取消 Task.Run 个作业

Question

我有 async 方法，如下所示。当用户决定停止工作时，我希望有可能 cancel 它。我怎么能在新按钮中做到这一点 CancelUpdate_Click

    Private Async Sub btnUpdate_Click(sender As Object, e As EventArgs) Handles btnUpdate.Click
            Await DoWorkAsync()
    End Sub
    
    Private Function DoWorkAsync() As Task(Of List(Of Vals))
                Return _
                    Task.Run(
                        Async Function() _
                                _migrator.PrepareMigration(CInt(Invoke(New Func(Of Integer)(Function() lbNumms.SelectedValue))), CInt(Invoke(New Func(Of Integer)(Function() lbPro.SelectedValue)))))
    End Function
    
Private Sub CancelUpdate_Click(sender As Object, e As EventArgs) Handles CancelUpdate.Click
            'Stop DoWorkAsync if running
    End Sub

更新：

Private _tokenSource As New CancellationTokenSource()
        Private ReadOnly _token = _tokenSource.Token

Private Async Sub btnUpdate_Click(sender As Object, e As EventArgs) Handles btnUpdate.Click
     Try
        Private ReadOnly _tokenSource As New CancellationTokenSource()
    Private ReadOnly _token = _tokenSource.Token
            Await DoWorkAsync()
        Catch oe As OperationCanceledException
                MessageBox.Show("Task has been cancelled")
      Finally
        _tokenSource.Dispose()
      End Try
    End Sub
        
        Private Function DoWorkAsync() As Task(Of List(Of Vals))
                    Return _
                        Task.Run(
                            Function()
                                Dim tokenSource = New CancellationTokenSource()
                                Dim token = tokenSource.Token
                                Return _migrator.PrepareMigration(CInt(Invoke(New Func(Of Integer)(Function() lbNumms.SelectedValue))), CInt(Invoke(New Func(Of Integer)(Function() lbProducent.SelectedValue))), _token)
                            End Function)
        End Function
            
        
        Public Function PrepareMigration(value1 As Integer, value2 As Integer, cancellationToken As CancellationToken) As List(Of Vals))
        
        cancellationToken.ThrowIfCancellationRequested() 
        MethodA()
        cancellationToken.ThrowIfCancellationRequested()
        MethodB()
        
        End Function
        
        Private Sub btnCancel_Click(sender As Object, e As EventArgs) Handles btnCancel.Click
                      _tokenSource.Cancel()
        End Sub

Answer 1

取消并不是神奇的事情。它必须明确编码。在这种情况下，您的 PrepareMigration 例程需要采用 CancellationToken 参数并且需要定期检查该参数的 IsCancellationRequested 属性已设置为 True.

这样做的一个捷径是定期调用令牌上的 ThrowIfCancellationRequested；如果设置了属性，这将导致 OperationCanceledException。

如果您的日常工作 return 是 Task，您可以 return Task.FromCanceled.

你会得到 CancellationToken 从 CancellationTokenSource 传入。然后取消按钮的事件处理程序将在源上调用 Cancel 以发出已发出令牌的信号。

例如，如果您的处理涉及多个步骤，可能会像这样工作：

'Include other parameters as appropriate.
Sub PrepareMigration(ByVal token As CancellationToken)
    DoStep1()

    token.ThrowIfCancellationRequested()
    DoStep2()

    '...

    token.ThrowIfCancellationRequested()
    DoStepn()
End Sub

或者，如果您要处理一组项目，可能看起来像这样：

Sub PrepareMigration(ByVal token As CancellationToken)
    For Each item In itemsToProcess
        token.ThrowIfCancellationRequested()
        DoTheItem(item)
    Next
End Sub

根据需要混合搭配。如果在特定步骤中调用的例程特别是 long-running，则该例程本身可能需要是 cancellation-aware.

最终，检查的位置取决于计算步骤的详细信息（包括可能的“快速”与“慢速”步骤）以及您要对用户必须等待多长时间设置的限制请求取消生效。

取消 Task.Run 个作业

Cancel Task.Run job

vb.net

asynchronous

task

winforms

async-await