取消令牌源和嵌套任务
本文关键字:嵌套 任务 令牌 取消 | 更新日期: 2023-09-27 18:32:24
我对我正在使用的取消令牌源有疑问,如下面的代码所示:
void Process()
{
//for the sake of simplicity I am taking 1, in original implementation it is more than 1
var cancellationToken = _cancellationTokenSource.Token;
Task[] tArray = new Task[1];
tArray[0] = Task.Factory.StartNew(() =>
{
cancellationToken.ThrowIfCancellationRequested();
//do some work here
MainTaskRoutine();
}, cancellationToken);
try
{
Task.WaitAll(tArray);
}
catch (Exception ex)
{
//do error handling here
}
}
void MainTaskRoutine()
{
//for the sake of simplicity I am taking 1, in original implementation it is more than 1
//this method shows that a nested task is created
var cancellationToken = _cancellationTokenSource.Token;
Task[] tArray = new Task[1];
tArray[0] = Task.Factory.StartNew(() =>
{
cancellationToken.ThrowIfCancellationRequested();
//do some work here
}, cancellationToken);
try
{
Task.WaitAll(tArray);
}
catch (Exception ex)
{
//do error handling here
}
}
编辑:进一步阐述
最终目标是:当用户取消操作时,所有立即挂起的任务(子项或孙项)都应取消。
场景:根据上面的代码:1. 我首先检查用户是否要求取消2. 如果用户没有要求取消,则只继续执行任务(请参阅处理方法)。示例代码在此处仅显示一个任务,但实际上可以有三个或更多
假设 CPU 开始处理 Task1,而其他任务仍在任务队列中等待某个 CPU 来执行它们。用户请求取消:处理方法中的任务 2,3 将立即取消,但任务 1 将继续工作,因为它已在处理中。
在任务 1 中,它调用方法 MainTaskRoutine,这反过来又会创建更多任务。
在 MainTaskRoutine 的函数中,我写了:cancelToken.ThrowIfCancelRequest();
所以问题是:使用CancelTokenSource的正确方法,因为它依赖于Task.WaitAll()?
[已编辑] 当您在代码中使用数组时,我假设可能有多个任务,而不仅仅是一个。我还假设在你从Process
开始的每个任务中,你想先做一些 CPU 密集型工作(//do some work here
),然后运行 MainTaskRoutine
。
如何处理任务取消异常由项目设计工作流决定。 例如,您可以在Process
方法中执行此操作,或者从调用Process
的位置执行此操作。如果您唯一关心的是从跟踪挂起任务的数组中删除 Task 对象,则可以使用 Task.ContinueWith 完成此操作。无论任务的完成状态如何(Cancelled
、Faulted
或RanToCompletion
),都将执行延续:
Task Process(CancellationToken cancellationToken)
{
var tArray = new List<Task>();
var tArrayLock = new Object();
var task = Task.Run(() =>
{
cancellationToken.ThrowIfCancellationRequested();
//do some work here
return MainTaskRoutine(cancellationToken);
}, cancellationToken);
// add the task to the array,
// use lock as we may remove tasks from this array on a different thread
lock (tArrayLock)
tArray.Add(task);
task.ContinueWith((antecedentTask) =>
{
if (antecedentTask.IsCanceled || antecedentTask.IsFaulted)
{
// handle cancellation or exception inside the task
// ...
}
// remove task from the array,
// could be on a different thread from the Process's thread, use lock
lock (tArrayLock)
tArray.Remove(antecedentTask);
}, TaskContinuationOptions.ExecuteSynchronously);
// add more tasks like the above
// ...
// Return aggregated task
Task[] allTasks = null;
lock (tArrayLock)
allTasks = tArray.ToArray();
return Task.WhenAll(allTasks);
}
您的MainTaskRoutine
可以以与Process
完全相同的方式构建,并具有相同的方法签名(返回Task
)。
然后,您可能希望对 Process
返回的聚合任务执行阻塞等待,或者异步处理其完成,例如:
// handle the completion asynchronously with a blocking wait
void RunProcessSync()
{
try
{
Process(_cancellationTokenSource.Token).Wait();
MessageBox.Show("Process complete");
}
catch (Exception e)
{
MessageBox.Show("Process cancelled (or faulted): " + e.Message);
}
}
// handle the completion asynchronously using ContinueWith
Task RunProcessAync()
{
return Process(_cancellationTokenSource.Token).ContinueWith((task) =>
{
// check task.Status here
MessageBox.Show("Process complete (or cancelled, or faulted)");
}, TaskScheduler.FromCurrentSynchronizationContext());
}
// handle the completion asynchronously with async/await
async Task RunProcessAync()
{
try
{
await Process(_cancellationTokenSource.Token);
MessageBox.Show("Process complete");
}
catch (Exception e)
{
MessageBox.Show("Process cancelled (or faulted): " + e.Message);
}
}
经过一些研究,我发现了这个链接。
代码现在如下所示:请参阅下面的代码中CancelTokenSource.CreateLinkedTokenSource的用法
void Process()
{
//for the sake of simplicity I am taking 1, in original implementation it is more than 1
var cancellationToken = _cancellationTokenSource.Token;
Task[] tArray = new Task[1];
tArray[0] = Task.Factory.StartNew(() =>
{
cancellationToken.ThrowIfCancellationRequested();
//do some work here
MainTaskRoutine(cancellationToken);
}, cancellationToken);
try
{
Task.WaitAll(tArray);
}
catch (Exception ex)
{
//do error handling here
}
}
void MainTaskRoutine(CancellationToken cancellationToken)
{
//for the sake of simplicity I am taking 1, in original implementation it is more than 1
//this method shows that a nested task is created
using (var cancellationTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken))
{
var cancelToken = cancellationTokenSource.Token;
Task[] tArray = new Task[1];
tArray[0] = Task.Factory.StartNew(() =>
{
cancelToken.ThrowIfCancellationRequested();
//do some work here
}, cancelToken);
try
{
Task.WaitAll(tArray);
}
catch (Exception ex)
{
//do error handling here
}
}
}
注意:我没有使用它,但一旦完成,我会通知您:)