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Dispatching the Event & No Handlers


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Back in DeleteImagePostHandler, we need to dispatch our new ImagePostDeletedEvent message. Earlier, we created a second message bus service. We now have a bus that we're using as a command bus called messenger.bus.default and another one called event.bus. Thanks to this, when we run:

php bin/console debug:autowiring mess

we can now autowire either of these services. Just using the MessageBusInterface type-hint will give us the main command bus. But using that type-hint plus naming the argument $eventBus will give us the other.

Inside DeleteImagePostHandler, change the argument to $eventBus. I don't have to, but I'm also going to rename the property to $eventBus for clarity. Oh, and variables need a $ in PHP. Perfect!

// ... lines 1 - 11
class DeleteImagePostHandler implements MessageHandlerInterface
private $eventBus;
// ... lines 15 - 16
public function __construct(MessageBusInterface $eventBus, EntityManagerInterface $entityManager)
$this->eventBus = $eventBus;
// ... line 20
// ... lines 22 - 32

Inside __invoke(), it's really the same as before: $this->eventBus->dispatch() with new ImagePostDeletedEvent() passing that $filename.

// ... lines 1 - 5
use App\Message\Event\ImagePostDeletedEvent;
// ... lines 7 - 11
class DeleteImagePostHandler implements MessageHandlerInterface
// ... lines 14 - 22
public function __invoke(DeleteImagePost $deleteImagePost)
// ... lines 25 - 30
$this->eventBus->dispatch(new ImagePostDeletedEvent($filename));

That's it! The end result of all of this work... was to do the same thing as before, but with some renaming to match the "event bus" pattern. The handler performs its primary task - deleting the record from the database - then dispatches an event that says:

An image post was just deleted! If anyone cares... do something!

Routing Events

In fact, unlike with commands, when we dispatch an event... we don't actually care if there are any handlers for it. There could be zero, 5, 10 - we don't care! We're not going to use any return values from the handlers and, unlike with commands, we're not going to expect that anything specific happened. You're just screaming out into space:

Hey! An ImagePost was deleted!

Anyways, the last piece we need to fix to make this truly identical to before is, in config/packages/messenger.yaml, down under routing, route App\Message\Event\ImagePostDeletedEvent to the async transport.

34 lines | config/packages/messenger.yaml
// ... lines 3 - 29
// ... lines 31 - 32
'App\Message\Event\ImagePostDeletedEvent': async

Let's try this! Find your worker and restart it. All of this refactoring was around deleting images so... let's delete a couple of things, move back over and... yea! It's working great! ImagePostDeletedEvent is being dispatched and handled.

Handle Some Handlers Async?

Oh, and side note about routing. When you route a command class, you know exactly which one handler it has. And so, it's super easy to think about what that handler does and determine whether or not it can be handled async.

With events, it's a bit more complicated: this one event class could have multiple handlers. And, in theory, you might want some to be handled immediately and others later. Because Messenger is built around routing the messages to transports - not the handlers - making some handlers sync and others async isn't natural. However, if you need to do this, it is possible: you can route a message to multiple transports, then configure Messenger to only call one handler when it's received from transport A and only the other handler when it's received from transport B. It's a bit more complex, so I don't recommend doing this unless you need to. We won't talk about how in this tutorial, but it's in the docs.

Events can have No Handlers

Anyways, I mentioned before that, for events, it's legal on a philosophical level to have no handlers... though you probably won't do that in your application because... what's the point of dispatching an event with no handlers? But... for the sake of trying it, open RemoveFileWhenImagePostDeleted and take off the implements MessageHandleInterface part.

// ... lines 1 - 8
class RemoveFileWhenImagePostDeleted
// ... lines 11 - 21

I'm doing this temporarily to see what happens if Symfony sees zero handlers for an event. Let's... find out! Back in the browser, try to delete an image. It works! