How to Stop and Restart a BackGroundWorker

[LeoVBNET]

Hi. I'm working on a project in which use a BackGroundWorker control between two forms.

I need to Cancel and immediately to restart the BackGroundorker from the same event. The problem is that: when BackGroundWorker.CancelAsync(), apparently the thread does not cancel and when it gets to BackGroundWorker.RunWorkerAsync(), the exception "BackgroundWorker is busy" is thrown.

So, how can I cancel and restart the backgroundworker from the same event.

(Note: I tried with the Threading Class too and I got a similar problem. When use Thread.Abort() or Thread.Suspended(), the thread is stopped but it cannot restart again.)

[Mihail]

I try a response only because your post is 3 days older.
Define a public variable. Say: Public StopRun As Boolean.
Before starting the BackGroundorker set this variable to False.
From the BackGroundorker verify from time to time this variable:
If StopRun Then Exit

Set to True this variable from the main code when you wish to stop the BackGroundorker.

Sorry, my English is not very good. Hope you get the idea.

[LeoVBNET]

Thanks Mihail but it does not work propertly. I used a boolean 'StopRun' and when it is applied from a button or from another independent method works, but remember I need to stop and restart the thread from the same method.
Basically, I did StopRun on True, then sleep some milisecs and then StopRun on False and call the thread again. But the called method does not start from the begining or it just does not stop until finish.

I'm still looking for.

[Mihail]

What you are trying to do ?
Why you need to stop the BackGroundorker for miliseconds.
Do you know the DoEvents statement ?

If you explain WHY you wish to pause that BackGroundorker, maybe we can find a solution.

[LeoVBNET]

Ok. I will try to be clear.
See the code below:

Form_List
  -ListView
  Public Sub Fill_ListView()
    (every code to fill the ListView)
     Linq
     ImageList --> Module1.FindImage(Image As Bitmap)
     ListViewItem
     etc.
 
---------------------
 
Form_PPAL
  Private MyList_1 As Form_List
  Private MyList_2 As Form_List
 
  Me.MyList_1.Fill_ListView
  Me.MyList_2.Fill_ListView
 

Ok. The application of course has a lot of things I do not show. I just show you the important issue.

The problem comes when it has to find each image of each item and there are a lot of items to show. It takes a lot of time. So, I suppose to use a thread to populate the ListViews.

In the first sight the thread works fine but the user can switch a radios buttons ir order to visualize another items.

So, just imagine when the user click on a radio button, the application has to stop the loading of items in either ListViews and then needs to start over again with populate de ListView with other items. That's all.

Finally, I've heard about DoEvents but never have used it. I'm gonna try.

Thanks.

[Mihail]

Yep.
Seems that I have had a good feeling.
I'm pretty sure that the DoEvents statement is the answer to your issue

Say you have a long cycle:

For i = 1 to 1000000000000000000000000000000000
    For j = 1 to 1000000000000000000000000000000000
        For k = 1 to 1000000000000000000000000000000000
            DoEvents
            'Other code
        Next k
    Next j
Next i
MsgBox("OK - I have finished

The DoEvents statement will allow you to do anything else with your computer WHILE the cycle IS RUNNING.
No need to stop this cycle.
And, when I say anything is really ANYTHING:
You can see a movie, run other procedures, drink a beer :)

Cheers !

[LeoVBNET]

jeje... I appreciate all your good intention. Really I do. But, my friend, that´s not the solution.

When user changes a radiobutton option or wants to go to another place inside application, he should not wait for ListView loading. Besides, if user make two, three or more changes (clicks, etc), all of them are placed inside memory waiting to act. It is like when a program does not respond and suddenly every click you did appers one behind other and other.

That's it. When user clicks radiobutton or any other button, the app must terminate the ListView loading, take de new values and then populates the ListView once again from benining with new values.

It would be great if we can do something like this:

Private Sub Fill_ListView()
 
 If Thread1.ThreadState = Threading.ThreadState.Running Then
    Thread1.Abort()
    <some code>
 End If
 
 Thread1.Start()
 
End Sub
 

[Mihail]

And this is fairly easy to obtain using DoEvents.

