Threading Question

**Tom Shelton** · Sep 30 '08, 10:15 PM

Re: Threading Question

On 2008-09-30, Bill Schanks <wschanks@gmail .comwrote:

I have a form that gets opened, and it a separate thread I want a
timer at the bottom of the form to update while the datagridview is
updating.
>
In my main form I have this:
Dim t As New Thread(AddressO f ChildForm.Updat eRetrieveTimer)
t.Start()
><<Snip>>
>
Here is the code for the thread:
Public Sub UpdateRetrieveT imer()
Dim ts As TimeSpan
Dim s As String
Do While Not g_bRetreiveDone (g_ChildFormNum ber)
ts = Date.Now - g_dtStartQuery
s = String.Format(" {0:d2}:{1:d2}:{ 2:d2}", ts.Hours,
ts.Minutes, ts.Seconds)
Me.rssRetrieveT ime.Text = s
Loop
End Sub
>
The problem is on the line 'Me.rssRetrieve Time.Text = s'. It pauses
here.

You can not access any form controls on a background thread - period. If you
want the update to occur, then you must use Me.Invoke to marshal the call back
to the forms thread.

A better alternative, if you are using VB2005 and up is to use the
BackgroundWorke r component to handle this. Just make sure, that you do your
interface updates in ProgressChanged event.

--
Tom Shelton

**Bill Schanks** · Oct 2 '08, 06:55 PM

Re: Threading Question

I have been trying to get the backgroundworke r to work. Here is what I
have. The problem is the .dowork section runs for 5 seconds or so but
the datagridview doesn't have any results on it.

Private WithEvents ReportWorker As
System.Componen tModel.Backgrou ndWorker

<<snip>>
'Get the results (Background worker)
_dtStartQuery = Date.Now
ReportWorker = New System.Componen tModel.Backgrou ndWorker
ReportWorker.Wo rkerReportsProg ress = True
ReportWorker.Ru nWorkerAsync()
_ChildForm.tsBt nStop.Enabled = True

'Show timer while data is retrieved
While ReportWorker.Is Busy
_ts = Date.Now - _dtStartQuery
_ChildForm.rssR etrieveTime.Tex t =
String.Format(" {0:d2}:{1:d2}:{ 2:d2}.{3:d3}", _
_ts.Hours, _ts.Minutes, _ts.Seconds,
_ts.Millisecond s)
Application.DoE vents()
End While
<<snip>>

Private Sub ReportWorker_Do Work(ByVal sender As Object, _
ByVal e As System.Componen tModel.DoWorkEv entArgs) _
Handles ReportWorker.Do Work

_ChildForm.Spoc _ev_resultsTabl eAdapter.Fill(_ ChildForm.RCT_E V_QADataSet.spo c_ev_results,
_
_dtAsOf, _
_sRptID, _
_sDB, _
_sBL, _
_sCID, _
_sMgr, _
_bCommentsMissi ng)

End Sub

Tom Shelton wrote:

On 2008-09-30, Bill Schanks <wschanks@gmail .comwrote:

I have a form that gets opened, and it a separate thread I want a
timer at the bottom of the form to update while the datagridview is
updating.

In my main form I have this:
Dim t As New Thread(AddressO f ChildForm.Updat eRetrieveTimer)
t.Start()
<<Snip>>

Here is the code for the thread:
Public Sub UpdateRetrieveT imer()
Dim ts As TimeSpan
Dim s As String
Do While Not g_bRetreiveDone (g_ChildFormNum ber)
ts = Date.Now - g_dtStartQuery
s = String.Format(" {0:d2}:{1:d2}:{ 2:d2}", ts.Hours,
ts.Minutes, ts.Seconds)
Me.rssRetrieveT ime.Text = s
Loop
End Sub

The problem is on the line 'Me.rssRetrieve Time.Text = s'. It pauses
here.

>
You can not access any form controls on a background thread - period. If you
want the update to occur, then you must use Me.Invoke to marshal the call back
to the forms thread.
>
A better alternative, if you are using VB2005 and up is to use the
BackgroundWorke r component to handle this. Just make sure, that you do your
interface updates in ProgressChanged event.
>
--
Tom Shelton

**Branco Medeiros** · Oct 3 '08, 03:25 AM

Re: Threading Question

Hi, Bill.

It seems you're doing the same thing again (i. e., updating a control
in a background thead), only now it's the Datagrid.

