title: Extending MXML custom components via MXML
body: <p>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</p>
  
  <p>create a component bc.mxml</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;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"&gt;
  
      &lt;s:Button id="b1" label="button1"/&gt;
  &lt;/s:BorderContainer&gt;
  </code></pre>
  
  <p>and then extend it to a separate component mc.mxml</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;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.*"&gt;
  
      &lt;s:Button id="b2" y="100" label="button2"/&gt;
  &lt;/borderContainerX:bc&gt;
  </code></pre>
  
  <p>and get a component with 2 buttons.</p>
  
  <p>I've seen various threads on how this is either not possible (<a href="http://forums.adobe.com/message/2756043" rel="noreferrer">1</a>) or on workarounds to accomplish this (<a href="http://paulofierro.com/archives/538/" rel="noreferrer">2</a>, <a href="http://www.tink.ws/blog/extending-your-own-mxml-components/" rel="noreferrer">3</a>) 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? </p>