Symfony EventListener How does it exactly work

Question

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.

Answer 1

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.

Answer 2

这与Symfony事件无关，当您使用$event->getException您将获得Exception类的实例，此类已定义了这些方法。

Answer 3

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.