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How do I find out which handles should I use when I want to customize a certain page?

For example, by searching online, I found out that checkout_onepage_index is the handle to modify the Checkout page. I don't know about this magic, but somehow somebody just know it and I want to know how the learn this magic.

<?xml version="1.0"?>

<layout>

    <checkout_onepage_index>

        <reference name="head"></reference>

    </checkout_onepage_index>

</layout>

How do I know what handles if let say I want to modify the success page instead? or the frontend order detail page? etc...?

In the simplest case, layout handles represent frontName_controller_action.

So, in your example:

• 
  frontName is checkout for the Mage_Checkout module (as defined by its config.xml file in the <routers></routers> section).


• 
  controller is onepage for Mage_Checkout_OnepageController.


• 
  action is index for Mage_Checkout_OnepageController::indexAction()
  

Note that this is only the simplest case. Layout handles can be created/added in many other places and ways, such as the catalog logic (PRODUCT_TYPE_x layout handles for different product types).

A good way of finding layout handles to use is to check the files in design/frontend/base/default/layout - this directory stores all of the layout files of the base package/theme, and it has most (if not all) of the out of the box layout handles that you can utilize.

You can add this to your controller action.
It will show handles in more neat way than var_dump.
Strange that this answer is not already on stack.

Zend_Debug::dump($this->getLayout()->getUpdate()->getHandles());

We wrote down, how the handle is built (and many other things) here:

http://theslidesareonline.de/magento-in-30-minutes-en.pdf

and we wrote down a few layout handles which one should know, like customer_logged_in|out

Custom Action

Layout handle will be build with the NODE name!

<routers>

    <companyModule_frontend>

        <use>standard</use>

        <args>

            <module>Company_Module</module>

            <frontName>frontend</frontName>

        </args>

    </companyModule_frontend>

</routers>

Route: frontend//
Layout Handle: companyModule_frontend_controllerName_actionName

Phtandard Layout-Handle

^ Magento uses only parts of the Layout-XML file, depending on the requested page. These parts are called layout handle. There are a few which are used every time, and others which are belonging to exactly one URL

The most important Layout-Handles
which don't belong to a page:

• 
  (all pages)


• 
  (customer status)


• 
  (category w/o LN)


• 
  (category with LN)


• 
  (all categories)


• 
  (product detail page)

Special Layout-Handle

<PRODUCT_TYPE_(simple|configurable|grouped|virtual|downloadable|bundle{|giftcard})

Custom Layout handle

Easiest: add one with an observer

public function addMyOwnLayoutHandle(){

    Mage::app()->getLayout()->addHandle('my_own_handle');

}

than you can use in layout XML

<layout>

    <my_own_handle>

The convention is made this way because before loading the layout magento first search for the handle with this type of handle,so that it can find the template from the controller.But you can make your own custom handles

public function controllerActionLayoutLoadBefore(Varien_Event_Observer $observer)

        {

            /** @var $layout Mage_Core_Model_Layout */

            $layout = $observer->getEvent()->getLayout();



        $id = Mage::app()->getRequest()->getParam('id');



        /* or */



        if($ourCoolObject = Mage::registry('our_cool_object'))

        {

            $id = $ourCoolObject->getId();

        }



        $layout->getUpdate()->addHandle('OUR_COOL_OBJECT_'.$id);

    }

By the way there are 5 type of handles briefed below and we are talking about the fourth one here.

default

STORE_bare_us

THEME_frontend_default_default

helloworld_index_index

customer_logged_out

For more info refer

http://alanstorm.com/layouts_blocks_and_templates