[fpc-pascal] Why TSdfDataSet doesn't simply return FCurRec as RecNo?

[Michael Van Canneyt]

On Thu, 30 Oct 2014, Joost van der Sluis wrote:

> On 10/30/2014 04:34 PM, Michael Van Canneyt wrote:
>> 
>> 
>> On Thu, 30 Oct 2014, Michael Van Canneyt wrote:
>> 
>>> 
>>> 
>>> On Thu, 30 Oct 2014, Luca Olivetti wrote:
>>> 
>>>> Hello,
>>>> 
>>>> TFixedFormatDataSet (ancestor of TSdfDataSet) overrides GetRecNo
>>>> (correct) but it does a strange contortion instead of simply returning
>>>> FCurRec:
>>>> 
>>>> function TFixedFormatDataSet.GetRecNo: Longint;
>>>> var
>>>>  BufPtr: TRecordBuffer;
>>>> begin
>>>>  Result := -1;
>>>>  if GetActiveRecBuf(BufPtr) then
>>>>    Result := PRecInfo(BufPtr + FRecInfoOfs)^.RecordNumber;
>>> 
>>> The records in the buffer may be in a different order than they are
>>> displayed in the grid.
>> 
>> Forget this. I was too quick to respond.
>> 
>> I have done some digging, but cannot find a reason for the current
>> behaviour.
>
> Easy: Some systems use the recno as some sort of 'bookmark'. And some people 
> insist that this is the correct behaviour for a dataset. (Note that for 
> TBufDataset a call to recno is really slow, as it has to iterate over all 
> records to get the correct recno.)

AFAIK RecNo has no meaning when you work with multiple indexes and filters.

>
> So some datasets have recno's that uniquely identify a record. I've tried to 
> unify that once, but that was not easily done. So I decided to leave this 
> decision to the author of each dataset. Hence the GetRecNo.

I understand why the GetRecNo is there, just not why it is implemented as it is in TSDFDataset.
(which is what I was investigating)

> But that TDBGrid uses the recno is troublesome... But I don't see a better 
> solution.

Delphi DBGrid does not use recno, so it must be possible... 
AFAIK TDatalink has special provisions to provide a window buffer.

Michael.