flex弹出窗体的应用 PopUpManager类

一，Flex的主程序 PopUpDemo.mxml代码如下：

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
<mx:Script>
<![CDATA[
import mx.managers.PopUpManager;

private var tw:titlewindow=new titlewindow();
private function mytw_click():void{
  tw.owner = this;
PopUpManager.addPopUp(tw,this);
  PopUpManager.centerPopUp(tw);
}
]]>
</mx:Script>
<mx:Panel x="94" y="178" width="503" height="347" layout="absolute">
<mx:TextInput x="134" y="64" id="tit_usr" text="username"/>
    <mx:TextInput x="134" y="125" id="tit_psw" text="password"/>
    <mx:Button x="171" y="209" label="Submit" click="mytw_click()"/>
</mx:Panel>
</mx:Application>

二，点击主程序button弹出窗体  titlewindow.mxml代码如下：

<?xml version="1.0" encoding="utf-8"?>
<mx:TitleWindow xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" width="498" height="368" showCloseButton="true" close="PopUpManager.removePopUp
(this)">
<mx:Script>
<![CDATA[
import mx.managers.PopUpManager;

private function btn_click():void{
          dispatchEvent(new Event("tw_click"));
var a:PopUpDemo = this.owner as PopUpDemo;
  a.tit_usr.text = this.tw_usr.text;
  a.tit_psw.text = this.tw_psw.text;
PopUpManager.removePopUp(this);
}
]]>
</mx:Script>
<mx:Label x="96" y="67" text="用户名" width="97" height="26"/>
<mx:Label x="96" y="128" text="密码" width="97" height="24"/>
<mx:TextInput x="217" y="65" id="tw_usr"/>
<mx:TextInput x="217" y="126" id="tw_psw"/>
<mx:Button x="228" y="239" label="Click" click="btn_click()"/>
</mx:TitleWindow>

代码解释：点击主程序的button 弹出窗体TitleWindow，在弹出的窗体中输入username和password，点击Click，弹出窗体TitleWindow关闭，主程序上显示弹出窗体输入的username和password。

你好，有个问题请教一下，假如titlewindow.mxml不知道调用它的父窗体是谁，而且调用titlewindow.mxml时给它传一个字符串做为参数，这种调用情况能帮我说说思路么，谢谢