OnColumnChanged and OnRowChanged events to TStringGrid

Question

I need to know when the row/column property of a grid was changed in order to do some processing. In TStringGrid Row property is

property Row: Longint read FCurrent.Y write SetRow;

But, unfortunately I cannot override the SetRow as it is private. SelectCell is not private BUT it is called BEFORE the new column and row attribute is set. The only solution will be to replace all calls to Row property with my own property

property MyRow: Longint read Row write SetMyRow;

but it is not the most elegant solution. Any ideas?

Delphi 7, Win 7 32 bit

DelphiXE has an onSelectCell event which does just this. Unfortunately I do not have Delphi 7 available to verify if it was available in that version or not. — skamradt
TStringGrid has an OnSelectCell event in all Delphi versions, AFAIK. — Remy Lebeau
@Remy: Indeed. For one thing, it would be very strange if not. Second, when was this component last updated? It really has the old Win 9x look... — Andreas Rejbrand
@Andreas: TStringGrid is a custom-made control, not a wrapper for any OS control. It is completely custom-drawn, and the drawing logic was never updated to account for theming. — Remy Lebeau
@Remy: Yes, of course I know that. That is also what I meant. If the control would have been actively developed, it wouldn't look so odd in a modern Windows environment. — Andreas Rejbrand

Andreas Rejbrand Andreas Rejbrand · Accepted Answer · 2010-10-20T20:07:00

I just had a look at the source of TStringGrid. The Row property is inherited from TCustomGrid (via TDrawGrid and TCustomDrawGrid), where it is defined as

property Row: Longint read FCurrent.Y write SetRow;

as you say. SetRow calls FocusCell whichs calls MoveCurrent. This one calls SelectCell. This is a virtual function, and although it is highly trivial in TCustomGrid, where it is defined as

function TCustomGrid.SelectCell(ACol, ARow: Longint): Boolean;
begin
  Result := True;
end;

in TCustomDrawGrid, we have

function TCustomDrawGrid.SelectCell(ACol, ARow: Longint): Boolean;
begin
  Result := True;
  if Assigned(FOnSelectCell) then FOnSelectCell(Self, ACol, ARow, Result);
end;

Hence, OnSelectCell is called every time Row or Col is changed, as Skamradt wrote in a comment.

Yes, this event is called before the new cell is selected, but we have

FOnSelectCell: TSelectCellEvent;

where

type
  TSelectCellEvent = procedure (Sender: TObject; ACol, ARow: Longint;
var CanSelect: Boolean) of object;

ACol and ARow contain the new "values-to-be". You can even disallow the change of selected cell by setting CanSelect to false. Consequently, there is no need to override anything.

(Also, you cannot override SetRow because it is not virtual. It is very possible to override private and protected members, but only virtual methods can be overridden.)

OnColumnChanged and OnRowChanged events to TStringGrid

1 Answers