Button 直到btnStop.Click=True；

问题：btnClick触发重复循环，我希望在单击btnStop时重复循环停止，然后lblLetter将显示单击btnStop时屏幕上的字母

 TForm1 = class(TForm)
    lblLetter: TLabel;
    btnSelect: TButton;
    btnStop: TButton;

const
arrLetter:array[1..22] of string = ('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','R','S','T','U','V','W');

procedure TForm1.btnSelectClick(Sender: TObject);
var
k, letter:integer;
bstop: boolean;

begin

repeat
 k:= Random(22)+1 ;
 lblLetter.Caption:= inttostr (k);
 Sleep(300)

until

 btnStop.Click = true;
 lblLetter.Caption:= inttostr (k);