取消Task.Run作業

我有async如下所示的方法。cancel當用戶決定停止作業時，我希望有可能。我怎么能在新按鈕中做到這一點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

uj5u.com熱心網友回復：

取消不是神奇的東西。它必須明確編碼。在這種情況下，您的PrepareMigration例程需要獲取一個CancellationToken引數，并且需要定期檢查該引數的IsCancellationRequested屬性是否已設定為True.

這樣做的一種捷徑是定期呼叫ThrowIfCancellationRequested令牌；這將導致一個OperationCanceledExceptionif 屬性被設定。

如果您正在處理回傳 a 的例程，Task則可以回傳Task.FromCanceled.

你會得到CancellationToken從 a 傳入的CancellationTokenSource。然后取消按鈕的事件處理程式將呼叫Cancel源以發出發出令牌的信號。

例如，如果您有涉及多個步驟的處理，它可能會像這樣作業：

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

    token.ThrowIfCancellationRequested()
    DoStep2()

    '...

    token.ThrowIfCancellationRequested()
    DoStepn()
End Sub

或者，如果你有一些東西正在處理一組專案，也許它看起來像這樣：

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

根據需要混合搭配。如果在特定步驟中呼叫的例程運行時間特別長，則該例程本身可能需要取消感知。

最終，您放置檢查的位置取決于計算步驟的詳細資訊（包括可能的“快速”與“緩慢”步驟）以及您希望設定的用戶必須等待多長時間的限制，直到請求的取消生效。

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