vb.net 后台工作程序 CancelAsync() 不起作用

Backgroundworker CancelAsync() won't work

提问by Ponch0

I am trying to cancel my Backgroundworker with WorkerClass.bw.CancelAsync(). But it wont work at all.

我正在尝试使用WorkerClass.bw.CancelAsync(). 但它根本不起作用。

//edit! I posted the full Code here. May this will help. Okay, i added some Msgboxes to know if the Worker is still busy and the wired thing is, that i get a falsewhile the worker is doing things!?!?

//编辑！我在这里发布了完整的代码。愿这会有所帮助。好的，我添加了一些 Msgboxes 来知道 Worker 是否仍然很忙，而有线的事情是，我有一段false时间 Worker 正在做事！？！？

Public Class Form1

Private Sub btn_start_click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_start.Click
        Dim WorkerClass As New BGWClass
        WorkerClass.bw.WorkerSupportsCancellation = True
        WorkerClass.bw.WorkerReportsProgress = True
        If btn_start.Text = "Start" Then
            btn_start.Image = My.Resources.Resources.gem_remove
            btn_add_addy.Enabled = False
            btn_start.Text = "Stop"
            WorkerClass.Start()
            WorkerClass.bw.RunWorkerAsync()
            MsgBox(WorkerClass.bw.IsBusy & " " & WorkerClass.bw.WorkerSupportsCancellation)
        Else
            btn_start.Image = My.Resources.Resources.error_fwor
            btn_add_addy.Enabled = True
            btn_start.Enabled = False
            MsgBox(WorkerClass.bw.IsBusy & " " & WorkerClass.bw.WorkerSupportsCancellation)
            WorkerClass.bw.CancelAsync()
        End If
    End Sub
End Class

Public Class BGWClass
    Public bw As BackgroundWorker = New BackgroundWorker

    Sub Start()
        AddHandler bw.DoWork, AddressOf bw_DoWork
        AddHandler bw.ProgressChanged, AddressOf bw_ProgressChanged
        AddHandler bw.RunWorkerCompleted, AddressOf bw_RunWorkerCompleted
    End Sub

    Private Sub bw_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs)
         For x As Integer = 1 To 15
            If bw.CancellationPending Then
                e.Cancel = True
                Exit Sub
            End If
            bw.ReportProgress(x)
            Threading.Thread.Sleep(1000)
        Next
    End Sub

    Private Sub bw_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs)
        Dim myObject As Object = e.UserState
        Form1.Prgs_error.Text = "- Error: " + e.ProgressPercentage.ToString
    End Sub

    Private Sub bw_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs)
        '...
    End Sub
End Class

采纳答案by mclark1129

I think your problem is that you are only evaluating CancellationPendingonce, at the start of your bw_DoWorkmethod. Since there's not really a way to cancel the BackgroundWorkerbefore it starts, CancellationPendingwill always be false and will never interrupt the work. The BackgroundWorker doesn't use some magic to take over the program counter when you call CancelAsync.

我认为你的问题是你只CancellationPending在你的bw_DoWork方法开始时评估一次。由于没有真正的方法可以BackgroundWorker在它开始之前取消，CancellationPending将始终为假并且永远不会中断工作。当您调用 .BackgroundWorker 时，BackgroundWorker 不会使用一些魔法来接管程序计数器CancelAsync。

In order for this to work, the core logic of your DoWorkmethod will have to be implemented in a way that allows for the frequent polling of CancellationPendingso that the code will know exactly when to exit out of what it's doing. You will need to go from this:

为了使其工作，您的DoWork方法的核心逻辑必须以允许频繁轮询的方式实现，CancellationPending以便代码准确知道何时退出正在执行的操作。你需要从这里开始：

Private Sub bw_DoWork(ByVal sender As Object, 
                      ByVal e As System.ComponentModel.DoWorkEventArgs)

    If bw.CancellationPending = True Then
        e.Cancel = True
    Else
        'do stuff here
    End If

End Sub

To something more like this:

更像这样的事情：

Private Sub bw_DoWork(ByVal sender As Object, 
                      ByVal e As System.ComponentModel.DoWorkEventArgs)

    Dim workIsCompleted As Boolean = False
    While (Not bw.CancellationPending) AndAlso (Not workIsCompleted) Then

        ' Do stuff here, but incrementally so that the while loop can
        ' periodically check to see if CancelAsync has been called.
        ' Also, be sure to set workIsCompleted = True when the work is done.
        ' Otherwise, you will just spin forever until someone cancels it
        ' (Which may or may not be a bad thing, depending on your requirements)

    End While

End Sub

回答by dbasnett

Here is the basic model of a BackgroundWorker. Is this how yours look.

这是 BackgroundWorker 的基本模型。这是你的样子吗。

Dim WithEvents worker As New System.ComponentModel.BackgroundWorker

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    'start
    If Not worker.IsBusy Then
        worker.WorkerReportsProgress = True
        worker.WorkerSupportsCancellation = True
        worker.RunWorkerAsync()
    End If
End Sub

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
    'cancel
    If worker.IsBusy AndAlso worker.WorkerSupportsCancellation Then
        worker.CancelAsync()
    End If
End Sub

Private Sub foo_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles worker.DoWork
    For x As Integer = 1 To 15
        If worker.CancellationPending Then
            e.Cancel = True
            Exit For
        End If
        worker.ReportProgress(x)
        Threading.Thread.Sleep(1000)
    Next
End Sub

Private Sub foo_ProgressChanged(sender As Object, e As System.ComponentModel.ProgressChangedEventArgs) Handles worker.ProgressChanged
    Label1.Text = e.ProgressPercentage.ToString
End Sub

Private Sub foo_RunWorkerCompleted(sender As Object, e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles worker.RunWorkerCompleted
    If e.Cancelled Then
        Label1.Text = "canceled"
    ElseIf Not IsNothing(e.Error) Then
        Label1.Text = "error " & e.Error.Message
    Else
        Label1.Text = "done"
    End If
End Sub