CountdownEventCountdownEvent

System.Threading.CountdownEvent é um primitivo de sincronização que desbloqueia seus threads em espera após ser sinalizado um determinado número de vezes.System.Threading.CountdownEvent is a synchronization primitive that unblocks its waiting threads after it has been signaled a certain number of times. CountdownEvent foi projetado para cenários em que, caso contrário, seria necessário usar um ManualResetEvent ou um ManualResetEventSlim e diminuir manualmente uma variável antes de sinalizar o evento.CountdownEvent is designed for scenarios in which you would otherwise have to use a ManualResetEvent or ManualResetEventSlim and manually decrement a variable before signaling the event. Por exemplo, em um cenário de bifurcação/junção, basta criar um CountdownEvent que tenha uma contagem de sinal de 5 e, em seguida, iniciar cinco itens de trabalho no pool de threads e fazer com que cada item de trabalho chame Signal quando ele for concluído.For example, in a fork/join scenario, you can just create a CountdownEvent that has a signal count of 5, and then start five work items on the thread pool and have each work item call Signal when it completes. Cada chamada para Signal diminui a contagem de sinal em 1.Each call to Signal decrements the signal count by 1. No thread principal, a chamada para Wait será bloqueada até que a contagem de sinal seja zero.On the main thread, the call to Wait will block until the signal count is zero.

Observação

No caso do código que não têm que interagir com as APIs de sincronização herdadas do .NET Framework, considere o uso de System.Threading.Tasks.Task objetos ou o método Invoke para uma abordagem ainda mais fácil para expressar o paralelismo bifurcação-junção.For code that does not have to interact with legacy .NET Framework synchronization APIs, consider using System.Threading.Tasks.Task objects or the Invoke method for an even easier approach to expressing fork-join parallelism.

CountdownEvent tem estes recursos adicionais:CountdownEvent has these additional features:

  • A operação de espera pode ser cancelada usando tokens de cancelamento.The wait operation can be canceled by using cancellation tokens.

  • Sua contagem de sinal pode ser incrementada depois que a instância for criada.Its signal count can be incremented after the instance is created.

  • As instâncias podem ser reutilizadas após Wait ser retornado ao chamar o método Reset.Instances can be reused after Wait has returned by calling the Reset method.

  • As instâncias expõem um WaitHandle para integração com outras APIs de sincronização do .net, como WaitAll .Instances expose a WaitHandle for integration with other .NET synchronization APIs, such as WaitAll.

Uso básicoBasic Usage

O exemplo a seguir demonstra como usar um CountdownEvent com itens de trabalho ThreadPool.The following example demonstrates how to use a CountdownEvent with ThreadPool work items.

IEnumerable<Data> source = GetData();
using (CountdownEvent e = new CountdownEvent(1))
{
    // fork work:
    foreach (Data element in source)
    {
        // Dynamically increment signal count.
        e.AddCount();
        ThreadPool.QueueUserWorkItem(delegate(object state)
         {
             try
             {
                 ProcessData(state);
             }
             finally
             {
                 e.Signal();
             }
         },
         element);
    }
    e.Signal();

    // The first element could be run on this thread.

    // Join with work.
    e.Wait();
}
// .,.
Dim source As IEnumerable(Of Data) = GetData()
Dim e = New CountdownEvent(1)

' Fork work:
For Each element As Data In source
    ' Dynamically increment signal count.
    e.AddCount()

    ThreadPool.QueueUserWorkItem(Sub(state)
                                     Try
                                         ProcessData(state)
                                     Finally
                                         e.Signal()
                                     End Try
                                 End Sub,
                                  element)
Next
' Decrement the signal count by the one we added
' in the constructor.
e.Signal()

' The first element could also be run on this thread.
' ProcessData(New Data(0))

' Join with work:
e.Wait()

CountdownEvent com cancelamentoCountdownEvent With Cancellation

O exemplo a seguir mostra como cancelar a operação de espera em CountdownEvent usando um token de cancelamento.The following example shows how to cancel the wait operation on CountdownEvent by using a cancellation token. O padrão básico segue o modelo para cancelamento unificado, que foi introduzido no .NET Framework 4.The basic pattern follows the model for unified cancellation, which was introduced in .NET Framework 4. Para obter mais informações, consulte cancelamento em threads gerenciados.For more information, see Cancellation in Managed Threads.

class CancelableCountdownEvent
{
    class Data
    {
        public int Num { get; set; }
        public Data(int i) { Num = i; }
        public Data() { }
    }

    class DataWithToken
    {
        public CancellationToken Token { get; set; }
        public Data Data { get; private set; }
        public DataWithToken(Data data, CancellationToken ct)
        {
            this.Data = data;
            this.Token = ct;
        }
    }
    static IEnumerable<Data> GetData()
    {
        return new List<Data>() { new Data(1), new Data(2), new Data(3), new Data(4), new Data(5) };
    }
    static void ProcessData(object obj)
    {
        DataWithToken dataWithToken = (DataWithToken)obj;
        if (dataWithToken.Token.IsCancellationRequested)
        {
            Console.WriteLine("Canceled before starting {0}", dataWithToken.Data.Num);
            return;
        }

        for (int i = 0; i < 10000; i++)
        {
            if (dataWithToken.Token.IsCancellationRequested)
            {
                Console.WriteLine("Cancelling while executing {0}", dataWithToken.Data.Num);
                return;
            }
            // Increase this value to slow down the program.
            Thread.SpinWait(100000);
        }
        Console.WriteLine("Processed {0}", dataWithToken.Data.Num);
    }

    static void Main(string[] args)
    {
        EventWithCancel();

