How to add function to modify sitemap

Question

I currently created an extension which does a series of works to the category and product url.

Then I need to tell Magento not to include some of the categories and products if they have certain attributes.

But I'm a bit new so I need to know where do I setup my function. Is it in Observer.php or do I need to create another observer?

How to hook the event to my function. I think that would be in the config.xml like below:

<frontend>
    <events>
        <core_block_abstract_prepare_layout_before>
            <observers>
                <updater>
                    <type>model</type>
                    <class>My_Updater_Model_Observer</class>
                    <method>beforeLoadLayout</method>
                </updater>
            </observers>
        </core_block_abstract_prepare_layout_before>
    </events>
</frontend>

Thanks.

Answer 1

Hope this helps a bit .... in this expample we exclude all products with color attribute red:

app\etc\modules\My_Module.xml

<?xml version="1.0"?>
<config>
    <modules>
        <My_Module>
            <active>true</active>
            <codePool>community</codePool>
        </My_Module>
    </modules>
</config>

app\code\community\My\Module\etc\config.xml

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <modules>
        <My_Module>
            <version>0.0.1</version>
        </My_Module>
    </modules>

    <global>
        <models>
            <My_Module>
                <class>My_Module_Model</class>
            </My_Module>
        </models>
        <events>
            <!-- product sitemap -->
            <sitemap_products_generating_before>
                <observers>
                    <sitemap_products>
                        <class>My_Module/observer</class>
                        <method>excludeProductsFromSitemap</method>
                    </sitemap_products>
                </observers>
            </sitemap_products_generating_before>
            <!-- category sitemap -->
            <sitemap_categories_generating_before>
                <observers>
                    <sitemap_products>
                        <class>My_Module/observer</class>
                        <method>excludeCategoriesFromSitemap</method>
                    </sitemap_products>
                </observers>
            </sitemap_categories_generating_before>
        </events>
    </global>
</config>

app\code\community\My\Module\Model\Observer.php

<?php

class My_Module_Model_Observer extends Mage_Core_Model_Observer
{
    public function excludeProductsFromSitemap(Varien_Event_Observer $observer)
    {
        $event = $observer->getEvent();
        $collection = $event->getCollection();
        $items = $collection->getItems();

        $excludeIds = Mage::getModel('catalog/product')
            ->getCollection()
            ->setStoreId($event->getStoreId())
            // example filter
            ->addAttributeToFilter('color', 'red')
            ->getAllIds();

        foreach ($excludeIds as $id) {
            unset($items[$id]);
        }

        $collection->setItems($items);
    }
    public function excludeCategoriesFromSitemap(Varien_Event_Observer $observer)
    {
        $event = $observer->getEvent();
        $collection = $event->getCollection();
        $items = $collection->getItems();

        // your logic to filter categories here

        $collection->setItems($items);
    }
}

Answer 2

You can create new function in same observer and mention that function name in config.xml file as you did.

So you can write your new function in Model/Observer.php as below

...
public function beforeLoadLayout($observer)
{
// your function body here
}
...