[DUG] TListitems disappearing and reappearing causing access violations

Kyley Harris Kyley at harrissoftware.com
Wed Apr 10 00:47:39 NZST 2013 

I think the problem is that right now you are running on the Theory that
they are being reconstructed as you state, and you have not created a test
scenario to validate what you are saying.

it could just be a coding error, and the delete event will tell you when a
list item is being destroyed. That list item will not be destroyed if the
window handle is reconstructed. Not in the source code I can see.,


On Wed, Apr 10, 2013 at 12:41 AM, Ross Levis <ross at stationplaylist.com>wrote:

> Actually that doesn’t help so much.  I really need to know when Windows is
> deleting the items.  My app also deletes items.  I guess I could remove the
> OnDeletion event property before coding a delete and reinstate if
> afterwards.  A little messy.****
>
> ** **
>
> I really need to associate the individual deleted items with the identical
> new items that will be created just afterwards.  Ideally I would want my
> app to stop processing all timers and functions while the deletions and
> recreations are done.****
>
> ** **
>
> Getting too complicated.****
>
> ** **
>
> *From:* delphi-bounces at listserver.123.net.nz [mailto:
> delphi-bounces at listserver.123.net.nz] *On Behalf Of *Kyley Harris
>
> *Sent:* Wednesday, 10 April 2013 12:06 a.m.
> *To:* NZ Borland Developers Group - Delphi List
> *Subject:* Re: [DUG] TListitems disappearing and reappearing causing
> access violations****
>
> ** **
>
> 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 ****
>
>
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>
>
> --
> Kyley Harris
> Harris Software
> +64-21-671-821 ****
>
> _______________________________________________
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> Post: delphi at listserver.123.net.nz
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-- 
Kyley Harris
Harris Software
+64-21-671-821
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