RemoveEventListener无法移除事件监听器的情况

小杜前几天问了我一个很基础的问题，居然就被问住了。

为什么removeEventListener不起效果，查了一下，原来对于MXML的属性中定义的事件监听无法用removeEventListener来移除的。以前看书居然没有看到，罪过罪过。

In the example
below flex removeEventListener method working is
demonstrated. This method removes the events from the flex components. Here
mouse click event are removed by the method. In the example method doesn't able
to remove the event from the button with id b3
because on this flex control, the associated event is built with the button
click attribute.
And this button control tag is under mxml tags, this shows
that events cannot be removed that are added inside the mxml tags.

<?xml version = '1.0' encoding = 'utf-8'?>
<mx:Application xmlns:mx = 'http://www.adobe.com/2006/mxml'
initialize = 'addEvent();'>
<mx:Script>
import mx.controls.Alert;

public function addEvent():void{
b1.addEventListener(MouseEvent.CLICK, alertbox, false, 0);
}

public function removeEvent():void{
b1.removeEventListener(MouseEvent.CLICK, alertbox);
b2.removeEventListener(MouseEvent.CLICK, alertbox);
b3.removeEventListener(MouseEvent.CLICK, alertbox);

}

public function alertbox(event:Event):void{
Alert.show('Mouse click event occured');
}

</mx:Script>

<mx:HBox>

<mx:Button id = 'b1' label = 'b1 button'/>
<mx:Button id = 'b2' label = 'b2 button'
initialize = 'b2.addEventListener(MouseEvent.CLICK, alertbox, false,0);'/>
<mx:Button id = 'b3' label = 'b3 button' click = 'alertbox(event);'/>

</mx:HBox>

<mx:TextInput text = 'click to remove click action'
click = 'removeEvent()' editable = 'false'/>

</mx:Application>