I suspect that when you call
_ChildForm.Spoc _ev_resultsTabl eAdapter.Fill(. ..) from inside the
DoWork method, the update gets blocked, 'cause anything running inside
the DoWork method is in a secondary thread.

As Tom suggested, one way to do what you want could be to use the
ProgressChanged event of the BackgroundWorke r (which kicks in on the
main thread) to update the display. In this case, the filling of the
grid should be left in the main thread (as you were originally doing
it seems).

Another approach, as Tom also pointed out, is to use the form's invoke
method. It turns out a control (and the form) has the method
InvokeRequired which returns true if the current thread can't update
the user interface. If so, then you must resort to calling the form's
Invoke method, passing the address of the method to execute and an
array with the method's parameters. Invoke will then ensure that the
method is executed in a thread that can update the UI (usually the
main thread).

As a side note, considering your first example, it seems also you are
acessing, from your secondary thread, a variable which gets data
updated from the main thread (g_bRetreiveDon e(g_ChildFormNu mber)).
Ideally, all access to variables shared between threads should be
"interlocke d". In your case, maybe you don't get into trouble, cause
it seems it's only the main thread that updates the variable, but keep
this in mind for future reference.

Finally, notice that you have a **very tight** loop running inside
UpdateRetrievet imer. I mean, the thing is running *non-stop* probably
hundreds or even thousands of times per second without much need,
since its only job is to update the screen once per second. It would
be nice if you put it to rest at regular intervals, say 500 ms...

Using your original approach as reference, here's a possible (but not
tested, mind you) solution:

<example>
'In the child form
Delegate Sub Updater(Text As String)

Private mFinishUpdate As Boolean
'used to lock shared access to the variable above
Private mLock As New Object

Private Property Finished As Boolean
Get
Dim Result As Boolean
SyncLock(mLock)
Result = mFinished
End SyncLock
Return Result
End Get
Set(Value As Boolean)
SyncLock(mLock)
mFinished = Value
End SyncLock
End Set
End Property

Sub UpdateTimer(Tex t As String)
Static DoUpdate As Updater = AddressOf UpdateTimer
If Me.InvokeRequir ed Then
'this runs in the secondary thread
Me.Invoke(DoUpd ate, New Object(){Text})
Else
'this runs in the main thread
Me.rssRetrieveT ime.Text = Text
End If
End Sub

Public Sub UpdateRetrieveT imer()
Dim ts As TimeSpan
Dim s As String
Do While Not Finished
ts = Date.Now - g_dtStartQuery
s = String.Format(" {0:d2}:{1:d2}:{ 2:d2}", _
ts.Hours, ts.Minutes, ts.Seconds)
UpdateTimer(s)
System.Threadin g.Thread.Sleep( 500)
Loop
End Sub
</example>

Hope this helps.

Regards,

Branco.

------------------
Bill Schanks wrote:

I have been trying to get the backgroundworke r to work. Here is what I
have. The problem is the .dowork section runs for 5 seconds or so but
the datagridview doesn't have any results on it.
>
Private WithEvents ReportWorker As
System.Componen tModel.Backgrou ndWorker
>
<<snip>>
'Get the results (Background worker)
_dtStartQuery = Date.Now
ReportWorker = New System.Componen tModel.Backgrou ndWorker
ReportWorker.Wo rkerReportsProg ress = True
ReportWorker.Ru nWorkerAsync()
_ChildForm.tsBt nStop.Enabled = True
>
'Show timer while data is retrieved
While ReportWorker.Is Busy
_ts = Date.Now - _dtStartQuery
_ChildForm.rssR etrieveTime.Tex t =
String.Format(" {0:d2}:{1:d2}:{ 2:d2}.{3:d3}", _
_ts.Hours, _ts.Minutes, _ts.Seconds,
_ts.Millisecond s)
Application.DoE vents()
End While
<<snip>>
>
Private Sub ReportWorker_Do Work(ByVal sender As Object, _
ByVal e As System.Componen tModel.DoWorkEv entArgs) _
Handles ReportWorker.Do Work
>
_ChildForm.Spoc _ev_resultsTabl eAdapter.Fill(_ ChildForm.RCT_E V_QADataSet.spo c_ev_results,
_
_dtAsOf, _
_sRptID, _
_sDB, _
_sBL, _
_sCID, _
_sMgr, _
_bCommentsMissi ng)
>
End Sub