Mechanism to cancel a query

rbuser

Hi,

I am using RB 6.03 with a custom ODBC TDataset and DBPipeline.

With the end user query designer the user can easily create a query that
is very time consuming, which ends up calling TDataset.Open.

With ODBC it is possible cancel a query request. Is there a mechanism in
RB for such a cancel? The problem I am having is that the report
designer can hang in preview mode for minutes (even hours) and the user
interface is unresponsive. Would someone have any suggestions on how to
handle this?

Thanks,

Michel

digitalmetaphors

You can use the OnCancel event of the report to detect when the user presses
the cancel button in the preview.

If that doesn't work, then you can create a custom preview plugin that can
provide the ability to do this in the OnClick of the cancel/close button in
your TppPreview descendent. Here is an example of a custom preview to add a
new button. All you want to do in your descendent is to override the Cancel
method though to make a call to access you ODBC connection to cancel the
query.

http://www.digital-metaphors.com/tips/AddControlsToPreview.zip

Cheers,

Jim Bennett
Digital Metaphors

rbuser

Thanks! That is plenty of information to get going.

Michel

rbuser

Hi Jim,

I have advanced quite a bit now...

I am running my query in another thread to that it maybe cancelled.
Although I am fighting with some ODBC drivers which don't want to cancel
a query, this is working rather well.

So will the query is executing in another thread in
TDataset.InternalOpen I call Application.ProcessMessages in a loop and
the report designer is responding. I confirmed that the report designer
is responding because I can display the about box while the query is
executing in another thread.

The only problem is I can press the Cancel button (visually I see it
depressed), but the Report.OnCancel event is not triggered. For testing
I have been using a timeout in the ProcessMessages loop and it cancels fine.

Would you have any idea why the OnCancel event does not trigger in this
situation.

Best regards,

Michel

rbuser

Hi,

Maybe this will help find the problem.

When the Cancel button is pressed on the Preview here is the sequence of
calls:

TppPreview.ToolButtonClickEvent
TppCustomPreview.PerformPreviewAction
TppCustomPreview.Cancel
TppViewer.Cancel
TppScreenDevice.Cancel
TppReport.Cancel

but in TppReport.Cancel

if (FActiveReport <> nil) and (FActiveReport <> Self) then
FActiveReport.Cancel;

FActiveReport is nil, so the processing stops.

I was not able to find out where FActiveReport is supposed to be set. It
seems that it is set to nil in the constructor, and then never assigned.

I did a global search in the code an the only place where ActiveReport
is assigned seems to be in ppSubRpt.Pas, so I am not sure where to go
from here.

Thanks again,

Michel

digitalmetaphors

Ok, create a preview plugin that sends a notification in the cancel click.
This way you don't have to rely on Report.OnCancel being triggered. Here is
one simple example of a custom preview replacement.

http://www.digital-metaphors.com/tips/CustomPrintFromPreview.zip

Modify this example to only have one routine which is to override the Cancel
method and send a message to perform the cancel from that method.

Look in RBuilder\Source\ppPreview.pas for the ancestor interface in the
TppCustomPreview class for more info.


Cheers,

Jim Bennett
Digital Metaphors

rbuser

Hi Jim,

I understand this is a workaround, but all I want is to have the
OnCancel event work. I don't need to create a custom preview.

All I added in TppReport.Cancel is,

if Assigned(TNotifyEvent(OnCancel)) then
OnCancel(Self);

which seems quite logical and seems to work fine.

But I don't like patching third party libraries because it will cause
problems when the library is updated.

Any chance of having this fixed, rather than having to create a custom
preview to get the OnCancel event working.

Thanks,

Michel

digitalmetaphors

Sorry, I looked at the code and I suggested using the OnCancel when I
shouldn't have, based on the current context for the OnCancel event which is
for printing only. There is a cancel dialog when printing to the printer
which is responsible for the OnCancel event being triggered. The cancel
button in the preview doesn't trigger the OnCancel as I thought it should.
Since the context of the cancel is different and the device is different, we
can't trigger the OnCancel in the preview cancel button's click event, as
this may break code out in the field where other customers are relying on
this assumption in their OnCancel event handlers. The solution is to use a
preview plugin replacement or for you to continue using the modified source
to trigger the OnCancel event.

Cheers,

Jim Bennett
Digital Metaphors

rbuser

Ok, thanks.

I'm just trying to make my life simpler as my todo list is growing
faster than the work being done.

Michel