Практическое руководство. Отмена задачи и ее дочерних элементов
В следующих примерах показано выполнение следующих задач.
Создание и запуск отменяемой задачи.
Передача токена отмены пользовательскому делегату и при необходимости экземпляру задачи.
Рассмотрение и ответ на запрос отмены в пользовательском делегате.
При необходимости проверка отмены задачи в вызывающем потоке.
Вызывающий поток не завершает задачу принудительно. Он только сообщает о запросе отмены. Если задача уже выполняется, за рассмотрение запроса и соответствующую реакцию отвечает пользовательский делегат. При запросе отмены до запуска задачи пользовательский делегат никогда не выполняется, и объект задачи переходит в состояние Canceled.
Пример
В этом примере показано, как завершить задачу Task и ее дочерние задачи в ответ на запрос отмены. В нем также показано, что при завершении пользовательского делегата путем выдачи исключения OperationCanceledException вызывающий поток может дополнительно использовать метод Wait или метод WaitAll для ожидания завершения задач. В этом случае делегат должен использовать блок try catch для обработки исключений в вызывающем потоке.
' How to: Cancel a Task and Its Children
Imports System.Threading
Imports System.Threading.Tasks
Module CancelATask
Sub Main()
Console.WriteLine("Press any key to start. Press 'c' to cancel.")
Console.ReadKey()
Dim tokenSource As New CancellationTokenSource()
Dim token As CancellationToken = tokenSource.Token
' Store references to the tasks so that we can wait on them and
' observe their status after cancellation.
Dim tasks(10) As Task
' Request cancellation of a single task when the token source is canceled.
' Pass the token to the user delegate, and also to the task so it can
' handle the exception correctly.
tasks(0) = Task.Factory.StartNew(Sub() DoSomeWork(1, token), token)
' Request cancellation of a task and its children. Note the token is passed
' to (1) the user delegate and (2) as the second argument to StartNew, so
' that the task instance can correctly handle the OperationCanceledException.
tasks(1) = Task.Factory.StartNew(Sub()
' Create some cancelable child tasks.
For i As Integer = 2 To 10
' For each child task, pass the same token
' to each user delegate and to StartNew.
tasks(i) = Task.Factory.StartNew(Sub(iteration) DoSomeWork(iteration, token), i, token)
' Passing the same token again to do work on the parent task.
' All will be signaled by the call to tokenSource.Cancel below.
DoSomeWork(2, token)
Next
End Sub _
, token)
' Give the tasks a second to start.
Thread.Sleep(1000)
' Request cancellation from the UI thread.
If Console.ReadKey().KeyChar = "c"c Then
tokenSource.Cancel()
Console.WriteLine("\nTask cancellation requested.")
' Optional: Observe the change in the Status property on the task.
' It is not necessary to wait on tasks that have canceled. However,
' if you do wait, you must enclose the call in a try-catch block to
' catch the OperationCanceledExceptions that are thrown. If you do
' not wait, no OCE is thrown if the token that was passed to the
' StartNew method is the same token that requested the cancellation.
End If
Try
Task.WaitAll(tasks)
Catch e As AggregateException
' For demonstration purposes, show the OCE message.
For Each v In e.InnerExceptions
Console.WriteLine("msg: " + v.Message)
Next
End Try
' Prove that the tasks are now all in a canceled state.
For i As Integer = 0 To tasks.Length
Console.WriteLine("task(0) status is now 1", i, tasks(i).Status)
Next
' Keep the console window open while the
' task completes its output.
Console.ReadLine()
End Sub
Sub DoSomeWork(ByVal taskNum As Integer, ByVal ct As CancellationToken)
' Was cancellation already requested?
If ct.IsCancellationRequested = True Then
Console.WriteLine("We were cancelled before we got started.")
Console.WriteLine("Press Enter to quit.")
ct.ThrowIfCancellationRequested()
End If
Dim maxIterations As Integer = 1000
' NOTE!!! An "OperationCanceledException was unhandled
' by user code" error will be raised here if "Just My Code"
' is enabled on your computer. On Express editions JMC is
' enabled and cannot be disabled. The exception is benign.
' Just press F5 to continue executing your code.
For i As Integer = 0 To maxIterations
' Do a bit of work. Not too much.
