenSource?.Cancel(false);
            SetResult(TaskResult.Success);
        }
    }
    enum TaskResult { Failed, Success, Cancelled }
    private void SetResult(TaskResult result)
    {
        if (result == TaskResult.Success)
            _taskCompletionSource.SetResult(true);
        else if (result == TaskResult.Failed)
            _taskCompletionSource.SetResult(false);
        else if (result == TaskResult.Cancelled)
            _taskCompletionSource.SetCanceled();
        Dispose();

    }
    public void Dispose()
    {
        RemoveHandler();
        _elapsedCancellationTokenSource?.Dispose();
        _linkedCancellationTokenSource?.Dispose();
    }
}

它基本上依赖于CancellationTokenSource来报告结果。 它使用一些 IL 注入来创建一个委托来匹配事件的签名。 然后使用一些反射将该委托添加为该事件的处理程序。 generate 方法的主体只是调用 EventAwaiter 类上的另一个函数，然后使用CancellationTokenSource报告成功。

注意，不要在产品中按原样使用它。 这是一个工作示例。

例如，IL 生成是一个昂贵的过程。 您应该避免一遍又一遍地重新生成相同的方法，而是缓存它们。

Answer 11

AsyncEx 为此具有AsyncManualResetEvent 。 你可以：

var signal = new AsyncManualResetEvent();
await signal.WaitAsync();

并通过以下方式触发它：

signal.Set();