Why don't child controls of a TStringGrid work properly?

Question

I am placing checkboxes (TCheckBox) in a string grid (TStringGrid) in the first column. The checkboxes show fine, positioned correctly, and respond to mouse by glowing when hovering over them. When I click them, however, they do not toggle. They react to the click, and highlight, but finally, the actual Checked property does not change. What makes it more puzzling is I don't have any code changing these values once they're there, nor do I even have an OnClick event assigned to these checkboxes. Also, I'm defaulting these checkboxes to be unchecked, but when displayed, they are checked.

The checkboxes are created along with each record which is added to the list, and is referenced inside a record pointer which is assigned to the object in the cell where the checkbox is to be placed.

String grid hack for cell highlighting:

type
  THackStringGrid = class(TStringGrid); //used later...

Record containing checkbox:

  PImageLink = ^TImageLink;
  TImageLink = record
    ...other stuff...
    Checkbox: TCheckbox;
    ShowCheckbox: Bool;
  end;

Creation/Destruction of checkbox:

function NewImageLink(const AFilename: String): PImageLink;
begin
  Result:= New(PImageLink);
  ...other stuff...
  Result.Checkbox:= TCheckbox.Create(nil);
  Result.Checkbox.Caption:= '';
end;

procedure DestroyImageLink(AImageLink: PImageLink);
begin
  AImageLink.Checkbox.Free;
  Dispose(AImageLink);
end;

Adding rows to grid:

//...after clearing grid...
//L = TStringList of original filenames
if L.Count > 0 then
  lstFiles.RowCount:= L.Count + 1
else
  lstFiles.RowCount:= 2; //in case there are no records
for X := 0 to L.Count - 1 do begin
  S:= L[X];
  Link:= NewImageLink(S); //also creates checkbox
  Link.Checkbox.Parent:= lstFiles;
  Link.Checkbox.Visible:= Link.ShowCheckbox;
  Link.Checkbox.Checked:= False;
  Link.Checkbox.BringToFront;
  lstFiles.Objects[0,X+1]:= Pointer(Link);
  lstFiles.Cells[1, X+1]:= S;
end;

Grid's OnDrawCell Event Handler:

procedure TfrmMain.lstFilesDrawCell(Sender: TObject; ACol, ARow: Integer;
  Rect: TRect; State: TGridDrawState);
var
  Link: PImageLink;
  CR: TRect;
begin
  if (ARow > 0) and (ACol = 0) then begin
    Link:= PImageLink(lstFiles.Objects[0,ARow]); //Get record pointer
    CR:= lstFiles.CellRect(0, ARow); //Get cell rect
    Link.Checkbox.Width:= Link.Checkbox.Height;
    Link.Checkbox.Left:= CR.Left + (CR.Width div 2) - (Link.Checkbox.Width div 2);
    Link.Checkbox.Top:= CR.Top;
    if not Link.Checkbox.Visible then begin
      lstFiles.Canvas.Brush.Color:= lstFiles.Color;
      lstFiles.Canvas.Brush.Style:= bsSolid;
      lstFiles.Canvas.Pen.Style:= psClear;
      lstFiles.Canvas.FillRect(CR);
      if lstFiles.Row = ARow then
        THackStringGrid(lstFiles).DrawCellHighlight(CR, State, ACol, ARow);
    end;
  end;
end;

Here's how it looks when clicking...

What could be causing this? It's definitely not changing the Checked property anywhere in my code. There's some strange behavior coming from the checkboxes themselves when placed in a grid.

EDIT

I did a brief test, I placed a regular TCheckBox on the form. Check/unchecks fine. Then, in my form's OnShow event, I changed the Checkbox's Parent to this grid. This time, I get the same behavior, not toggling when clicked. Therefore, it seems that a TCheckBox doesn't react properly when it has another control as its parent. How to overcome this?

Answer 1

TStringGrid's WMCommand handler doesn't allow children controls to handle messages (except for InplaceEdit).

So you can use e.g. an interposed class (based on code by Peter Below) or draw controls by hands, as some people have adviced. Here is the code of the interposed class:

uses
  Grids;

type
  TStringGrid = class(Grids.TStringGrid)
  private
    procedure WMCommand(var AMessage: TWMCommand); message WM_COMMAND;
  end;

implementation

procedure TStringGrid.WMCommand(var AMessage: TWMCommand);
begin
  if EditorMode and (AMessage.Ctl = InplaceEditor.Handle) then
    inherited
  else
  if AMessage.Ctl <> 0 then
  begin
    AMessage.Result := SendMessage(AMessage.Ctl, CN_COMMAND,
      TMessage(AMessage).WParam, TMessage(AMessage).LParam);
  end;
end;

Answer 2

In Delphi7 at least I do this:

You need to draw a checkbox on the cell, and keep it in sync with an array of boolean (here fChecked[]) that indicates the state of the checkbox in each row. Then, in the DrawCell part of the TStringGrid:

var
 cbstate: integer;
begin
...
if fChecked[Arow] then cbState:=DFCS_CHECKED else cbState:=DFCS_BUTTONCHECK;
DrawFrameControl(StringGrid.canvas.handle, Rect, DFC_BUTTON, cbState);
...
end;

To get the checkbox to respond to the space-bar, use the KeyDown event, and force a repaint:

if (Key = VK_SPACE) And (col=ColWithCheckBox) then begin
  fChecked[row]:=not fChecked[row];
  StringGrid.Invalidate;
  key:=0;
end;

A similar approach is needed for the OnClick method.