在循环中，如何加入“按Esc键中止”功能？

Question

窗体中有一ContextMenuStrip，其中某个ToolStripMenuItem.Click事件中，执行类似如下代码：

For each ...

'提示用户：正在......按Esc键中止

'执行代码

Next

也就是说，在执行过程中，用户可随时按Esc键中止。

但是，点击菜单项后，它失去焦点，无法捕捉KeyDown事件，是不是要在窗体的KeyDown事件中处理呢？如何实现这一功能比较好呢？

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韩立学

Answer 1

尝试把Form的KeyPriview设置为true（窗体捕获按键）。

另外执行一个长时间的任务可能导致卡死，建议这样做功能：

Imports System.Threading.Tasks
Imports System.Threading
 
Public Class Form1
    '取消标识符
    Private sourceCancel As New CancellationTokenSource
    Private flag As CancellationToken = sourceCancel.Token
 
 
    Private Sub LoopingToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles LoopingToolStripMenuItem.Click
        Dim task As New Task(Sub()
                                 For index = 1 To 100
                                     If (flag.IsCancellationRequested) Then
                                         MsgBox("正在取消……")
                                         flag.ThrowIfCancellationRequested()
                                     End If
                                     '模拟大数据量缓慢处理的进程，如果需要和UI交互，请直接使用BeginInvoke方法处理
                                     Threading.Thread.Sleep(1000)
                                 Next
                             End Sub, flag)
        task.Start()
    End Sub
 
    Private Sub Form1_KeyDown(sender As Object, e As KeyEventArgs) Handles MyBase.KeyDown
        If (e.KeyCode = Keys.Escape) Then
            '取消
            sourceCancel.Cancel()
        End If
    End Sub
End Class

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Answer 2

另外参考使用BackGroundWorker：

代码

尝试把Form的KeyPriview设置为true（窗体捕获按键）。

Imports System.Threading.Tasks
Imports System.Threading
Imports System.ComponentModel
 
Public Class Form1
    '是否取消标识符
    Private flag As Boolean = False
 
    Private Sub DoHeavyWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs)
        Dim bgWork As BackgroundWorker = CType(sender, BackgroundWorker)
 
        For i As Integer = 1 To 100
            If (flag) Then
                '取消
                e.Cancel = True
                Exit For
            End If
            Thread.Sleep(1000)
            bgWork.ReportProgress(i)
        Next
    End Sub
 
    Private Sub LoopingToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles LoopingToolStripMenuItem.Click
        bgBusy.RunWorkerAsync()
    End Sub
 
    Private Sub Form1_KeyDown(sender As Object, e As KeyEventArgs) Handles MyBase.KeyDown
        '取消
        flag = True
    End Sub
 
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
 
    End Sub
 
    Private Sub bgBusy_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles bgBusy.DoWork
        DoHeavyWork(sender, e)
    End Sub
 
    Private Sub bgBusy_ProgressChanged(sender As Object, e As ProgressChangedEventArgs) Handles bgBusy.ProgressChanged
        ProgressBar1.Value = e.ProgressPercentage
    End Sub
 
    Private Sub bgBusy_RunWorkerCompleted(sender As Object, e As RunWorkerCompletedEventArgs) Handles bgBusy.RunWorkerCompleted
        If (e.Cancelled) Then
            MsgBox("Cancelled")
        Else
            MsgBox("Finished")
        End If
    End Sub
End Class

运行效果：

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Answer 3

注意第二个示例代码中不要忘记设置以上属性！

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Answer 4

谢谢回复！

我用的是VS2010，单线程有无更好的办法？

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韩立学

Answer 5

谢谢回复！

我用的是VS2010，单线程有无更好的办法？

---

韩立学

参考我的第二个回复（BackGroundWorker）

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Answer 6

谢谢回复！

我用的是VS2010，单线程有无更好的办法？

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韩立学

其实VS2010照样可以使用这个功能，选择net framework4即可。

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