[crazyone]

Это не просто окно, а диалоговое окно. Поэтому логично сделать

loginWindow=new DialogWindow();
loginWindow.addEventListener(DialogEvent.ANSWER,loginAnswerHandler);
...
function loginAnswerHandler(event:DialogEvent){
  switch(event.btnCode){
    case DialogButtons.OK:
      login();
    break;
    case DialogButtons.CANCEL:
    case DialogButtons.CLOSE:
      if(!canCloseLoginDialog()){
        event.preventDefault();
        showWarning("Thou shalt not close this window!");
      }
    break;
  }
}

public class DialogWindow extends ...{
...
   private function buttonClick(btnCode:String){
      var event:DialogEvent=new DialogEvent(DialogEvent.ANSWER,btnCode);
      dispatchEvent(event);
      if(!event.isDefaultPrevented()){
         close();
      }
   }
...
}