Magento2 : How create an Observer

Question

I want to create an Observer for an event.
i.e. controller_action_postdispatch_checkout_onepage_saveBilling
How to create it in Magento2 ?

Answer 1

Create events.xml at app\code\YOURNAMSPACE\MODULENAME\etc

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="controller_action_postdispatch_checkout_onepage_saveBilling">
        <observer name="YOUR_EVENT_NAME" instance="YOURNAMSPACE\MODULENAME\Observer\YourObserverClass" />
    </event>
</config>

cretae YourObserverClass.php at app\code\YOURNAMSPACE\MODULENAME\Observer

<?php
namespace YOURNAMSPACE\MODULENAME\Observer;

use Magento\Framework\Event\Observer as EventObserver;
use Magento\Framework\Event\ObserverInterface;

class YourObserverClass implements ObserverInterface
{

    public function execute(EventObserver $observer)
    {
        //Do your code here
    }
}