How to implement NotifyPropertyChanged method onPropertyChanged? - symfony

Question

How to implement NotifyPropertyChanged for retrieving entity changes (in Symfony 2.8 project with doctrine)?
I have found several examples, but they are not full:
http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/change-tracking-policies.html
http://www.boostr.in/26896/is-there-a-built-in-way-to-get-all-of-the-changed-updated-fields-in-a-doctrine-2%23googleads
For me it is not clear if i have to implement the own onPropertyChanged function, how ? I would like to retrive the property changes naming the last and the current properties in the controller and format them as a message for the user.
When i search in the documentation, NotifyPropertyChanged is not described or described minimally:
http://symfony2-document.readthedocs.io/en/latest/search.html?q=NotifyPropertyChanged&check_keywords=yes&area=default
http://api.symfony.com/2.8/search.html?search=NotifyPropertyChanged
http://docs.w3cub.com/symfony~2.8/
http://phpdox.de/demo/Symfony2/interfaces/Doctrine_Common_NotifyPropertyChanged.xhtml
It is written that addPropertyChangedListener() — Adds a listener that wants to be notified about property changes. I do not understand how i should put the custom code in this listener and how to execute this listener in such a way, that i could retrieve the entity changes to the controller action.
// src\MeetingBundle\Entity\EventLisObject.php
<?php
namespace MeetingBundle\Entity;
use Doctrine\Common\NotifyPropertyChanged;
use Doctrine\Common\PropertyChangedListener;
abstract class EventLisObject implements NotifyPropertyChanged
{
private $listeners = array();
public function addPropertyChangedListener(PropertyChangedListener $listener) {
$this->listeners[] = $listener;
}
/** Notifies listeners of a change. */
protected function onPropertyChanged($propName, $oldValue, $newValue) {
if ($this->listeners) {
foreach ($this->listeners as $listener) {
$listener->propertyChanged($this, $propName, $oldValue, $newValue);
}
}
}
}
//C src\MeetingBundle\Entity\Event.php
<?php
namespace MeetingBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;
use Symfony\Component\Validator\Constraints as Assert;
/**
* Event
*
* #ORM\Table(name="tevent", indexes={#ORM\Index(columns={"keywords"}, flags={"fulltext"})})
* #ORM\Entity(repositoryClass="MeetingBundle\Repository\EventRepository" )
*/
class Event extends EventLisObject
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255)
* #Assert\NotBlank()
*/
private $title;
/**
* Set title
*
* #param string $title
*
* #return Event
*/
public function setTitle($title) {
// $this->title = $title;
if( $title != $this->title ) {
$this->_onPropertyChanged("title", $this->title, $title);
$this->title = $title; }
return $this;
}
// src\MeetingBundle\Controller\EventMapControllerACL.php
....
/**
* Event controller.
*
* #Route("/eventmapacl")
*/
class EventMapControllerACL extends Controller
{
/**
* #Route("/edit/{id}", name="event_jsMap_edit")
* #Method("GET|POST")
* #Template("MeetingBundle:Event:ev_jsMap_edit.html.twig")
*/
public function editAction($id, Request $request)
{
$em = $this->getDoctrine()->getManager();
$entity = $em->getRepository('MeetingBundle:Event')->find($id);
if (!$entity) {
throw $this->createNotFoundException('Unable to find Event entity.');
}
$editForm = $this->createEditForm($entity);
$editForm->handleRequest($request);
// $deleteForm = $this->createDeleteForm($id);
if ($editForm->isValid()) {
$em = $this->getDoctrine()->getManager();
$em->persist($entity);
$em->flush();
// here or before flush or before persist i would like to estimate the changes and generate the message to user informing about them.
return $this->redirect($this->generateUrl('event_jsMap_edit', array('id' => $entity->getId())));
}
return array(
'entity' => $entity,
'edit_form' => $editForm->createView(),
);
}