Add block with observer event

Question

I want to add a block inside another block I have this structure:

in config.xml for the observer part I have this:

......
 <frontend>
       <events>
            <core_block_abstract_to_html_before>
                <observers>
                    <cart_shopping>
                        <type>model</type>
                        <class>cart_shopping/observer</class>
                        <method>insertBlock</method>
                    </cart_shopping>
                </observers>
            </core_block_abstract_to_html_before>
        </events>
....

and in observer I search for block and then I create the new block:

class Cart_Shopping_Model_Observer
{

        public function insertBlock($observer)
        {
            /** @var $_block Mage_Core_Block_Abstract */
            /*Get block instance*/
            $_block = $observer->getBlock();
            /*get Block type*/
            $_type = $_block->getType();
           /*Check block type*/
            if ($_type == 'catalog/product_price') {
                /*Clone block instance*/
                $_child = clone $_block;
                /*set another type for block*/
                $_child->setType('test/block');
                /*set child for block*/
                $_block->setChild('child', $_child);
                /*set our template*/
                $_block->setTemplate('at.phtml');
            }
        }
    }

my theme is rwd so: I have put at.phtml in app/design/frontend/rwd/default/template/catalog/product/at.phtml

and here is at.phtml:

<?php echo $this->getChildHtml('child') ?>
<h1>Hello</h1>

It is not adding the block, why? do I need any custom layout file or changing any local.xml file?! I don't know what else I should do I tried to do observer solution with this link

Answer 1

I understood my problem it was in config.xml, I changed class like this:

 <core_block_abstract_to_html_before>
                <observers>
                    <cart_shopping>
                        <type>model</type>
                        <class>Cart_Shopping_Model_Observer</class>
                        <method>insertBlock</method>
                    </cart_shopping>
                </observers>
            </core_block_abstract_to_html_before>

and also I did what Amit said about the template

Answer 2

Your template path location is wrong:

It should be $_block->setTemplate('catalog/product/at.phtml'); as you define template location

app/design/frontend/rwd/default/template/catalog/product/at.phtml

As magento every template file location is define at layout and php code after template/.

Note: you must need to use RWD Package

As you have using cart_shopping/observe that means you have used xml pattern for call observer class that why need define Model class prefix in xml

  <global>
        <models>
            <cart_shopping><!-- model class prefix -->
                <class>Cart_Shopping_Model</class>
            </cart_shopping>
        </models>
    </global>

Answer 3

The other option is to add the model definition to your config.xml

<global>
    <models>
        <cart_shopping>
            <class>Cart_Shopping_Model</class>
        </cart_shopping>
    </models>
</global>

This will mean that the observer can use the config cart_shopping/observer or as you have commented you can switch to the full path Cart_Shopping_Model_Observer