如何：取消链接数据流块

  • 项目

本文档介绍如何取消目标数据流块与其源的链接。

注意

TPL 数据流库（System.Threading.Tasks.Dataflow 命名空间）不随 .NET 一起分发。 若要在 Visual Studio 中安装 System.Threading.Tasks.Dataflow 命名空间，请打开项目，选择“项目”菜单中的“管理 NuGet 包”，再在线搜索 System.Threading.Tasks.Dataflow 包。 或者，若要使用 .NET Core CLI 进行安装，请运行 dotnet add package System.Threading.Tasks.Dataflow。

示例

下面的示例创建了三个 TransformBlock<TInput,TOutput> 对象，每个对象调用 TrySolution 方法来计算值。 此示例只需使用第一次调用 TrySolution 的结果即可完成。

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

// Demonstrates how to unlink dataflow blocks.
class DataflowReceiveAny
{
   // Receives the value from the first provided source that has
   // a message.
   public static T ReceiveFromAny<T>(params ISourceBlock<T>[] sources)
   {
      // Create a WriteOnceBlock<T> object and link it to each source block.
      var writeOnceBlock = new WriteOnceBlock<T>(e => e);
      foreach (var source in sources)
      {
         // Setting MaxMessages to one instructs
         // the source block to unlink from the WriteOnceBlock<T> object
         // after offering the WriteOnceBlock<T> object one message.
         source.LinkTo(writeOnceBlock, new DataflowLinkOptions { MaxMessages = 1 });
      }
      // Return the first value that is offered to the WriteOnceBlock object.
      return writeOnceBlock.Receive();
   }

   // Demonstrates a function that takes several seconds to produce a result.
   static int TrySolution(int n, CancellationToken ct)
   {
      // Simulate a lengthy operation that completes within three seconds
      // or when the provided CancellationToken object is cancelled.
      SpinWait.SpinUntil(() => ct.IsCancellationRequested,
         new Random().Next(3000));

      // Return a value.
      return n + 42;
   }

   static void Main(string[] args)
   {
      // Create a shared CancellationTokenSource object to enable the
      // TrySolution method to be cancelled.
      var cts = new CancellationTokenSource();

      // Create three TransformBlock<int, int> objects.
      // Each TransformBlock<int, int> object calls the TrySolution method.
      Func<int, int> action = n => TrySolution(n, cts.Token);
      var trySolution1 = new TransformBlock<int, int>(action);
      var trySolution2 = new TransformBlock<int, int>(action);
      var trySolution3 = new TransformBlock<int, int>(action);

      // Post data to each TransformBlock<int, int> object.
      trySolution1.Post(11);
      trySolution2.Post(21);
      trySolution3.Post(31);

      // Call the ReceiveFromAny<T> method to receive the result from the
      // first TransformBlock<int, int> object to finish.
      int result = ReceiveFromAny(trySolution1, trySolution2, trySolution3);

      // Cancel all calls to TrySolution that are still active.
      cts.Cancel();

      // Print the result to the console.
      Console.WriteLine("The solution is {0}.", result);

      cts.Dispose();
   }
}

/* Sample output:
The solution is 53.
*/

Imports System.Threading
Imports System.Threading.Tasks.Dataflow

' Demonstrates how to unlink dataflow blocks.
Friend Class DataflowReceiveAny
    ' Receives the value from the first provided source that has 
    ' a message.
    Public Shared Function ReceiveFromAny(Of T)(ParamArray ByVal sources() As ISourceBlock(Of T)) As T
        ' Create a WriteOnceBlock<T> object and link it to each source block.
        Dim writeOnceBlock = New WriteOnceBlock(Of T)(Function(e) e)
        For Each source In sources
            ' Setting MaxMessages to one instructs
            ' the source block to unlink from the WriteOnceBlock<T> object
            ' after offering the WriteOnceBlock<T> object one message.
            source.LinkTo(writeOnceBlock, New DataflowLinkOptions With {.MaxMessages = 1})
        Next source
        ' Return the first value that is offered to the WriteOnceBlock object.
        Return writeOnceBlock.Receive()
    End Function

    ' Demonstrates a function that takes several seconds to produce a result.
    Private Shared Function TrySolution(ByVal n As Integer, ByVal ct As CancellationToken) As Integer
        ' Simulate a lengthy operation that completes within three seconds
        ' or when the provided CancellationToken object is cancelled.
        SpinWait.SpinUntil(Function() ct.IsCancellationRequested, New Random().Next(3000))

        ' Return a value.
        Return n + 42
    End Function

    Shared Sub Main(ByVal args() As String)
        ' Create a shared CancellationTokenSource object to enable the 
        ' TrySolution method to be cancelled.
        Dim cts = New CancellationTokenSource()

        ' Create three TransformBlock<int, int> objects. 
        ' Each TransformBlock<int, int> object calls the TrySolution method.
        Dim action As Func(Of Integer, Integer) = Function(n) TrySolution(n, cts.Token)
        Dim trySolution1 = New TransformBlock(Of Integer, Integer)(action)
        Dim trySolution2 = New TransformBlock(Of Integer, Integer)(action)
        Dim trySolution3 = New TransformBlock(Of Integer, Integer)(action)

        ' Post data to each TransformBlock<int, int> object.
        trySolution1.Post(11)
        trySolution2.Post(21)
        trySolution3.Post(31)

        ' Call the ReceiveFromAny<T> method to receive the result from the 
        ' first TransformBlock<int, int> object to finish.
        Dim result As Integer = ReceiveFromAny(trySolution1, trySolution2, trySolution3)

        ' Cancel all calls to TrySolution that are still active.
        cts.Cancel()

        ' Print the result to the console.
        Console.WriteLine("The solution is {0}.", result)

        cts.Dispose()
    End Sub
End Class

' Sample output:
'The solution is 53.
'

为了接收完成的第一个 TransformBlock<TInput,TOutput> 对象的值，该示例定义了 ReceiveFromAny(T) 方法。 ReceiveFromAny(T) 方法接受一个 ISourceBlock<TOutput> 对象的数组，并将其中每个对象链接到 WriteOnceBlock<T> 对象。 当使用 LinkTo 方法将源数据流块链接到目标块时，源会在数据可用时将消息传播到目标。 因为 WriteOnceBlock<T> 类只接受为其提供的第一条消息，ReceiveFromAny(T) 方法通过调用 Receive 方法来生成其结果。 这将生成提供给 WriteOnceBlock<T> 对象的第一条消息。 LinkTo 方法有一个重载版本，其含有一个具有 MaxMessages 属性的 DataflowLinkOptions 对象，当该属性设置为 1 时，则指示源块在目标收到来自源的一条消息后取消与目标的链接。 取消 WriteOnceBlock<T> 对象与其源的链接非常重要，因为当 WriteOnceBlock<T> 对象收到一条消息后，便不再需要源数组与 WriteOnceBlock<T> 对象之间的关系。

为了使 TrySolution 的剩余调用能够在其中一个调用计算了一个值后结束，TrySolution 方法采用一个 CancellationToken 对象，该对象在对 ReceiveFromAny(T) 的调用返回后将被取消。 当此 SpinUntil 对象取消时，CancellationToken 方法将返回。

请参阅