Magento 2 – Custom Product List Template for Category Layout Update

If you just want to change the template for the product_list which based on the comments above I think is what your trying to achieve you can just reference the original name and then set a new template file name.

In your local.xml under whatever layout handle .eg <catalog_category_layered> you want this to update on you can do this:

<reference name="product_list">
<action method="setTemplate"><template>catalog/product/subcatlist.phtml</template></action>
</reference>

If you want to do this from the backend it's the same apart from you don't need to wrap a layout handle around the code.

I'm trying to solve a similar issue, so I'm reading through the code pretty extensively to get to the root of this. I'll start at the end and work backward:

The call to addUpdate() you reference actually does end up calling addUpdate() on the \Magento\Framework\View\Model\Layout\Merge class. Hold onto your seat: it's a bumpy road. The $page variable is an instance of \Magento\Framework\View\Result, and it inherits the definition of addUpdate() from its parent class Layout (in the same namespace). That class in turn calls addUpdate() on a ProcessorInterface, whose default implementation is none other than: Magento\Framework\View\Model\Layout\Merge.

So then to your question about it seeming to pass the entire XML string: you are correct, and the behavior is correct. The Merge class should theoretically be taking care of parsing the <update> inside _fetchRecursiveUpdates() and then loading the layout XML from the referenced handle.

The catch is that _fetchRecursiveUpdates() is never called on XML chunks that are added via addUpdate(). They are assumed to be ready-to-go (non-recursive) layout instructions. Layout XML from external addUpdate() calls are included directly via a call to $this->asString() at the very end of Merge::load(). They never pass through the inner _merge() function which is what indirectly calls _fetchRecursiveUpdates().

A simplistic solution would be to create a new class that extends the built-in Merge class and set it as the preferred implementation of ProcessorInterface. You would then change the addUpdate() method to perform the same logic as _fetchRecursiveUpdates() before saving its argument to the internal array, something like the following:

public function addUpdate($update)
{
    $updateXml = $this->_loadXmlString($update);
    if (strtolower($updateXml->getName()) == 'update' && isset($updateXml['handle'])) {
        $this->_merge((string)$updateXml['handle']);
    } else {
        $this->updates[] = $update;
    }
    return $this;
}