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.
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