Canceling a While loop prematurely.

Question

I'm using a While loop that loops a certain number of cycles (1-576) based on a value entered by a user. It's activated by the user clicking a "Start" button, But I would like it to be able to be canceled using, preferably, the "Escape" key.

However, when the loop is going I can't get the program to recognize any keypresses.

Private Sub OnGlobalKeyDown(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles kh.KeyDown

        lblInput.Text = String.Format("'{0}' Code:{1}", e.KeyCode, CInt(e.KeyCode).ToString())


        If e.KeyCode = CType(27, Keys) Then
                    count = 0
                    loops = 0
        End If
    End Sub

My Loop

Private Sub RUNLOOP()
            While loops >= 1
                  ' my code that runs in the loop
                  loop = loop - 1
            End While
End Sub

While the loop is running my keypresses don't register, otherwise they register fine.

Answer 1

Inside your loop place the following code:

Application.DoEvents()

Answer 2

While counter <= 10
         ' skip remaining code in loop only if counter = 7
         If counter = 7 Then
            Exit While
         End If

         counter += 1
      End While

Answer 3

Have you thought of using BackgroundWorker to perform the while loop code in a background thread, making sure your GUI keeps responsive (=resizing/redrawing the form and letting the user "reach" the way you implemented the cancelation of the running work) without needing Application.DoEvents inside the loop to "fake" this behavior?

If you are already on .NET Framework 4.0 you could try and use parallel programming instead of the BackgroundWorker.

Answer 4

There are two ways do to this. You can either use a separate thread for your loop or use Application.DoEvents(). The thread approach will generally give you the best results.

Threaded:

You simply call StopLoop() to tell the loop to stop running. You don't want to set loop = 0 because then you would have to use the Interlocked class to ensure loop was set properly between threads.

Private Sub RunLoop()
  Dim loop As Action = AddressOf InternalRunLoop
  loop.BeginInvoke(null, null)
End Sub

Private bStopLoop As Boolean

Private Sub InternalRunLoop()
  While loops >= 1 And Not bStopLoop
    ' my code that runs in the loop
    loop = loop - 1
  End While
End Sub

Private Sub StopLoop()
  bStopLoop = True
End Sub

DoEvents:

Calling Application.DoEvents() will cause all the pending events on the window's thread to be processed. If each iteration of the loop takes some time, then the app will still appear to the user like it is non-responsive, which is why the threaded approach is preferred.

Private SubRunLoop()
  While loops >= 1 And Not bStopLoop
    ' my code that runs in the loop
    loop = loop - 1
    Application.DoEvents()
  End While
End Sub