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Decoration: Override Core Services & AsDecorator

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In Symfony, decoration has a secret super-power: it allows us to customize nearly any service inside of Symfony. Woh.

For example, imagine that there's a core Symfony service and we need to extend its behavior with our own. How could we do that? Well, we could subclass the core service... and reconfigure things so that Symfony's container uses our class instead of the core one. That might work... but this is where decoration shines.

So, as a challenge, let's extend the behavior of Symfony's core EventDispatcher service so that whenever an event is dispatched, we dump a debugging message.

Investigating the Event Dispatcher

The ID of the service that we want to decorate is event_dispatcher

php ./bin/console debug:container event_dispatcher

And, fortunately, this class does implement an interface. Over on GitHub... on the symfony/symfony repository, hit t and open EventDispatcher.php.

And... yup! This implements EventDispatcherInterface. Decoration will work!

Creating the Decorator Class

Let's go make our decorator class. I'll create a new Decorator/ directory... and inside, a new PHP class called... how about DebugEventDispatcherDecorator.

Step one, is always to implement the interface: EventDispatcherInterface... though this is a little tricky because there are three of them! There's Psr, which is the smallest... one from Contract, and this one from Component. The one from Component extends the one from Contract... which extends the one from Psr.

Which do we want? The "biggest" one: the one from Symfony\Component:

14 lines | src/Decorator/DebugEventDispatcherDecorator.php
// ... lines 1 - 2
namespace App\Decorator;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;
class DebugEventDispatcherDecorator implements EventDispatcherInterface
{
// ... lines 9 - 12
}

The reason is that, if our EventDispatcher decorator is going to be passed around the system in place of the real one, it needs to implement the strongest interface: the interface that has the most methods on it.

Go to "Code"->"Generate" - or Command+N on a Mac - and select "Implement methods" to add the bunch we needed. Whew... there we go!

55 lines | src/Decorator/DebugEventDispatcherDecorator.php
// ... lines 1 - 7
class DebugEventDispatcherDecorator implements EventDispatcherInterface
{
// ... lines 10 - 14
public function dispatch(object $event, string $eventName = null): object
{
// ... line 17
}
public function addListener(string $eventName, $listener, int $priority = 0)
{
// ... line 22
}
public function addSubscriber(EventSubscriberInterface $subscriber)
{
// ... line 27
}
public function removeListener(string $eventName, $listener)
{
// ... line 32
}
public function removeSubscriber(EventSubscriberInterface $subscriber)
{
// ... line 37
}
public function getListeners(string $eventName = null): array
{
// ... line 42
}
public function getListenerPriority(string $eventName, $listener): ?int
{
// ... line 47
}
public function hasListeners(string $eventName = null): bool
{
// ... line 52
}
}

The other thing we need to do is add a constructor where the inner EventDispatcherInterface will be passed to us... and make that a property with private readonly:

55 lines | src/Decorator/DebugEventDispatcherDecorator.php
// ... lines 1 - 7
class DebugEventDispatcherDecorator implements EventDispatcherInterface
{
public function __construct(
private readonly EventDispatcherInterface $eventDispatcher
) {
}
// ... lines 14 - 53
}

Now that we have this, we need to call the inner dispatcher in all of these methods. This part is simple.... but boring. Say $this->eventDispatcher->addListener($eventName, $listener, $priority):

55 lines | src/Decorator/DebugEventDispatcherDecorator.php
// ... lines 1 - 7
class DebugEventDispatcherDecorator implements EventDispatcherInterface
{
// ... lines 10 - 19
public function addListener(string $eventName, $listener, int $priority = 0)
{
$this->eventDispatcher->addListener($eventName, $listener, $priority);
}
// ... lines 24 - 53
}

We also need to check whether or not the method should return a value. We don't need to return in this method... but there are methods down here that do have return values, like getListeners().

To avoid spending the next 3 minutes repeating what I just did 8 more times and putting you to sleep... bam! I'll just paste in the finished version:

55 lines | src/Decorator/DebugEventDispatcherDecorator.php
// ... lines 1 - 7
class DebugEventDispatcherDecorator implements EventDispatcherInterface
{
// ... lines 10 - 14
public function dispatch(object $event, string $eventName = null): object
{
return $this->eventDispatcher->dispatch($event, $eventName);
}
public function addListener(string $eventName, $listener, int $priority = 0)
{
$this->eventDispatcher->addListener($eventName, $listener, $priority);
}
public function addSubscriber(EventSubscriberInterface $subscriber)
{
$this->eventDispatcher->addSubscriber($subscriber);
}
public function removeListener(string $eventName, $listener)
{
$this->eventDispatcher->removeListener($eventName, $listener);
}
public function removeSubscriber(EventSubscriberInterface $subscriber)
{
$this->eventDispatcher->removeSubscriber($subscriber);
}
public function getListeners(string $eventName = null): array
{
return $this->eventDispatcher->getListeners($eventName);
}
public function getListenerPriority(string $eventName, $listener): ?int
{
return $this->eventDispatcher->getListenerPriority($eventName, $listener);
}
public function hasListeners(string $eventName = null): bool
{
return $this->eventDispatcher->hasListeners($eventName);
}
}

You can copy this from the code block on this page. We're simply calling the inner dispatcher in every method.

