Question

I’m trying to create a small bit of software to mess around with drag and drop a bit. I started out with only creating TButtons dynamically and it worked fine.

However when generalizing the functionality I’ve run into a problem due to the ‘protected’ status of the OnDragDrop and OnMouseDownEvent of the base class TControl.

procedure TForm1.FormDragDrop(Sender, Source: TObject; X, Y: Integer);
  var
    newControl: TControl;
    selClass: TControlClass;
    ctlName: string;
    selItem: string;
  begin
    if TControl(Sender).Parent = Self then
    begin
      with TWinControl(Source) do
      begin
        Left:= X;
        Top:= Y;
        EndDrag(True); {drop the control}
      end;
    end
    else begin
      selItem:= TypeList.Items[TypeList.ItemIndex];
      selClass:= TControlClass(GetClass(selItem));

      newControl:= selClass.Create(Self);
      newControl.Parent:= Self;

      ctlName:= newControl.ClassName + IntToStr(GetControlCount(selClass));
      Delete(ctlName, 1, 1); {Remove 'T' from name}

      newControl.Name:= ctlName;
      newControl.Left:= X;
      newControl.Top:= Y;

      { TODO : assign events onDragDrop and onMouseDown}
      (*
      newControl.OnMouseDown:= @ControlMouseDown;
      newControl.OnDragDrop:= @FormDragDrop;
      *)

    end;

  end;

Answer 1

Add the following declaration to your unit:

type
  TControlHack = class(TControl);

and use a type cast for the new control:

TControlHack(newControl).OnMouseDown := ...