CODEWITHSUNDEEP
phpsymfony
John
John

Reputation: 1655

Symfony EventListener How does it exactly work

I am trying to understand how to build a event Listener in Symfony and how it works.

so I have looked at the Example calss from Symfony Docs.

<?php

// src/AppBundle/EventListener/AcmeExceptionListener.php
namespace AppBundle\EventListener;

use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;

class AcmeExceptionListener
{
    public function onKernelException(GetResponseForExceptionEvent $event)
    {
// You get the exception object from the received event
        $exception = $event->getException();
        $message = sprintf(
            'My Error says: %s with code: %s',
            $exception->getMessage(),
            $exception->getCode()
        );

// Customize your response object to display the exception details
        $response = new Response();
        $response->setContent($message);

// HttpExceptionInterface is a special type of exception that
// holds status code and header details
        if ($exception instanceof HttpExceptionInterface) {
            $response->setStatusCode($exception->getStatusCode());
            $response->headers->replace($exception->getHeaders());
        } else {
            $response->setStatusCode(Response::HTTP_INTERNAL_SERVER_ERROR);
        }

// Send the modified response object to the event
        $event->setResponse($response);
    }
}

I understand that $event->getException() from GetResponseForExceptionEvent class picks a Event throwns in my application

What i don't quiet understand is how does why $exception can use getMessage() and getCode(). Can someone briefly explain these to me and Maybe a bit more about Symfony Event Listener.

Upvotes: 2

Views: 553

Answers (3)

Shairyar
Shairyar

Reputation: 3356

This is a huge topic and I found it hard to understand when I was learning it via Symfony documentation and other online blogs. I will recommend you to view this tutorial from KNP University and believe me you will have a very clear understanding of it.

Upvotes: 4

Cesar
Cesar

Reputation: 4116

That has nothing to do with Symfony Events, when yo use $event->getException you are getting an instance of class Exception, this class has those methods defined.

Upvotes: 1

egon12
egon12

Reputation: 810

The last question first.

why $exception can use getMessage() and getCode()

Symfony is a framework that use strict Objcet Oriented paradigm. So oftenly a function:

  • had an object type argument,
  • return also an object
  • and an object also can have it's own function and properties

In this example for simplicity we can think, $exception is an Exception object that have properties that can be see in here.

Also to understands how listeners works, it help if you have some knowledge about "Observer Design Pattern". This is a litle explanation.

  • there is an object called dispatcher (EventDispatcher)
  • this object had an array contains many listeners object
  • at point of time the kernel told dispatcher to dispatch
  • and dispatcher told all of listener to execute functions if exists

And to understanding your code, you create a listeners that active when Exception is thrown from anywhere. So you can choose to or not to show your error message.

Upvotes: 0

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