How to use event in Magento 2 Extension.

I have introduce How to use Helper and setting in Magento 2 extension . I will introduce how to use event in Magento. Event in Magento is very importance in Magento 2 extension because some time you have to change data, do somethings before or after action from Magento core . It include tow parts :
1. Create Event
2 . Catch Event

1. Create Event

To create event, Create Text Event Action and enter code bellow

<?php
namespace Magebay\Hello\Controller\Index;

use Magento\Framework\App\Action\Context;
use Magento\Framework\View\Result\PageFactory;

/**
 * Class Index
 * @package Magebay\Hello\Controller\Index
 */
class TestEvent extends \Magento\Framework\App\Action\Action
{
    protected $session;
    public function __construct(
        Context $context,
        PageFactory $resultPageFactory,
        \Magento\Customer\Model\Session $session
    )
    {
        parent::__construct($context);
        $this->resultPageFactory = $resultPageFactory;
        $this->session = $session;
    }

    /**
     * @return \Magento\Framework\App\ResponseInterface|\Magento\Framework\Controller\ResultInterface|void
     */
    public function execute()
    {
        $text = 'Hello ';
        $this->session->setTextMessage($text);
        $this->_eventManager->dispatch('magebay_hello_display_text_before', ['hello_message' => $text]);
        echo $this->session->getTextMessage();

    }
}

2. Catch Event.

Catch event is very importance because some time you have to custom data or action from core Magento . Example : Custom Price when adding product cart . Create event, you can do 2 step bellow:
Step 1 : create event in xml file . Create file app/code/Magebay/Hello/etc/frontend/events.xml.

<?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="magebay_hello_display_text_before">
        <observer name="mb_show_text" instance="Magebay\Hello\Observer\DisplayText" />
    </event>
</config>

Note: if you want to Catch events in frontend, can create events in etc/fontend/events.xml, you if want to Catch events in backend, you can create event in etc/adminhtml/events.xml and if you want to the both area, create events in etc/events.xml

Step 2 : Create file app/code/Magebay/Hello/Observer/DisplayText.php

<?php
/**
 * Created by PhpStorm.
 * User: maiuoc
 * Date: 2019-01-19
 * Time: 4:58 PM
 */
namespace Magebay\Hello\Observer;

use Magento\Framework\Event\Observer as EventObserver;
use Magento\Framework\Event\ObserverInterface;
class DisplayText implements ObserverInterface
{
    protected $session;
    function __construct(
        \Magento\Customer\Model\Session $session
    )
    {
        $this->session = $session;
    }


    public function execute(EventObserver $observer)
    {
        // TODO: Implement execute() method.
        $message  = $observer->getData('hello_message');
        $message .= ' Magebay'; // change text 
        $this->session->setTextMessage($message);
    }
}

If if focus the Tutorial , You can understand basic knowledge About Magento extension . I hope you can build a Magento extension by your self . if you have any question , you can comment under the post, I will check help you. I will introduce how to how to create admin grid in magento 2.