How to display NULL value as unchecked state in a TDBCheckBox descendant?

Question

Having a nullable dataset field of a boolean type, how to display its NULL value as an unchecked state in a TDBCheckBox control descendant linked to this field. By default, TDBCheckBox displays the NULL value of the field as a grayed check box:

but I need it to be displayed as an unchecked state in my TDBCheckBox control descendant:

Modifying the original TDBCheckBox source code is not an option for me, nor I cannot override the TDBCheckBox.GetFieldState because it's a private method.

So, how can I display the NULL value as an unchecked state in my TDBCheckBox descendant ?

Answer 1

Thanks everyone for help!

I implemented it following way:

  type
  TDBCheckBoxHack = class(TDBCheckBox)
    FDataLink: TFieldDataLink;
    procedure DataChange(Sender: TObject); virtual;
    function GetFieldState: TCheckBoxState; virtual;
  public
    constructor Create(AOwner: TComponent); override;
  end;

In constructor I used message CM_GETDATALINK for retrieve DataLink to my component

  constructor TDBCheckBoxHack.Create(AOwner: TComponent);
  var
    AMessage: TMessage;
  begin
    inherited;
    FillChar(AMessage, 0, sizeof(AMessage));
    AMessage.Msg := CM_GETDATALINK;
    Dispatch(AMessage);
    FDataLink := TFieldDataLink(Pointer(AMessage.Result));
    FDataLink.OnDataChange := DataChange;
  end;

and GetFieldState implementation:

  function TFsDBCheckBox.GetFieldState: TCheckBoxState;
  var
    Text: string;
  begin
    if FDatalink.Field <> nil then
      if FDataLink.Field.IsNull then
        Result := cbUnchecked
      else if FDataLink.Field.DataType = ftBoolean then
        if FDataLink.Field.AsBoolean then
          Result := cbChecked
        else
          Result := cbUnchecked
      else
      begin
        Result := cbGrayed;
        Text := FDataLink.Field.Text;
        if ValueMatch(ValueChecked, Text) then Result := cbChecked else
          if ValueMatch(ValueUnchecked, Text) then Result := cbUnchecked;
      end
   else
     Result := cbUnchecked;
 end;