Question

I am trying to synchronize the scrolling of two TDBGrid components in a VCL Forms application, I am having difficulties intercepting the WndProc of each grid component without some stack issues. I have tried sending WM_VSCROLL messages under scrolling events but this still results in the incorrect operation. It needs to work for clicking the scrollbar, as well as highlighting a cell, or an up or down mouse button. The whole idea is to have two grids next to each other displaying a sort of matching dialog.

Tried

SendMessage( gridX.Handle, WM_VSCROLL, SB_LINEDOWN, 0 );

Also

procedure TForm1.GridXCustomWndProc( var Msg: TMessage );
begin
Msg.Result := CallWindowProc( POldWndProc, gridX.Handle, Msg.Msg, Msg.wParam, Msg.lParam );

   if ( Msg.Msg = WM_VSCROLL ) then 
   begin
      gridY.SetActiveRow( gridX.GetActiveRow );
      gridY.Perform( Msg.Msg, Msg.wParam, Msg.lParam );
      SetScrollPos( gridY.Handle, SB_VERT, HIWORD( Msg.wParam ), True );
   end;
end;

And

procedure TForm1.GridxCustomWndProc( var Msg: TMessage );
begin
   if ( Msg.Msg = WM_VSCROLL ) then 
   begin
      gridY.SetActiveRow( gridX.GetActiveRow );
      gridY.Perform( Msg.Msg, Msg.wParam, Msg.lParam );
      SetScrollPos( gridY.Handle, SB_VERT, HIWORD( Msg.wParam ), True );
   end;
   inherited WndProc( Msg );
end;

The First is only a temporary solution, the second results in invalid memory reads, and the third results in a stack overflow. So none of these solutions seems to work for me. I’d love some input on how to accomplish this task! Thanks in advance.

UPDATE: Solution

  private
    [...]
    GridXWndProc, GridXSaveWndProc: Pointer;
    GridYWndProc, GridYSaveWndProc: Pointer;
    procedure GridXCustomWndProc( var Msg: TMessage );
    procedure GridYCustomWndProc( var Msg: TMessage );

procedure TForm1.FormCreate(Sender: TObject);
begin
  GridXWndProc := classes.MakeObjectInstance( GridXCustomWndProc );
  GridXSaveWndProc := Pointer( GetWindowLong( GridX.Handle, GWL_WNDPROC ) );
  SetWindowLong( GridX.Handle, GWL_WNDPROC, LongInt( GridXWndProc ) );

  GridYWndProc := classes.MakeObjectInstance( GridYCustomWndProc );
  GridYSaveWndProc := Pointer( GetWindowLong( GridY.Handle, GWL_WNDPROC ) );
  SetWindowLong( GridY.Handle, GWL_WNDPROC, LongInt( GridYWndProc ) );
end;

procedure TForm1.GridXCustomWndProc( var Msg: TMessage );
begin
   Msg.Result := CallWindowProc( GridXSaveWndProc, GridX.Handle, Msg.Msg, Msg.WParam, Msg.LParam );
   case Msg.Msg of
      WM_KEYDOWN:
      begin
         case TWMKey( Msg ).CharCode of VK_UP, VK_DOWN, VK_PRIOR, VK_NEXT:
            GridY.Perform( Msg.Msg, Msg.WParam, Msg.LParam );
         end;
      end;
      WM_VSCROLL:
         GridY.Perform( Msg.Msg, Msg.WParam, Msg.LParam );
      WM_HSCROLL:
         GridY.Perform( Msg.Msg, Msg.WParam, Msg.LParam );
      WM_MOUSEWHEEL:
      begin
         ActiveControl := GridY;
         GridY.Perform( Msg.Msg, Msg.WParam, Msg.LParam );
      end;
      WM_DESTROY:
      begin
         SetWindowLong( GridX.Handle, GWL_WNDPROC, Longint( GridXSaveWndProc ) );
         Classes.FreeObjectInstance( GridXWndProc );
      end;
  end;
end;

procedure TForm1.GridXMouseDown( Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer );
begin
   GridY.SetActiveRow( GridX.GetActiveRow );
end;

procedure TForm1.GridYCustomWndProc( var Msg: TMessage );
begin
   Msg.Result := CallWindowProc( GridYSaveWndProc, GridY.Handle, Msg.Msg, Msg.WParam, Msg.LParam );
   case Msg.Msg of
      WM_KEYDOWN:
      begin
         case TWMKey( Msg ).CharCode of VK_UP, VK_DOWN, VK_PRIOR, VK_NEXT:
            GridX.Perform( Msg.Msg, Msg.WParam, Msg.LParam );
         end;
      end;
      WM_VSCROLL:
         GridX.Perform( Msg.Msg, Msg.WParam, Msg.LParam );
      WM_HSCROLL:
         GridX.Perform( Msg.Msg, Msg.WParam, Msg.LParam );
      WM_MOUSEWHEEL:
      begin
         ActiveControl := GridX;
         GridX.Perform( Msg.Msg, Msg.WParam, Msg.LParam );
      end;
      WM_DESTROY:
      begin
         SetWindowLong( GridY.Handle, GWL_WNDPROC, Longint( GridYSaveWndProc ) );
         Classes.FreeObjectInstance( GridYWndProc );
      end;
   end;
end;

procedure TForm1.GridYMouseDown( Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer );
begin
   GridX.SetActiveRow( GridY.GetActiveRow );
end;

Thanks to – Sertac Akyuz for the solution. When integrated into a VCL forms application using grids, they will mimmic each other in scrolling, and highlighting the selected record.