        Console.WriteLine("Press enter to exit.");
        Console.ReadLine();
    }

    static void EventWithCancel()
    {
        IEnumerable<Data> source = GetData();
        CancellationTokenSource cts = new CancellationTokenSource();

        //Enable cancellation request from a simple UI thread.
        Task.Factory.StartNew(() =>
             {
                 if (Console.ReadKey().KeyChar == 'c')
                     cts.Cancel();
             });

        // Event must have a count of at least 1
        CountdownEvent e = new CountdownEvent(1);

        // fork work:
        foreach (Data element in source)
        {
            DataWithToken item = new DataWithToken(element, cts.Token);
            // Dynamically increment signal count.
            e.AddCount();
            ThreadPool.QueueUserWorkItem(delegate(object state)
             {
                 ProcessData(state);
                 if (!cts.Token.IsCancellationRequested)
                     e.Signal();
             },
             item);
        }
        // Decrement the signal count by the one we added
        // in the constructor.
        e.Signal();

        // The first element could be run on this thread.

        // Join with work or catch cancellation.
        try
        {
            e.Wait(cts.Token);
        }
        catch (OperationCanceledException oce)
        {
            if (oce.CancellationToken == cts.Token)
            {
                Console.WriteLine("User canceled.");
            }
            else
            {
                throw; //We don't know who canceled us!
            }
        }
        finally {
            e.Dispose();
            cts.Dispose();
        }
        //...
    } //end method
} //end class
Option Strict On
Option Explicit On

Imports System.Collections
Imports System.Collections.Generic
Imports System.Linq
Imports System.Threading
Imports System.Threading.Tasks

Module CancelEventWait

    Class Data
        Public Num As Integer
        Public Sub New(ByVal i As Integer)
            Num = i
        End Sub
        Public Sub New()

        End Sub
    End Class

    Class DataWithToken
        Public Token As CancellationToken
        Public _data As Data
        Public Sub New(ByVal d As Data, ByVal ct As CancellationToken)
            Me._data = d
            Me.Token = ct
        End Sub
    End Class

    Class Program
        Shared Function GetData() As IEnumerable(Of Data)
            Dim nums = New List(Of Data)
            For i As Integer = 1 To 5
                nums.Add(New Data(i))
            Next
            Return nums
        End Function

        Shared Sub ProcessData(ByVal obj As Object)
            Dim dataItem As DataWithToken = CType(obj, DataWithToken)
            If dataItem.Token.IsCancellationRequested = True Then
                Console.WriteLine("Canceled before starting {0}", dataItem._data.Num)
                Exit Sub
            End If

            ' Increase this value to slow down the program.
            For i As Integer = 0 To 10000

                If dataItem.Token.IsCancellationRequested = True Then
                    Console.WriteLine("Cancelling while executing {0}", dataItem._data.Num)
                    Exit Sub
                End If
                Thread.SpinWait(100000)
            Next
            Console.WriteLine("Processed {0}", dataItem._data.Num)


        End Sub

        Shared Sub Main()
            DoEventWithCancel()
            Console.WriteLine("Press the enter key to exit.")
            Console.ReadLine()
        End Sub

        Shared Sub DoEventWithCancel()
            Dim source As IEnumerable(Of Data) = GetData()
            Dim cts As CancellationTokenSource = New CancellationTokenSource()

            ' Enable cancellation request from a simple UI thread.
            Task.Factory.StartNew(Sub()
                                      If Console.ReadKey().KeyChar = "c"c Then
                                          cts.Cancel()
                                      End If
                                  End Sub)

            ' Must have a count of at least 1 or else it is signaled.
            Dim e As CountdownEvent = New CountdownEvent(1)

            For Each element As Data In source
                Dim item As DataWithToken = New DataWithToken(element, cts.Token)

                ' Dynamically increment signal count.
                e.AddCount()

                ThreadPool.QueueUserWorkItem(Sub(state)
                                                 ProcessData(state)
                                                 If cts.Token.IsCancellationRequested = False Then
                                                     e.Signal()
                                                 End If
                                             End Sub,
                                            item)
            Next
            ' Decrement the signal count by the one we added
            ' in the constructor.
            e.Signal()
            ' The first element could be run on this thread.
            ' ProcessData(source(0))

            ' Join with work or catch cancellation exception
            Try
                e.Wait(cts.Token)
            Catch ex As OperationCanceledException
                If ex.CancellationToken = cts.Token Then
                    Console.WriteLine("User canceled.")
                Else : Throw ' we don't know who canceled us.

                End If
            Finally
                e.Dispose()
                cts.Dispose()
            End Try
        End Sub
    End Class
End Module

Observe que a operação de espera não cancela os threads que a estão sinalizando.Note that the wait operation does not cancel the threads that are signaling it. Normalmente, o cancelamento é aplicado a uma operação lógica e isso pode incluir aguardar o evento e todos os itens de trabalho que a espera está sincronizando.Typically, cancellation is applied to a logical operation, and that can include waiting on the event as well as all the work items that the wait is synchronizing. Neste exemplo, cada item de trabalho passa uma cópia do mesmo token de cancelamento para que ele possa responder à solicitação de cancelamento.In this example, each work item is passed a copy of the same cancellation token so that it can respond to the cancellation request.

Confira tambémSee also