Finally, now that our decorator is doing all the things it must do, we can add our custom stuff. Right before the inner dispatch() method is called, I'll paste in two dump() lines and also dump Dispatching event, $event::class:

59 lines | src/Decorator/DebugEventDispatcherDecorator.php
// ... lines 1 - 7
class DebugEventDispatcherDecorator implements EventDispatcherInterface
{
// ... lines 10 - 14
public function dispatch(object $event, string $eventName = null): object
{
dump('--------------------');
dump('Dispatching event: ' . $event::class);
dump('--------------------');
return $this->eventDispatcher->dispatch($event, $eventName);
}
// ... lines 23 - 57
}

AsDecorator: Making Symfony use OUR Service

Ok! Our decorator class is done! But, there are many places in Symfony that rely on the service whose ID is event_dispatcher. So here's the million dollar question: how can we replace that service with our own service... but still get the original event dispatcher passed to us?

Whelp, Symfony has a feature built specifically for this and you're going to love it! Go to the top of our decorator class, add a PHP 8 attribute called: #[AsDecorator()] and pass the ID of the service that we want to decorate: event_dispatcher:

61 lines | src/Decorator/DebugEventDispatcherDecorator.php
// ... lines 1 - 4
use Symfony\Component\DependencyInjection\Attribute\AsDecorator;
// ... lines 6 - 8
#[AsDecorator('event_dispatcher')]
class DebugEventDispatcherDecorator implements EventDispatcherInterface
{
// ... lines 12 - 59
}

That's it. Seriously! This says:

Hey Symfony! Thanks for being so cool! Also, please make me the real event_dispatcher service, but still autowire the original event_dispatcher service into me.

Let's try it! Run our app:

php ./bin/console app:game:play

And... it works! Look! You can see the event being dumped out! And there's our custom event too. And when I exit... another event at the bottom! We just replaced the core event_dispatcher service with our own by creating a single class. That's bananas!

Using AsDecorator with OutputtingXpCalculator

Could we have used this AsDecorator trick earlier for our own XpCalculator decoration situation? Yep! Here's how: In config/services.yaml, remove the manual arguments:

31 lines | config/services.yaml
// ... lines 1 - 7
services:
// ... lines 9 - 27
App\Service\OutputtingXpCalculator:
arguments:
$innerCalculator: '@App\Service\XpCalculator'

And change the interface to point to the original, undecorated service: XpCalculator:

27 lines | config/services.yaml
// ... lines 1 - 7
services:
// ... lines 9 - 25
App\Service\XpCalculatorInterface: '@App\Service\XpCalculator'

Basically, in the service config, we want to set things up the "normal" way, as if there were no decorators.

If we tried our app now, it would work, but it wouldn't be using our decorator. But now, go into OutputtingXpCalculator add #[AsDecorator()] and pass it XpCalculatorInterface::class, since that's the ID of the service we want to replace:

34 lines | src/Service/OutputtingXpCalculator.php
// ... lines 1 - 6
use Symfony\Component\DependencyInjection\Attribute\AsDecorator;
#[AsDecorator(XpCalculatorInterface::class)]
class OutputtingXpCalculator implements XpCalculatorInterface
{
// ... lines 12 - 32
}

Donezo! If we try this now:

php ./bin/console app:game:play

No errors. An even faster way to prove this is working is by running:

php ./bin/console debug:container XpCalculatorInterface --show-arguments

And... check it out! It says that this is an alias for the service OutputtingXpCalculator. So anyone that's autowiring this interface will actually get the OutputtingXpCalculator service. And if you look down here at the arguments, the first argument passed to OutputtingXpCalculator is the real XpCalculator. That's amazing!

Multiple Decoration

All right, the decorator pattern is done. What a cool pattern! One feature of the decorator pattern that we only mentioned is that you can decorate a service as many times as you want. Yep! If we created another class that implemented XpCalculatorInterface and gave it this #AsDecorator() attribute, there would now be two services decorating it. Which service would be on the outside? If you care enough, you could set a priority option on one of the attributes to control that.

Decoration in the Wild?

Where do we see decoration in the wild? The answer to that is... sort of all over! In API Platform, it's common to use decoration to extend core services like the ContextBuilder. And Symfony itself uses decoration pretty commonly to add debugging features while we're in the dev environment. For example, we know that this EventDispatcher class would be used in the prod environment. But in the dev environment - I'll hit t to search for a "TraceableEventDispatcher" - assuming that you have some debugging tools installed, this is the actual class that represents the event_dispatcher service. It decorates the real one!

I can prove it. Head back to your terminal and run:

php ./bin/console debug:container event_dispatcher --show-arguments

Scroll to the top and... check it out! The event_dispatcher service is an alias to debug.event_dispatcher... whose class is TraceableEventDispatcher! And if you scroll down to its arguments, ha! It's passed our DebugEventDispatcherDecorator as an argument. Yup, there are 3 event dispatchers in this case: Symfony's core TraceableEventDispatcher is on the outside, it calls into our DebugEventDispatcherDecorator... and then that ultimately calls the real event dispatcher. Inception!

Problems Solved by Decorator

And what problems does the decorator pattern solve? Simple: it allows us to extend the behavior of an existing class - like XpCalculator - even if that class does not contain any other extension points. This means we can use it to override vendor services when all else fails. The only downside to the decorator pattern is that we can only run code before or after the core method. And the service we want to decorate must implement an interface.

Okay, team. We're done! There are many more patterns out there in the wild: this was a collection of some of our favorites. If we skipped one or several that you really want to hear about, let us know! Until then, see if you can spot these patterns in the wild and figure out where you can apply them to clean up your own code... and impress your friends.

Thanks for coding with me, and I'll see you next time!