快速连续停止然后启动相同的后台辅助角色

Question

当 CancelAsync(( 调用和 RunWorkerAsync(( 调用之间只有非常小的间隔时，我在处理使用 VB.net 停止然后启动后台工作线程时遇到了一些困难。我创建了一个示例来说明下面的问题。如果单击按钮一次，然后快速连续单击两次，则会出现此示例中的问题。从本质上讲，据我了解，问题在于后台工作线程仍在迭代 for 循环(包括以模仿需要一些时间才能运行的代码(，并且还没有机会检查取消是否挂起。

我尝试使用的解决方案是等待后台工作线程关闭，然后重新启动工作线程。但是，这个 while 循环无限期迭代 - 后台工作线程永远不会终止。我假设将主线程置于睡眠状态也会使后台工作线程进入睡眠状态。这是对的吗？

问题：当检查挂起的取消无法比现在更频繁地进行检查时，停止后台工作程序然后在几分之一秒后可能需要再次启动它的建议方法是什么？

非常感谢建议和评论。

Public Class Form1
    Dim WithEvents bgWorker As System.ComponentModel.BackgroundWorker = New System.ComponentModel.BackgroundWorker
    Friend WithEvents startStopButton As Button = New Button
    Dim workerShouldBeRunning As Boolean = False
    Sub startStopButton_click() Handles startStopButton.Click
        If workerShouldBeRunning Then
            bgWorker.CancelAsync()
        Else
            While bgWorker.IsBusy
                Threading.Thread.Sleep(1)
            End While
            bgWorker.RunWorkerAsync()
        End If
        workerShouldBeRunning = Not workerShouldBeRunning
    End Sub
    Sub bgWorker_doWork() Handles bgWorker.DoWork
        While Not bgWorker.CancellationPending
            Dim sum As Long
            For counter = 1 To 100000
                sum += counter
            Next
            Threading.Thread.Sleep(1000)
        End While
    End Sub
    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        startStopButton.Location = New Point(10, 100)
        startStopButton.Size = New System.Drawing.Size(200, 23)
        startStopButton.Text = "Click" 'Click to start or stop the background dummy process.
        Me.Controls.Add(startStopButton)
        bgWorker.WorkerSupportsCancellation = True
    End Sub
End Class

编辑：

根据Ross Presser的建议，我改进了示例代码。不过，解决方案似乎有点差。

Public Class Form1
    Dim WithEvents bgWorker As System.ComponentModel.BackgroundWorker = New System.ComponentModel.BackgroundWorker
    Friend WithEvents startStopButton As Button = New Button
    Dim workerShouldBeRunning As Boolean = False
    Dim startQueued As Boolean = False
    Sub startStopButton_click() Handles startStopButton.Click
        If workerShouldBeRunning Then
            bgWorker.CancelAsync()
        Else
            If bgWorker.IsBusy Then
                startQueued = True
            Else
                bgWorker.RunWorkerAsync()
            End If
        End If
        workerShouldBeRunning = Not workerShouldBeRunning
    End Sub
    Sub bgWorker_doWork() Handles bgWorker.DoWork
        While Not bgWorker.CancellationPending
            Dim sum As Long
            For counter = 1 To 100000
                sum += counter
            Next
            Threading.Thread.Sleep(1000)
        End While
    End Sub
    Sub bgWorkerCompleted() Handles bgWorker.RunWorkerCompleted
        If startQueued Then
            startQueued = False
            bgWorker.RunWorkerAsync()
        End If
    End Sub

    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        startStopButton.Location = New Point(10, 100)
        startStopButton.Size = New System.Drawing.Size(200, 23)
        startStopButton.Text = "Click" 'Click to start or stop the background dummy process.
        Me.Controls.Add(startStopButton)
        bgWorker.WorkerSupportsCancellation = True
    End Sub
End Class

Answer 1

这种方法怎么样?...我们只需将旧的 BackgroundWorker 替换为新的：

Public Class Form1
    Private WithEvents startStopButton As Button = New Button
    Private WithEvents bgWorker As System.ComponentModel.BackgroundWorker = Nothing
    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        startStopButton.Location = New Point(10, 100)
        startStopButton.Size = New System.Drawing.Size(200, 23)
        startStopButton.Text = "Click" 'Click to start or stop the background dummy process.
        Me.Controls.Add(startStopButton)
    End Sub
    Private Sub startStopButton_click() Handles startStopButton.Click
        If IsNothing(bgWorker) Then
            Static bgwCounter As Integer = 0
            bgwCounter = bgwCounter + 1
            bgWorker = New System.ComponentModel.BackgroundWorker
            bgWorker.WorkerSupportsCancellation = True
            bgWorker.RunWorkerAsync(bgwCounter)
        Else
            bgWorker.CancelAsync()
            bgWorker = Nothing
        End If
    End Sub
    Private Sub bgWorker_doWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles bgWorker.DoWork
        Dim bgwCounter As Integer = e.Argument
        Debug.Print("Running #" & bgwCounter)
        Dim bgw As System.ComponentModel.BackgroundWorker = sender
        While Not bgw.CancellationPending
            Dim sum As Long
            For counter As Integer = 1 To 100000
                sum += counter
            Next
            Threading.Thread.Sleep(1000)
        End While
        Debug.Print("Cancelled #" & bgwCounter)
    End Sub
End Class

Answer 2

而不是检查IsBusy，我会处理BackgroundWorker.RunWorkerCompleted事件

当后台操作已完成、已取消或引发异常时发生。

并将重新启动辅助角色推迟到此事件的处理程序。