如何：透過輪詢接聽取消要求

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下列範例將示範一種方式：使用者程式碼可定期輪詢取消語彙基元，以查看呼叫執行緒是否已要求取消。 這個範例會使用 System.Threading.Tasks.Task 類型，但相同的模式適用於 System.Threading.ThreadPool 類型或 System.Threading.Thread 類型直接建立的非同步作業。

範例

輪詢需要某種可定期讀取布林值 IsCancellationRequested 屬性值的迴圈或遞迴程式碼。 如果您使用 System.Threading.Tasks.Task 類型且正在等候呼叫執行緒上的工作完成，您可以使用 ThrowIfCancellationRequested 方法來檢查屬性，並擲回例外狀況。 使用此方法，您可以確保會擲回正確的例外狀況來回應要求。 如果您使用 Task，則呼叫此方法優於手動擲回 OperationCanceledException。 如果您不需擲回例外狀況，則只需檢查屬性，如果屬性為 true，即從方法返回。

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

public struct Rectangle
{
   public int columns;
   public int rows;
}

class CancelByPolling
{
   static void Main()
   {
      var tokenSource = new CancellationTokenSource();
      // Toy object for demo purposes
      Rectangle rect = new Rectangle() { columns = 1000, rows = 500 };

      // Simple cancellation scenario #1. Calling thread does not wait
      // on the task to complete, and the user delegate simply returns
      // on cancellation request without throwing.
      Task.Run(() => NestedLoops(rect, tokenSource.Token), tokenSource.Token);

      // Simple cancellation scenario #2. Calling thread does not wait
      // on the task to complete, and the user delegate throws
      // OperationCanceledException to shut down task and transition its state.
      // Task.Run(() => PollByTimeSpan(tokenSource.Token), tokenSource.Token);

      Console.WriteLine("Press 'c' to cancel");
      if (Console.ReadKey(true).KeyChar == 'c') {
          tokenSource.Cancel();
          Console.WriteLine("Press any key to exit.");
      }

      Console.ReadKey();
      tokenSource.Dispose();
  }

   static void NestedLoops(Rectangle rect, CancellationToken token)
   {
      for (int col = 0; col < rect.columns && !token.IsCancellationRequested; col++) {
         // Assume that we know that the inner loop is very fast.
         // Therefore, polling once per column in the outer loop condition
         // is sufficient.
         for (int row = 0; row < rect.rows; row++) {
            // Simulating work.
            Thread.SpinWait(5_000);
            Console.Write("{0},{1} ", col, row);
         }
      }

      if (token.IsCancellationRequested) {
         // Cleanup or undo here if necessary...
         Console.WriteLine("\r\nOperation canceled");
         Console.WriteLine("Press any key to exit.");

         // If using Task:
         // token.ThrowIfCancellationRequested();
      }
   }
}

Imports System.Threading
Imports System.Threading.Tasks

Public Structure Rectangle
    Public columns As Integer
    Public rows As Integer
End Structure

Class CancelByPolling
    Shared Sub Main()
        Dim tokenSource As New CancellationTokenSource()
        ' Toy object for demo purposes
        Dim rect As New Rectangle()
        rect.columns = 1000
        rect.rows = 500

        ' Simple cancellation scenario #1. Calling thread does not wait
        ' on the task to complete, and the user delegate simply returns
        ' on cancellation request without throwing.
        Task.Run(Sub() NestedLoops(rect, tokenSource.Token), tokenSource.Token)

        ' Simple cancellation scenario #2. Calling thread does not wait
        ' on the task to complete, and the user delegate throws 
        ' OperationCanceledException to shut down task and transition its state.
        ' Task.Run(Sub() PollByTimeSpan(tokenSource.Token), tokenSource.Token)

        Console.WriteLine("Press 'c' to cancel")
        If Console.ReadKey(True).KeyChar = "c"c Then

            tokenSource.Cancel()
            Console.WriteLine("Press any key to exit.")
        End If

        Console.ReadKey()
        tokenSource.Dispose()
    End Sub

    Shared Sub NestedLoops(ByVal rect As Rectangle, ByVal token As CancellationToken)
        Dim col As Integer
        For col = 0 To rect.columns - 1
            ' Assume that we know that the inner loop is very fast.
            ' Therefore, polling once per column in the outer loop condition
            ' is sufficient.
            For col As Integer = 0 To rect.rows - 1
                ' Simulating work.
                Thread.SpinWait(5000)
                Console.Write("0',1' ", x, y)
            Next
        Next

        If token.IsCancellationRequested = True Then
            ' Cleanup or undo here if necessary...
            Console.WriteLine(vbCrLf + "Operation canceled")
            Console.WriteLine("Press any key to exit.")

            ' If using Task:
            ' token.ThrowIfCancellationRequested()
        End If
    End Sub
End Class

呼叫 ThrowIfCancellationRequested 相當快速，不會在迴圈中引進大量額外負荷。

如果您正在呼叫 ThrowIfCancellationRequested，當您要進行其他工作來回應取消 (擲回例外狀況除外) 時，只需明確檢查 IsCancellationRequested 屬性。 在此範例中，您可以看到程式碼會實際存取屬性兩次：一次是明確的存取，另一次則是在 ThrowIfCancellationRequested 方法中。 但由於讀取 IsCancellationRequested 屬性的動作牽涉到每次存取只能有一個暫時性讀取指令，因此，從效能觀點來看，兩次存取並不重要。 最好還是呼叫方法，而非手動擲回 OperationCanceledException。

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