Dim sw As New SpinWait()
For j As Integer = 0 To 3000
sw.SpinOnce()
Next
Console.WriteLine("...0 ", taskNum)
If ct.IsCancellationRequested Then
Console.WriteLine("bye from 0.", taskNum)
Console.WriteLine("\nPress Enter to quit.")
ct.ThrowIfCancellationRequested()
End If
Next
End Sub
End Module
namespace CancellationWithOCE
{
using System;
using System.Threading;
using System.Threading.Tasks;
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Press any key to start. Press 'c' to cancel.");
Console.ReadKey();
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
// Store references to the tasks so that we can wait on them and
// observe their status after cancellation.
Task[] tasks = new Task[10];
// Request cancellation of a single task when the token source is canceled.
// Pass the token to the user delegate, and also to the task so it can
// handle the exception correctly.
tasks[0] = Task.Factory.StartNew(() => DoSomeWork(1, token), token);
// Request cancellation of a task and its children. Note the token is passed
// to (1) the user delegate and (2) as the second argument to StartNew, so
// that the task instance can correctly handle the OperationCanceledException.
tasks[1] = Task.Factory.StartNew(() =>
{
// Create some cancelable child tasks.
for (int i = 2; i < 10; i++)
{
// For each child task, pass the same token
// to each user delegate and to StartNew.
tasks[i] = Task.Factory.StartNew(iteration =>
DoSomeWork((int)iteration, token), i, token);
}
// Passing the same token again to do work on the parent task.
// All will be signaled by the call to tokenSource.Cancel below.
DoSomeWork(2, token);
}, token);
// Give the tasks a second to start.
Thread.Sleep(1000);
// Request cancellation from the UI thread.
if (Console.ReadKey().KeyChar == 'c')
{
tokenSource.Cancel();
Console.WriteLine("\nTask cancellation requested.");
// Optional: Observe the change in the Status property on the task.
// It is not necessary to wait on tasks that have canceled. However,
// if you do wait, you must enclose the call in a try-catch block to
// catch the OperationCanceledExceptions that are thrown. If you do
// not wait, no OCE is thrown if the token that was passed to the
// StartNew method is the same token that requested the cancellation.
#region Optional_WaitOnTasksToComplete
try
{
Task.WaitAll(tasks);
}
catch (AggregateException e)
{
// For demonstration purposes, show the OCE message.
foreach (var v in e.InnerExceptions)
Console.WriteLine("msg: " + v.Message);
}
// Prove that the tasks are now all in a canceled state.
for (int i = 0; i < tasks.Length; i++)
Console.WriteLine("task[{0}] status is now {1}", i, tasks[i].Status);
#endregion
}
// Keep the console window open while the
// task completes its output.
Console.ReadLine();
}
static void DoSomeWork(int taskNum, CancellationToken ct)
{
// Was cancellation already requested?
if (ct.IsCancellationRequested)
{
Console.WriteLine("We were cancelled before we got started.");
Console.WriteLine("Press Enter to quit.");
ct.ThrowIfCancellationRequested();
}
int maxIterations = 1000;
// NOTE!!! A benign "OperationCanceledException was unhandled
// by user code" error might be raised here. Press F5 to continue. Or,
// to avoid the error, uncheck the "Enable Just My Code"
// option under Tools > Options > Debugging.
for (int i = 0; i < maxIterations; i++)
{
// Do a bit of work. Not too much.
var sw = new SpinWait();
for (int j = 0; j < 3000; j++) sw.SpinOnce();
Console.WriteLine("...{0} ", taskNum);
if (ct.IsCancellationRequested)
{
Console.WriteLine("bye from {0}.", taskNum);
Console.WriteLine("\nPress Enter to quit.");
ct.ThrowIfCancellationRequested();
}
}
}
}
}
Класс System.Threading.Tasks.Task полностью интегрирован с моделью отмены, основанной на типах System.Threading.CancellationTokenSource и System.Threading.CancellationToken. Дополнительные сведения см. в разделах Отмена и Отмена задач.
См. также
Ссылки
System.Threading.CancellationTokenSource
System.Threading.CancellationToken
System.Threading.Tasks.Task<TResult>
Основные понятия
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