Answer 1

You are probably implementing the message override for both of the grids. GridX scrolls GridY, which in turn scrolls GridX, which in turn … SO. You can protect the superficial scrolling code by surrounding the block with flags.

type
  TForm1 = class(TForm)
    [..] 
  private
    FNoScrollGridX, FNoScrollGridY: Boolean;
    [..]

procedure TForm1.GridXCustomWndProc( var Msg: TMessage );
begin
  Msg.Result := CallWindowProc(POldWndProc, gridX.Handle, Msg.Msg, Msg.wParam, Msg.lParam );

  if ( Msg.Msg = WM_VSCROLL ) then 
  begin
    if not FNoScrollGridX then
    begin
      FNoScrollGridX := True
      gridY.SetActiveRow( gridX.GetActiveRow );
      gridY.Perform( Msg.Msg, Msg.wParam, Msg.lParam );
//      SetScrollPos( gridY.Handle, SB_VERT, HIWORD( Msg.wParam ), True );
    end;
    FNoScrollGridX := False;
  end;
end;

Similiar code for the GridY. BTW, you shouln’t need the SetScrollPos.

---

edit:

TForm1 = class(TForm)
  [..]
private
  GridXWndProc, GridXSaveWndProc: Pointer;
  GridYWndProc, GridYSaveWndProc: Pointer;
  procedure GridXCustomWndProc(var Msg: TMessage);
  procedure GridYCustomWndProc(var Msg: TMessage);
  [..]

procedure TForm1.FormCreate(Sender: TObject);
begin
  [..]

  GridXWndProc := classes.MakeObjectInstance(GridXCustomWndProc);
  GridXSaveWndProc := Pointer(GetWindowLong(GridX.Handle, GWL_WNDPROC));
  SetWindowLong(GridX.Handle, GWL_WNDPROC, LongInt(GridXWndProc));

  GridYWndProc := classes.MakeObjectInstance(GridYCustomWndProc);
  GridYSaveWndProc := Pointer(GetWindowLong(GridY.Handle, GWL_WNDPROC));
  SetWindowLong(GridY.Handle, GWL_WNDPROC, LongInt(GridYWndProc));
end;

procedure TForm1.GridXCustomWndProc(var Msg: TMessage);
begin
  Msg.Result := CallWindowProc(GridXSaveWndProc, GridX.Handle,
      Msg.Msg, Msg.WParam, Msg.LParam);

  case Msg.Msg of
    WM_KEYDOWN:
      begin
        case TWMKey(Msg).CharCode of
          VK_UP, VK_DOWN, VK_PRIOR, VK_NEXT:
            GridY.Perform(Msg.Msg, Msg.WParam, Msg.LParam);
        end;
      end;
    WM_VSCROLL: GridY.Perform(Msg.Msg, Msg.WParam, Msg.LParam);
    WM_MOUSEWHEEL:
      begin
        ActiveControl := GridY;
        GridY.Perform(Msg.Msg, Msg.WParam, Msg.LParam);
      end;
    WM_DESTROY:
      begin
        SetWindowLong(GridX.Handle, GWL_WNDPROC, Longint(GridXSaveWndProc));
        Classes.FreeObjectInstance(GridXWndProc);
      end;
  end;
end;

procedure TForm1.GridYCustomWndProc(var Msg: TMessage);
begin
  Msg.Result := CallWindowProc(GridYSaveWndProc, GridY.Handle,
      Msg.Msg, Msg.WParam, Msg.LParam);

  case Msg.Msg of
    WM_KEYDOWN:
      begin
        case TWMKey(Msg).CharCode of
          VK_UP, VK_DOWN, VK_PRIOR, VK_NEXT:
            GridX.Perform(Msg.Msg, Msg.WParam, Msg.LParam);
        end;
      end;
    WM_VSCROLL: GridX.Perform(Msg.Msg, Msg.WParam, Msg.LParam);
    WM_MOUSEWHEEL:
      begin
        ActiveControl := GridX;
        GridY.Perform(Msg.Msg, Msg.WParam, Msg.LParam);
      end;
    WM_DESTROY:
      begin
        SetWindowLong(GridY.Handle, GWL_WNDPROC, Longint(GridYSaveWndProc));
        Classes.FreeObjectInstance(GridYWndProc);
      end;
  end;
end;