Comment : écouter plusieurs demandes d'annulation

Cet exemple montre comment écouter simultanément deux jetons d’annulation afin de pouvoir annuler une opération si un des jetons le demande.

Exemple

Dans l'exemple suivant, la méthode CreateLinkedTokenSource permet de fusionner deux jetons en un seul. Cela permet de transmettre le jeton à des méthodes qui n’acceptent qu’un seul jeton d’annulation comme argument. L’exemple illustre un scénario courant dans lequel une méthode doit observer à la fois un jeton reçu depuis l’extérieur de la classe et un jeton généré à l’intérieur de la classe.

using System;
using System.Threading;
using System.Threading.Tasks;

class LinkedTokenSourceDemo
{
    static void Main()
    {
        WorkerWithTimer worker = new WorkerWithTimer();
        CancellationTokenSource cts = new CancellationTokenSource();

        // Task for UI thread, so we can call Task.Wait wait on the main thread.
        Task.Run(() =>
        {
            Console.WriteLine("Press 'c' to cancel within 3 seconds after work begins.");
            Console.WriteLine("Or let the task time out by doing nothing.");
            if (Console.ReadKey(true).KeyChar == 'c')
                cts.Cancel();
        });

        // Let the user read the UI message.
        Thread.Sleep(1000);

        // Start the worker task.
        Task task = Task.Run(() => worker.DoWork(cts.Token), cts.Token);

        try
        {
            task.Wait(cts.Token);
        }
        catch (OperationCanceledException e)
        {
            if (e.CancellationToken == cts.Token)
                Console.WriteLine("Canceled from UI thread throwing OCE.");
        }
        catch (AggregateException ae)
        {
            Console.WriteLine("AggregateException caught: " + ae.InnerException);
            foreach (var inner in ae.InnerExceptions)
            {
                Console.WriteLine(inner.Message + inner.Source);
            }
        }

        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
        cts.Dispose();
    }
}

class WorkerWithTimer
{
    CancellationTokenSource internalTokenSource = new CancellationTokenSource();
    CancellationToken internalToken;
    CancellationToken externalToken;
    Timer timer;

    public WorkerWithTimer()
    {
        // A toy cancellation trigger that times out after 3 seconds
        // if the user does not press 'c'.
        timer = new Timer(new TimerCallback(CancelAfterTimeout), null, 3000, 3000);
    }

    public void DoWork(CancellationToken externalToken)
    {
        // Create a new token that combines the internal and external tokens.
        this.internalToken = internalTokenSource.Token;
        this.externalToken = externalToken;

        using (CancellationTokenSource linkedCts =
                CancellationTokenSource.CreateLinkedTokenSource(internalToken, externalToken))
        {
            try
            {
                DoWorkInternal(linkedCts.Token);
            }
            catch (OperationCanceledException)
            {
                if (internalToken.IsCancellationRequested)
                {
                    Console.WriteLine("Operation timed out.");
                }
                else if (externalToken.IsCancellationRequested)
                {
                    Console.WriteLine("Cancelling per user request.");
                    externalToken.ThrowIfCancellationRequested();
                }
            }
        }
    }

    private void DoWorkInternal(CancellationToken token)
    {
        for (int i = 0; i < 1000; i++)
        {
            if (token.IsCancellationRequested)
            {
                // We need to dispose the timer if cancellation
                // was requested by the external token.
                timer.Dispose();

                // Throw the exception.
                token.ThrowIfCancellationRequested();
            }

            // Simulating work.
            Thread.SpinWait(7500000);
            Console.Write("working... ");
        }
    }

    public void CancelAfterTimeout(object? state)
    {
        Console.WriteLine("\r\nTimer fired.");
        internalTokenSource.Cancel();
        timer.Dispose();
    }
}

Imports System.Threading
Imports System.Threading.Tasks

Class LinkedTokenSourceDemo
    Shared Sub Main()
        Dim worker As New WorkerWithTimer()
        Dim cts As New CancellationTokenSource()

        ' Task for UI thread, so we can call Task.Wait wait on the main thread.
        Task.Run(Sub()
                     Console.WriteLine("Press 'c' to cancel within 3 seconds after work begins.")
                     Console.WriteLine("Or let the task time out by doing nothing.")
                     If Console.ReadKey(True).KeyChar = "c"c Then
                         cts.Cancel()
                     End If
                 End Sub)
        ' Let the user read the UI message.
        Thread.Sleep(1000)

        ' Start the worker task.
        Dim t As Task = Task.Run(Sub() worker.DoWork(cts.Token), cts.Token)
        Try
            t.Wait()
        Catch ae As AggregateException
            For Each inner In ae.InnerExceptions
                Console.WriteLine(inner.Message)
            Next
        End Try

        Console.WriteLine("Press any key to exit.")
        Console.ReadKey()
        cts.Dispose()
    End Sub
End Class

Class WorkerWithTimer
    Dim internalTokenSource As CancellationTokenSource
    Dim token As CancellationToken
    Dim timer As Timer

    Public Sub New()
        internalTokenSource = New CancellationTokenSource()
        token = internalTokenSource.Token

        ' A toy cancellation trigger that times out after 3 seconds
        ' if the user does not press 'c'.
        timer = New Timer(New TimerCallback(AddressOf CancelAfterTimeout), Nothing, 3000, 3000)
    End Sub

    Public Sub DoWork(ByVal externalToken As CancellationToken)
        ' Create a new token that combines the internal and external tokens.
        Dim internalToken As CancellationToken = internalTokenSource.Token
        Dim linkedCts As CancellationTokenSource =
        CancellationTokenSource.CreateLinkedTokenSource(internalToken, externalToken)
        Using (linkedCts)
            Try
                DoWorkInternal(linkedCts.Token)
            Catch e As OperationCanceledException
                If e.CancellationToken = internalToken Then
                    Console.WriteLine("Operation timed out.")
                ElseIf e.CancellationToken = externalToken Then
                    Console.WriteLine("Canceled by external token.")
                    externalToken.ThrowIfCancellationRequested()
                End If
            End Try
        End Using
    End Sub

    Private Sub DoWorkInternal(ByVal token As CancellationToken)
        For i As Integer = 0 To 1000
            If token.IsCancellationRequested Then
                ' We need to dispose the timer if cancellation
                ' was requested by the external token.
                timer.Dispose()

                ' Output for demonstration purposes.
                Console.WriteLine(vbCrLf + "Cancelling per request.")

                ' Throw the exception.
                token.ThrowIfCancellationRequested()
            End If

            ' Simulating work.
            Thread.SpinWait(7500000)
            Console.Write("working... ")
        Next
    End Sub

    Public Sub CancelAfterTimeout(ByVal state As Object)
        Console.WriteLine(vbCrLf + "Timer fired.")
        internalTokenSource.Cancel()
        timer.Dispose()
    End Sub
End Class

Lorsque le jeton lié lève une OperationCanceledException, le jeton transmis à l’exception est le jeton lié, et non un des jetons prédécesseurs. Pour déterminer quel jeton a été annulé, vérifiez directement l’état des jetons prédécesseurs.

Dans cet exemple, AggregateException ne doit jamais être levée, mais elle est interceptée ici car, dans les scénarios réels, toutes les autres exceptions, hormis OperationCanceledException, levées à partir du délégué de tâche sont encapsulées dans une AggregateException.

Voir aussi

• Annulation dans les threads managés