Conditionally setting the template in Magento - php

Question

I would like to make a portion of a page available for getting via AJAX. I have in mind to use a URL parameter, bare, that would tell Magento to present a page with a different template applied to the root block. The bare template looks like this:
<?php echo $this->getChildHtml('content'); ?>
That's it! The idea is that a JavaScript method could grab just the content of another page and insert it into the DOM where appropriate. (I don't want this to be possible with just any page – only pages that have been marked to do so in layout xml.)
I've read elsewhere that I should avoid conditional layout xml. The only other approach I can think of is to override the Page/Html block itself, creating a modified setTemplate method, like below. Instinctively, I'm concerned about overriding such a core part of Magento.
public function setTemplate($template, $bareTemplate='')
{
$bareMode = Mage::app()->getRequest()->getParam('bare');
$targetTemplate = (!empty($bareTemplate) && $bareMode === '1') ? $bareTemplate : $template;
return parent::setTemplate($targetTemplate);
}
What better approaches haven't I thought of?

Answer 1

The key to getting what you want is removing root as an output block, replacing it with content. Output blocks are just entry points for renderLayout();
To do this in Magento without include-path-hacking Mage_Core_Controller_Varien_Action, observe the controller_action_layout_render_before_$this->getFullActionName() scoped events which are fired in the base action controller class (ref Mage_Core_Controller_Varien_Action::renderLayout() method).
First configure your model class group and frontend event observer. You'll need to determine the full action name of any route that needs this logic. See Mage_Core_Controller_Varien_Action::renderLayout(). Example config below.
<?xml version="1.0"?>
<config>
<global>
<models>
<your_classgroup>
<class>Your_Classgroup_Model</class>
</your_classgroup>
</models>
</global>
<frontend>
<events>
<controller_action_layout_render_before_FULL_ACTION_NAME...>
<observers>
<your_observer_config>
<type>model</type>
<class>your_classgroup/observer</class>
<method>makeContentBlockTheOutputBlock</method>
</your_observer_config>
</observers>
</controller_action_layout_render_before_FULL_ACTION_NAME...>
</events>
</frontend>
</config>
The event observer logic is easy. Do this:
public function makeContentBlockTheOutputBlock($observer)
{
//Edit: action not passed in to this event; passed in generic generate_blocks event
if( Mage::app()->getRequest()->getParam('bare') )
{
Mage::app()->getLayout()->removeOutputBlock('root')->addOutputBlock('content');
}
}
HTH.