What is the function of "_initLayout" in magento?

Question

What does this function do in magento?

$this->_initLayoutMessages('customer/session');
$this->_initLayoutMessages('catalog/session');

What _initLayoutMessages() return?

Answer 1

The method returns nothing. Well, actually it returns $this.
From different parts of the application (mostly controllers) you can set session messages that are displayed in the next page you visit.
Here is an example.
When subscribing to a newsletter, when the page refreshes you will see a message Thank you for your subscription..
This message is added from Mage_Newsletter_SubscriberController::newAction by this line:

$session->addSuccess($this->__('Thank you for your subscription.'));

Where $session = Mage::getSingleton('core/session').

By calling $this->_initLayoutMessages('customer/session'); inside an action you tell magento to show all session messages that were previously added by using:

Mage::getSingleton('customer/session')->addError(...) (or addWarning, addNotice and addSuccess).

Since the messages can be added by using different session parts (customer/session, catalog/session, checkout/session, core/session) this allows you to separate and control the messages that the user sees.

For example if on a previous action you added a message like this:

Mage::getSingleton('customer/session')->addError('Some error here');  

the user won't see this message until he visits a page that maps to an action that has $this->_initLayoutMessages('customer/session');.

Answer 2

The method _initLayoutMessages adds a message block to the page layout that outputs any error, notice or success messages stored in the session model stated as argument.

So $this->_initLayoutMessages('catalog/session'); will output a message added via

Mage::getSingleton('catalog/session')->addError('foo');
Mage::getSingleton('catalog/session')->addSuccess('foo');
Mage::getSingleton('catalog/session')->addNotice('foo');