Extending MXML custom components via MXML

What I'd like to do: create an MXML component with some children, then extend it via MXML to create a new component with more children, without losing the original set. In other words

create a component bc.mxml

<?xml version="1.0" encoding="utf开发者_如何学Python-8"?>
<s:BorderContainer xmlns:fx="http://ns.adobe.com/mxml/2009" 
                   xmlns:s="library://ns.adobe.com/flex/spark" 
                   xmlns:mx="library://ns.adobe.com/flex/mx" width="400" height="300">

    <s:Button id="b1" label="button1"/>
</s:BorderContainer>

and then extend it to a separate component mc.mxml

<?xml version="1.0" encoding="utf-8"?>
<borderContainerX:bc xmlns:fx="http://ns.adobe.com/mxml/2009" 
                     xmlns:s="library://ns.adobe.com/flex/spark" 
                     xmlns:mx="library://ns.adobe.com/flex/mx" xmlns:borderContainerX="borderContainerX.*">

    <s:Button id="b2" y="100" label="button2"/>
</borderContainerX:bc>

and get a component with 2 buttons.

I've seen various threads on how this is either not possible (1) or on workarounds to accomplish this (2, 3) and have been wondering if something has changed with the advent of Flex 4 or if we're still stuck with these workarounds the last reply in thread 3 seems to hint at Flex 4 fixing it all?

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In Flex 4, you will have to override your "mxmlContent" property setter in order to preserve your already defined children in a parent class

One of possible implementations of such a override is presented in the comment for this blog entry

Quick tip (Flex 4): Goodbye templates – hello mxmlContent

http://www.websector.de/blog/2009/10/02/quick-tip-flex-4-goodbye-templates-hello-mxmlcontent/