Using TBookmarklist question
lazarus / 16-Databases

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daveinhull · 2018-12-28 17:58

Hi,

I'm using a TBookmarklist to manage tickboxes in a DB Grid. So the
DBGrid has two fields, the TBookmarklist in the first one and the
actual database field I'm interested in selecting in the second.

All this is being done in a modal form. So OnCreate I
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   RecList1
:= TBookmarkList.Create(DbGrid1); 
where RecList1 is defined as
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i    
RecList1: TBookmarklist;
Then in the DBGrid UserCheckboxState I do
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm2.DBGrid1UserCheckboxState(Sender: TObject; Column: TColumn; var
AState: TCheckboxState);begin  if RecList1.CurrentRowSelected then   
AState := cbChecked  else    AState := cbUnchecked;end;
and in the DBGrid CellClick I do
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm2.DBGrid1CellClick(Column: TColumn);begin  If Column.Index=0
then    RecList1.CurrentRowSelected := not
RecList1.CurrentRowSelected;end;  
On the main form I press a button and it shows this form Modal and
everything works well. I have a button which cancels the Modal form as
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
ModalResult := mrCancel; 
But the next time I press the main form button to ShowModal this form
I get an error External SIGSERV

This only happens if I actually select some of the tickboxes the first
time I show the form, i.e. I can show and close the form as many times
as I want so long as I don't select any tickboxes.

I think it must have something to do with memory getting released
maybe when the form is hidden (I assume it is hidden when I
ModalResult := mrCancel as the create routine isn't called again.)

Anyone got any thoughts in this?

Thanks
Dave

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daveinhull · 2018-12-29 18:04

Some further information:

It would appear that the cause of this problem is if I close and then
re-open the dataset associated with the grid that is connected to the
Bookmarklist and the issue is in DBGrids (I think), but I'm just not
good enough to find out what is going on.

Any help?

Thanks

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daveinhull · 2018-12-29 18:16

Ok, so a little bit more confident than I thought and traced the code
through to

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
VisitAll;  begin    AIndex := 0;    i := 0;    while i<FList.Count do
begin      if FUseCompareBookmarks then        CompareRes :=
FDataset.CompareBookmarks(Item, TBookmark(FList[I]))      else       
CompareRes := MyCompareBookmarks(FDataset, pointer(Item), FList[I]); 
   if CompareRes=0 then begin        result := true;        AIndex :=
i;        exit;      end;      inc(i);    end;  end;   
It errors on the line (7)
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i        
CompareRes := FDataset.CompareBookmarks(Item, TBookmark(FList[I]))
Any thoughts?
Dave

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lucamar · 2018-12-29 18:34

A wild guess: either FList got freed/destroyed somewhere before
reaching this point or "I" is way out of bounds.

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daveinhull · 2018-12-30 13:49

I think I have found why it errors.

In DBGrids there is this code
Code: Pascal  [Select]

      if FGrid.DataSource.DataSet=FDataset then
        exit;
      FDataset := FGrid.DataSource.DataSet;
    
      // Note.
      //
      // Some dataset descendants do not implement CompareBookmarks,
for these we
      // use MyCompareBookmarks in the hope the allocated bookmark
memory is used
      // to hold some kind of record index.
      FUseCompareBookmarks := TMethod(@FDataset.CompareBookmarks).Code
<>pointer(@TDataset.CompareBookmarks);
    
      // Note.
      //
      // fpc help say CompareBookmarks should return -1, 0 or 1 ...
which imply that
      // bookmarks should be a sorted array (or list). In this
scenario binary search
      // is the prefered method for finding a bookmark.
      //
      // The problem here is that TBufDataset and TSQLQuery (and thus
TCustomSQLQuery
      // and TCustomBufDataset) CompareBookmarks only return 0 or -1
(some kind of
      // is this a valid bookmark or not), the result is that it
appears as an unsorted
      // list (or array) and binary search should not be used.
      //
      // The weird thing is that if we use MyCompareBookmarks which
deals with comparing
      // the memory reserved for bookmarks in the hope bookmarks are
just some kind of
      // reocord indexes, currently work fine for TCustomBufDataset
derived datasets.
      // however using CompareBookmarks is always the right thing to
use where implemented.
      //
      // As Dbgrid should be TDataset implementation agnostic this is
a way I found
      // to know if the dataset is derived from TCustomBufDataset or
not.
      // Once TCustomBufDataset is fixed, remove this ugly note &
hack.
      case FDataset.ClassName of
       
'TSQLQuery','TBufDataset','TCustomSQLQuery','TCustomBufDataset':
          FCanDoBinarySearch := false;
        else
          FCanDoBinarySearch := true;
      end;


Which suggests that TSQLQuery is a funny dataset and doesn't comply
with the CompareBookmarks return values (see comments). But TMethod
does return True.
So at the end of this routine, FUseCOmpareBookmarks is True and
FCanDoBinarySearch is False.

In the TBookmarklist.Find routine there is this code
Code: Pascal  [Select]

      procedure VisitAll;
      begin
        AIndex := 0;
        i := 0;
        while i<FList.Count do begin
          if FCanDoBinarySearch then
          //if FUseCompareBookmarks then
            CompareRes := FDataset.CompareBookmarks(Item,
TBookmark(FList))
          else
            CompareRes := MyCompareBookmarks(FDataset, pointer(Item),
FList);
          if CompareRes=0 then begin
            result := true;
            AIndex := i;
            exit;
          end;
          inc(i);
        end;
      end;
    
    begin
      {$ifdef dbgDBGrid}
      DebugLn('%s.Find', [ClassName]);
      {$endif}
    
      Result := False;
      if Item=nil then
        Exit;
      if FCanDoBinarySearch then
        BinarySearch
      else
        VisitAll;
    end;


The main part of the routine checks whether we can do a binary sourt
or not and for a TSQLQuery we should so we call VisitAll.

However in VisitAll it then uses FCanDoBinarySearch (which is true) to
call the FDataset.COmpareBookmarks routine, but we had already
established that this didn't work for a TSQLQuery.

So changing the If condition to FUseCompareBookmarks to get it to go
to MyCompareBookmarks works and we don't get an error.

However, it still doesn't work correctly as each time I call the
ShowModal form the bookmarklist is now what appears to be random.

Just to clarify what I'm doing. I have create a Form with a DBGrid on
it which I'm using for a Filter selection a bit like the MS Access
filter selection. The form is opened with a procedural call which
specifies which  column of the main DBGrid should be filtered.

So it closes the local SQLQuery, changes the select to the required
column and opens it again. It also makes sure that a seperate
Bookmarklist is used (I have an array of bookmarks corresponding to
the column index).

The problem appears to be in the closing of the SQLQuery whcih I think
calls TBookmarkList.Clear somewhere along the line, but can't see
where.

Can anyone take this a further as I've about exhausted my knowledge of
what is going on, I'm not clear how Bookmarks work.

I do have a work around and that is to have a separate Bookmarklist
AND SQLQuery for each column that might be filtered, but I'd like to
get some answer on this problem just for knowledge.

Thanks
Dave

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daveinhull · 2018-12-31 00:40

Anyone can help????