轉換的Thread.join（延時），以相當於非線程模式

Question

我有以下代碼：

Thread validationThread = new Thread(DoAsyncValidationMethod); 

validationThread.Start(threadParams); 

bool isAborted = false; 

int timeLeft = 5000; 

// wait for thread terminates itself 
if (!validationThread.Join(timeLeft)) 
{ 
    // timeout, abort thread 
    validationThread.Abort(); 
    isAborted = true; 
} 

換句話說，我做了應該在指定的時間間隔來完成（一些工作5000毫秒）。如果完成或不完成，採取的線程方法將保證代碼在啓動後不遲於5秒返回。

有沒有辦法用異步/等待模式來完成這個？或者因爲線程只是用於訣竅，還有其他方法可以採用嗎？

越優雅簡單越好。 謝謝你的幫助。

Answer 1

這是我在其中一個項目中使用的一段代碼。

更多的，看看Task.WhenAny(param Task[] task) method on MSDN。它返回已完成的任務。

var timeout = 5000; 
var task = Task.Run(() => { 
    Thread.Sleep(new Random().Next(3000, 7000));//Running time simulation! 
    return "Say my name!";//Breaking Bad! 
}); 
if (await Task.WhenAny(task, Task.Delay(timeout)) == task) { 
    // Task completed within timeout. 
    // What if the task is faulted or canceled. 
    // I did not supply a cancellation token but it can easily be added 
    // We re-await the task so that any exceptions/cancellation is rethrown. 
    await task; 
} 
else { 
    // timeout/cancellation logic 
} 

Answer 2

你可以用微軟的Reactive Framework（NuGet「System.Reactive」）很好地做到這一點。那麼你可以這樣做：

IObservable<bool> query = 
    Observable.Amb(
     Observable.Timer(TimeSpan.FromSeconds(5.0)).Select(_ => false), 
     Observable.Start(() => DoAsyncValidationMethod()).Select(_ => true)); 

IDisposable subscription = 
    query.Subscribe(flag => 
    { 
     /* `true` succeeded, `false` timed out */ 
    }); 

Answer 3

使用CancellationTokenSource。該來源是CancellationTokens的工廠。您將此令牌傳遞給您希望能夠以簡潔的方式取消其處理的所有人。如果要取消所有具有CancellationToken和CancellationTokenSource的進程，只需告知CancellationTokenSource將取消發送到它生成的所有CancellationTokens，從而取消所有具有來自此源令牌的進程。

而好的是，CancellationTokenSource有一個CancelAfter(some timeout)。

它的良好實踐，讓您的流程啓動器爲您提供cancellationToken，因此您的流程啓動器可以決定在一次調用中要取消哪些流程。

public async Task<MyResult> MyProcessAsync(CancellationToken cancellationToken, ...) 
{ 
    // do something lengthy processing, without await, regularly check if Cancellation requested 
    while (stillProcessing) 
    { 
     cancellationToken.ThrowIfCancellationRequested(); 
     ... // process 
    } 

    // do some processing with async-await: 
    await myDbContext.SaveChangesAsync(cancellationToken); 
} 

用法：

private async Task LengthyProcessing(...) 
{ 
    CancellationTokenSource tokenSource = new CancellationTokenSource(); 
    tokenSource.CancelAfter(TimeSpan.FromSeconds(5)); 

    CancellationToken myCancellationToken = tokenSource.Token 
    try 
    { 
     // Start one task, don't await yet: 
     var myTask = await MyProcessAsync(myCancellationToken, ...) 
     // during processing there will be regular check if cancellation requested 
     // meanwhile, whenever myTask has to await, I can do some processing 
     // I'll have to check for cancellation regularly also: 

     while(...) 
     { 
      myCancellationToken.ThrowIfCancellationRequested(); 
      DoSomeProcessing(); 
      ... 
     } 

     MyResult result = await myTask; 
     // if here, not cancelled. can use Result: 
     ProcessResult(result); 
    } 
    catch (OperationCanceledException exc) 
    { 
     ProcessOperationCanceled(); 
    } 
} 

請注意，只有一個CancelltionTokenSource使用。無論何時此源認爲某些內容應該被取消，所有來自該源的令牌都會收到有關取消請求的通知。

而不是通過CancellationToken.ThrowIfCancellationRequested()使用異常處理考慮使用bool CancellationToken.IsCancellationRequested。

MSDN how to cancel a task and its children