[Lazarus] Proper implementation of TDataLink descendant

[Michael Van Canneyt]

On Fri, 5 Aug 2011, Alexander Klenin wrote:

> After researching http://bugs.freepascal.org/view.php?id=19887, I have
> found the following:
>
> 0) For context, here is TDbChartSource desctiption:
> http://wiki.lazarus.freepascal.org/TAChart_documentation#Database_source
> 1) My implementation of TDbСhartSource is incorrect, because during
> the chart drawing
>  it accesses dataset directly from the TDataLink descendant.
>  Apparently this causes infinite loop, because such accesses generate
> DataSetChanged events,
>  which in turn invalidate the chart.
>  Even DisableControls/EnableControls do not help, since
> EnableControls calls DataSetChanged (!).
> 2) Looking at the code in TDBGrid, it seems that the correct way is to
> use DataLink's ActiveRecord property.
>  Unfortunately, in contrast to TDBGrid, TDbСhartSource needs to read
> the whole dataset.
>  It seems that the only way to do this is to set BufferCount :=
> RecordCount, i.e. buffer entire dataset in memory,
>  which I do not like.
>
> So, my questions are:
>
> 1) Is there a way to totally disable DataSetChanged events in the dataset?

No.

You only disable the treatment of the event while browsing.

Something like

If not Floading then
   LoadChart;

Procedure LoadChart;

begin
   Floading:=True;
   try
     // Browse data and build series.
   finally
     Floading:=False;
   end;
end;

> 2) Alternatively, is there a mechanism to read entire dataset via
> TDataLink without fully buffering it?

No, for obvious reasons. I think that for the tchart, you should not bother with the 
buffer mechanism. Just do a

   First;
   While not EOF do
     begin
     AddPointToSeries;
     Next;
     end;

A chart always sees the dataset as a whole. DB aware controls normally see only 1 record.
Normally, a grid sees as much records as it currently displays rows.

But many advanced grids also browse the whole dataset when building rows; 
they don't bother with Activebuffer and buffercount. (

Michael.