Magento: page/html_wrapper block is empty - magento

Question

Small part of Page.xml:
<layout version="0.1.0">
....
<default translate="label" module="page">
<label>All Pages</label>
<block type="page/html" name="root" output="toHtml" template="page/2columns-left.phtml">
<block type="page/html_header" name="header" as="header" /> <!-- work -->
<block type="page/html_wrapper" name="u.Top.Menu" as="u_Top_Menu" translate="label"> <!-- doesn't work -->
<label>top menu</label>
<action method="setElementTagName"><value>div</value></action>
<action method="setElementClass"><value>sub-menu</value></action>
</block>
...
</block>
....
And in the 2columns-left.phtml output it:
<?php echo $this->getChildHtml('u_Top_Menu'); ?>
But it always retuns empty value. I'm confused here a bit. What I'm doing wrong here?

Answer 1

This is what happens when you forget to add the children block to the html wrapper block. Look at this block class source code:
class Mage_Page_Block_Html_Wrapper extends Mage_Core_Block_Abstract
{
/**
* Whether block should render its content if there are no children (no)
* #var bool
*/
protected $_dependsOnChildren = true;
/**
* Render the wrapper element html
* Supports different optional parameters, set in data by keys:
* - element_tag_name (div by default)
* - element_id
* - element_class
* - element_other_attributes
*
* Renders all children inside the element.
*
* #return string
*/
protected function _toHtml()
{
$html = empty($this->_children) ? '' : trim($this->getChildHtml('', true, true));
if ($this->_dependsOnChildren && empty($html)) {
return '';
}
...

Answer 2

So far as I can tell you are doing everything correct. The thing that normally trips people up is the as vs name issue whereby a block is referred in xml via it's name, but in templates via it's as. You don't appear to have fallen into this trap, so my best guess is that your cache hasn't been cleaned. An rm -rf var/cache/mage-* should hopefully fix the issue.

Answer 3

Zyava is correct, but I have a workaround!
If you don't want to add children to the block, but still need it to render, there is a class function (dependsOnChildren) that allows you to set this _dependsOnChildren flag from your layout XML like so:
<block type="page/html_wrapper" name="u.Top.Menu" as="u_Top_Menu" translate="label"> <!-- doesn't work -->
<label>top menu</label>
<action method="setElementTagName"><value>div</value></action>
<action method="setElementClass"><value>sub-menu</value></action>
<!-- This will tell PhP to call $blockClass->dependsOnChildren(0); before rendering. -->
<action method="dependsOnChildren"><value>0</value></action>
</block>
Here is that function (for reference)
app/code/core/Mage/Page/Block/Html/Wrapper.php # Line 80
/**
* Setter whether this block depends on children
* #param $depends
* #return Mage_Page_Block_Html_Wrapper
*/
public function dependsOnChildren($depends = '0')
{
$this->_dependsOnChildren = (bool)(int)$depends;
return $this;
}