[DUG] TListitems disappearing and reappearing causing access violations

Wed Apr 10 00:06:18 NZST 2013

the listview has an Ondeletion event. when that fires, you should check the
index, and remove any listitem references from your tlist.

The issue seems to be that you are mixing view objects with data objects.
the system might go more smoothly if you were containing your data objects
disctinct from your view (tlistview) and synchronizing the view to the
source.

On Tue, Apr 9, 2013 at 11:58 PM, Ross Levis <ross at stationplaylist.com>wrote:

> It isn’t a user initiated procedure.  It’s a broadcast media player in
> automation mode and each track is a listitem.  Items can be dropped off the
> top and more added to the bottom as it plays automated.****
>
> ** **
>
> There is a TTimer which accesses all the items in the list once a second
> to provide totals and such.  So the items are accessed very regularly.****
>
> ** **
>
> Some of the list items are timed events which need to occur at a specific
> time.  These item pointers are stored in a TList so the software can easily
> access the next one without having to look for it in the list.****
>
> ** **
>
> This user has had issues with the code that accesses this TList of
> TListitem pointers, so that sounds like the pointers to the listitems have
> been changed.****
>
> ** **
>
> Ideally the listitems shouldn’t be accessed directly and a separate data
> store should be used that is linked to each item via the Data property.  A
> bit too late for that now though.****
>
> ** **
>
> So it sounds like I need to find the Windows message that is being sent to
> the app to recreate windows and do some housekeeping.****
>
> ** **
>
> Any ideas what that might be?****
>
> ** **
>
> ** **
>
> *From:* delphi-bounces at listserver.123.net.nz [mailto:
> delphi-bounces at listserver.123.net.nz] *On Behalf Of *Kyley Harris
> *Sent:* Tuesday, 9 April 2013 11:35 p.m.
>
> *To:* NZ Borland Developers Group - Delphi List
> *Subject:* Re: [DUG] TListitems disappearing and reappearing causing
> access violations****
>
> ** **
>
> If that was the case.. then its unlikely to happen midstream during a very
> small short procedure call. Especially if that procedure call is a user
> initiated one. ****
>
> if that is the case.. its extremely simple to test this with remote
> desktop and some logging of the construction/destruction of the list items.
> ****
>
> ** **
>
> On Tue, Apr 9, 2013 at 11:29 PM, Ross Levis <ross at stationplaylist.com>
> wrote:****
>
> I’ve had someone suggest this elsewhere…****
>
>  ****
>
> *The user connects a monitor to a laptop, or connects remotely via
> terminal services.
> Basically, anything that changes the display resolution has that kind of
> consequences.*****
>
>  ****
>
> I have seen a TeamViewer in one of the many screen shots.  So I guess
> Windows is recreating the windows and allocating new handles to the list
> items as I thought.  I’ll check with them if the crashes coincide with use
> of TeamViewer.****
>
>  ****
>
> Ross.****
>
>  ****
>
> *From:* Ross Levis [mailto:ross at stationplaylist.com]
> *Sent:* Tuesday, 9 April 2013 11:17 p.m.
> *To:* 'NZ Borland Developers Group - Delphi List'
> *Subject:* RE: [DUG] TListitems disappearing and reappearing causing
> access violations****
>
>  ****
>
> The list is populated by the software.  It’s still in Delphi 7.****
>
>  ****
>
> No threads are deleting any items.  Only the main thread adds and removes
> items.****
>
>  ****
>
> There is no one procedure this has occurred in.  It can work for 2 days
> and suddenly this happens in a random procedure that accesses a list item.
> ****
>
>  ****
>
> In this particular case, it is in a procedure where one specific listitem
> is sent as a parameter.****
>
>  ****
>
> Here are some of the relevant lines of code in the procedure being
> executed.****
>
>  ****
>
>     if Item <> nil then****
>
>     begin****
>
>       if Item.Selected then UpdateButtons;****
>
>        …****
>
>       Item.ImageIndex := 8;****
>
>     end;****
>
>  ****
>
> There are no deletions in the procedure.****
>
>  ****
>
> The Item.Selected test works fine but Item.ImageIndex := 8 crashes at
> TListItem.SetImage with an access violation.****
>
>  ****
>
> The item existed and then no items exist based on the screen shot.****
>
>  ****
>
> I’ve been having to put validations all over the place like this…****
>
> if ListView.Items.IndexOf(Item) >= 0 then Listitem.ImageIndex := 8.****
>
>  ****
>
> This user has had a crash where it does this validation up the procedure
> somewhere which validated, but further down it will be accessing Item.Index
> and this crashes.****
>
>  ****
>
> It’s a nightmare.  I’m wondering if it could be corrupt RAM in the PC or
> something.****
>
>  ****
>
> Cheers,****
>
> Ross.****
>
>  ****
>
> *From:* delphi-bounces at listserver.123.net.nz [
> mailto:delphi-bounces at listserver.123.net.nz<delphi-bounces at listserver.123.net.nz>]
> *On Behalf Of *Kyley Harris
> *Sent:* Tuesday, 9 April 2013 10:33 p.m.
> *To:* NZ Borland Developers Group - Delphi List
> *Subject:* Re: [DUG] TListitems disappearing and reappearing causing
> access violations****
>
>  ****
>
> without seeing the 10lines above, and whats inbetween (if you were being
> literal) its very hard to say. ****
>
> The only time I've ever had something dissapear from a list is when I
> deleted it though a callback, method, thread etc.. or freeing something. *
> ***
>
> what version of delphi is the code now? How does the list get populated or
> edited by the application.. ****
>
> I'd start by looking at how many locations of code insert or delete the
> list items, and verify none are being called while your procedure is
> executing its for-loop, (assuming the access violation is an index based
> one because of the list count change)****
>
>  ****
>
>  ****
>
> On Tue, Apr 9, 2013 at 10:21 PM, Ross Levis <ross at stationplaylist.com>
> wrote:****
>
> Wondering if anyone has any ideas.****
>
>  ****
>
> I've had this problem a long time from when I started writing an app about
> 7 or 8 years ago, but it happens so rarely it's not usually an issue. But
> one user in particularly is regularly sending me MadExcept reports which
> either have access violation errors or "the application appears to be
> frozen". In all cases it's during an access of a TListItem which was
> perfectly valid 10 lines of code up a procedure but suddenly doesn't exist.
> ****
>
>  ****
>
> The screenshot created at the same time shows the ListView has no visible
> items, but I know for a fact there are 30 or more items in the list. I
> believe sometimes the items visually disappear and reappear shortly
> afterwards, like a Desktop refresh.****
>
>  ****
>
> I think I've heard something about Windows recreating the window handles
> or some such thing. I don't understand why it would do so. There are no
> changes to the listview that would require re-creating the window.****
>
>  ****
>
> Any ideas?****
>
>  ****
>
>
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>
>
>
> --
> Kyley Harris
> Harris Software
> +64-21-671-821 ****
>
>
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>
>
>
>
> --
> Kyley Harris
> Harris Software
> +64-21-671-821 ****
>
> _______________________________________________
> NZ Borland Developers Group - Delphi mailing list
> Post: delphi at listserver.123.net.nz
> Admin: http://delphi.org.nz/mailman/listinfo/delphi
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-- 
Kyley Harris
Harris Software
+64-21-671-821
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