Note that (see my previous post) you must allow events in the most interior cycle to ensure that DoEvents is executed with the maximum frequency.

Put in a form 5 command buttons:
cmdStart
cmdPause
cmdContinue
cmdRestart
cmdStop

and in the Form module copy this code:

Option Compare Database
Option Explicit
 
Dim ThreadAction As String
 
Private Sub cmdContinue_Click()
    ThreadAction = "Continue"
End Sub
 
Private Sub cmdPause_Click()
    ThreadAction = "Pause"
End Sub
 
Private Sub cmdRestart_Click()
    ThreadAction = "Restart"
End Sub
 
Private Sub cmdStart_Click()
    Call FillListView
End Sub
 
Private Sub cmdStop_Click()
    ThreadAction = "Stop"
End Sub
 
Private Sub FillListView()
    MsgBox ("Start cycle.")
 
    Do
        MsgBox ("Now I read (new) values for variables and (new) settings")
        ThreadAction = ""
        Do
            DoEvents
            If ThreadAction = "Pause" Then
                MsgBox ("I'll take a break")
                Do While ThreadAction = "Pause"
                    DoEvents
                Loop
                MsgBox ("The break was too short for me")
            End If
            If (ThreadAction = "Restart") Or (ThreadAction = "Stop") Then
        Exit Do
            End If
        Loop
    Loop Until ThreadAction = "Stop"
 
    MsgBox ("Stoped by user")
End Sub
 

Be sure that every command button "see" it's events.
.
.

[LeoVBNET]

It's ok when buttons is used. But that's not the idea.
If you want you can download a simple example I made in the next link:

http://leoweb.hol.es/VB/

'Threarding_1.zip'

It is a very simple example, but it has the principal idea.

When user click on either button or changes radiobuttons, both ListView should stop their work and restart again from the begining.

[Mihail]

I understand that.
But the radio button also has an event you can use: OnClick, OnChange, MouseDown, MouseUp etc.
I give you an idea about how DoEvents work.
If you don't wish even to try or you think that is a better way... Good luck !

[LeoVBNET]

In fact, I did try DoEvent but it was the same problem. Definitively, I gonna have to change the way I'm trying to do it.

Thanks again for your support.

[Killer42]

I'd just like to point out that you're likely to find much more useful information about threading in the VB.Net forum. This one is for VB6. VB6 is incredibly old, and threading was never much of an issue.

[txc2004]

Hello!

If you still need it,
Check this...

add 2 radiobuttons
add one textbox << this only for example put now
add one listbox << u later put listview
add one module

and use this code:

Public Class Form1
 
    Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged
        txt = CType(sender, Control).Text
 
        If work = False Then myEventHandler() Else restart = True '<< Important this line at the end only
    End Sub
 
    Private Sub RadioButton1_Click(sender As Object, e As EventArgs) Handles RadioButton1.Click, RadioButton2.Click
        txt = CType(sender, RadioButton).Text
 
        If work = False Then myEventHandler() Else restart = True '<< Important this line at the end only
    End Sub
End Class
 
 
Module Module1
    Public work As Boolean
    Public restart As Boolean
    Public i As Long
    Public txt As String
 
    Public Sub myEventHandler()
        ' hear put the startup staff
        i = 0 : Form1.ListBox1.Items.Clear() : restart = False '<< start/restart staff
        Form1.Text = "Start"
        work = True
 
        ' hear begin the core... !
        Do Until i = 10000
            Application.DoEvents()
 
            ' hear put everything u need for restart
            If restart = True Then 'Reset
                i = 0 : Form1.ListBox1.Items.Clear() : restart = False '<< start/restart staff
            End If
 
            ' hear is main work
            i = i + 1 : Form1.ListBox1.Items.Add(txt & "  " & Int(Rnd() * 999999))
            ' in this example put 10000 random numbers with sorted order
        Loop
 
        ' and hear put the finish staff
        work = False
        Form1.Text = "Finish"
    End Sub
 
End Module
 

this is a basic idea...
propably if use this idea with background worker
have better results...