Create Custom Shape Component

Hi, I need help on create a new component

This is my source code:
My problem is When I use method
   What should I do to make shape repaint when I add string to top items


thanks




unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,extctrls,
  StdCtrls;

type
  tmy=class(tshape)
  topitem:tstrings;
  bottomitem:tstrings;
  constructor Create(Owner: TComponent); override;
  procedure paint;override;
  end;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  a:tmy;
implementation

{$R *.DFM}


constructor Tmy.Create(Owner: TComponent);
begin
  inherited Create(Owner);  // Initialize inherited parts
  topitem:=tstringlist.create;
end;

procedure tmy.paint;
begin
   inherited paint;
   if topitem.count>0 then
   canvas.textout(1,1,topitem.strings[0]);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
a:=tmy.create(self);
a.parent:=form1;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
   a.topitem.Add('Hello Hello');
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
a.paint;
end;

end.