正在取消.Net 4.0中的任务延迟

Question

我目前正试图在一个必须以.Net 4.0为目标的程序中实现.Net 4.5的Task.Delay()方法的替代品。我在这个博客上找到了以下代码。

    /* You can write Task-based asynchronous methods by utilizing a TaskCompletionSource.
A TaskCompletionSource gives you a 'slave' Task that you can manually signal.
Calling SetResult() signals the task as complete, and any continuations kick off. */
void Main()
{    
    for (int i = 0; i < 10000; i++)
    {
        Task task = Delay (2000);
        task.ContinueWith (_ => "Done".Dump());
    }
}
Task Delay (int milliseconds)        // Asynchronous NON-BLOCKING method
{
    var tcs = new TaskCompletionSource<object>();
    new Timer (_ => tcs.SetResult (null)).Change (milliseconds, -1);
    return tcs.Task;
}

Tasks对我来说相当新鲜。System.Threading.Timer和TaskCompletionSource对我来说是全新的（截至今天），我有点挣扎。除此之外，我想知道如何将CancellationToken功能添加到该代码中。我假设我可以向Delay()方法添加一个参数，如下所示：

Task Delay (int milliseconds, CancellationToken token)        // Asynchronous NON-BLOCKING method
{
    var tcs = new TaskCompletionSource<object>();
    new Timer (_ => tcs.SetResult (null)).Change (milliseconds, -1);
    return tcs.Task;
}

但是，我该把检查令牌和退出该方法的逻辑放在哪里呢？回拨中的某个位置？这可能吗？

Answer 1

我试图尽可能少地更改您的代码，但这里有一个与Task.Delay.行为相同的工作示例

需要注意的是，我使用TrySetCanceled和TrySetResult，因为计时器可以在任务取消后完成。理想情况下，您希望停止计时器。

另请注意，取消的任务将引发TaskCanceledException

static void Main(string[] args)
{
    // A cancellation source that will cancel itself after 1 second
    var cancellationTokenSource = new CancellationTokenSource(TimeSpan.FromSeconds(1));
    try
    {
        // This will only wait 1 second because as it will be cancelled.
        Task t = Delay(5000, cancellationTokenSource.Token);                
        t.Wait();
        Console.WriteLine("The task completed");
    }
    catch (AggregateException exception)
    {
        // Expecting a TaskCanceledException
        foreach (Exception ex in exception.InnerExceptions)
            Console.WriteLine("Exception: {0}", ex.Message);
    }
    Console.WriteLine("Done");
    Console.ReadLine();
}
private static Task Delay(int milliseconds, CancellationToken token)
{
    var tcs = new TaskCompletionSource<object>();
    token.Register(() => tcs.TrySetCanceled());
    Timer timer = new Timer(_ => tcs.TrySetResult(null));
    timer.Change(milliseconds, -1);            
    return tcs.Task;
}

仔细阅读你的问题。如果您需要Task.Delay，并且您的目标是.NET 4.0，那么您应该使用http://www.nuget.org/packages/Microsoft.Bcl.Async/它包含方法TaskEx.Delay

Answer 2

像这样：

token.Register(() => tcs.TrySetCancelled());

Answer 3

这里是一个阻止垃圾收集器处理计时器的版本

    public static Task Delay(int milliseconds, CancellationToken token)
    {
        var tcs = new TaskCompletionSource<object>();
        var timer = new OneShotTimer((t) => {
            using ((OneShotTimer)t)
                tcs.SetResult(null);
        });
        token.Register(() => {
            if (timer.TryCancel())
            {
                using (timer)
                    tcs.SetCanceled();
            }
        });
        timer.Start(milliseconds);
        return tcs.Task;
    }

    public class OneShotTimer : IDisposable
    {
        private readonly object sync = new object();
        private readonly TimerCallback oneShotCallback;
        private readonly Timer timer;
        private bool isActive;
        public OneShotTimer(TimerCallback oneShotCallback, int dueTime = Timeout.Infinite)
        {
            this.oneShotCallback = oneShotCallback;
            this.isActive = dueTime != Timeout.Infinite;
            this.timer = new Timer(callback, this, dueTime, Timeout.Infinite);
        }

        public void Dispose()
        {
            timer.Dispose();
        }

        public void Start(int dueTime)
        {
            if (!tryChange(true, dueTime))
                throw new InvalidOperationException("The timer has already been started");
        }

        public bool TryCancel()
        {
            return tryChange(false, Timeout.Infinite);
        }

        public bool tryChange(bool targetIsActive, int dueTime)
        {
            bool result = false;
            lock (sync)
            {
                if (isActive != targetIsActive)
                {
                    result = true;
                    isActive = targetIsActive;
                    timer.Change(dueTime, Timeout.Infinite);
                }
            }
            return result;
        }

        private static void callback(object state)
        {
            var oneShotTimer = (OneShotTimer)state;
            if (oneShotTimer.TryCancel())
                oneShotTimer.oneShotCallback(oneShotTimer);
        }
    }