Standalone Symfony Event Dispatcher from the Scratch

2019-08-05

What is the Main Purpose of Event Dispatcher?

• Extend an application in a certain place without putting any code right there.

This way, you can extend 3rd party packages without rewriting them. And also allow other users to reach your code without even touching it.

Not sure how that looks? You will - at the end of this article.

Event Dispatcher

This is the brain. It stores all subscribers and calls events when you need to.

Event

This is the name of a place. When something has happened in the application: order is sent, or user is deleted.

Event Subscriber

This is the action that happens when we come to a specific event. When an order is sent (= Event), send me a confirmation SMS (= Event Subscriber). And check that all the ordered products are on stock.

1 event can invoke MORE Event Subscribers.

Create First Subscriber in 3 Steps

1. Install via Composer

composer require symfony/event-dispatcher

2. Create Event Dispatcher

// index.php
require_once __DIR__ . '/vendor/autoload.php';

// 1. create the Dispatcher
$eventDispatcher = new Symfony\Component\EventDispatcher\EventDispatcher;

// 2. some event happend, we dispatch it
$eventDispatcher->dispatch('youtube.newVideoPublished'); // oh: event is just a string

Try it:

php index.php

Wow! Nothing happened...

That's ok because there is no Subscriber. So let's...

3. Create and Register Subscriber

use Symfony\Component\EventDispatcher\EventSubscriberInterface;

final class NotifyMeOnVideoPublishedEventSubscriber implements EventSubscriberInterface
{
    public bool $isUserNotified = false;

    public static function getSubscribedEvents(): array
    {
        // in format ['event name' => 'public function name that will be called']
        return ['youtube.newVideoPublished' => 'notifyUserAboutVideo'];
    }

    public function notifyUserAboutVideo()
    {
        // some logic to send notification
        $this->isUserNotified = true;
    }
}

Let the Dispatcher know about the Subscriber.

$eventDispatcher = new Symfony\Component\EventDispatcher\EventDispatcher;

$eventSubscriber = new NotifyMeOnVideoPublishedEventSubscriber;
$eventDispatcher->addSubscriber($eventSubscriber);

// nothing happened, default value
var_dump($eventSubscriber->isUserNotified);
// false

// this calls our Subscriber
$eventDispatcher->dispatch('youtube.newVideoPublished');

// now it's changed
var_dump($eventSubscriber->isUserNotified);
// true

Run the code again from command line:

$ php index.php
int(0)
int(1)

And now you understand EventDispatcher. At least in 90 % use cases.

---

Still on? Let's get advanced.

What if we need to get the name of the Youtuber into the Subscriber?

Event Objects to the Rescue!

The Event objects are basically Value Objects. Pass a value in the constructor and get it with getter.

1. Create an Event Object

use Symfony\Component\EventDispatcher\Event;

final class YoutuberNameEvent extends Event
{
    private string $youtuberName;

    public function __construct(string $youtuberName)
    {
        $this->youtuberName = $youtuberName;
    }

    public function getYoutuberName(): string
    {
        return $this->youtuberName;
    }
}

2. Use Event Object in Event Subscriber

use Symfony\Component\EventDispatcher\EventSubscriberInterface;

final class NotifyMeOnVideoPublishedEventSubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents(): array
    {
        return ['youtube.newVideoPublished' => 'notifyUserAboutVideo'];
    }

    // Event Object is passed as method argument
    public function notifyUserAboutVideo(YoutuberNameEvent $youtuberNameEvent)
    {
        var_dump($youtuberNameEvent->getYoutuberName());
    }
}

3. Create an Object and Dispatch It

$youtuberNameEvent = new YoutuberNameEvent('Jirka Král');

$eventDispatcher->dispatch($youtuberNameEvent);

And here is the result:

$ php index.php
string('Jirka Král')

We Are 1 Step Further Now

You can now:

• understand basic Event workflow
• know what EventDispatcher and EventSubscriber are for
• and know-how to pass parameters via the Event object

Where to go next?

Still hungry for knowledge? Check Symfony documentation then.

But remember: practice is the best teacher.

Happy coding!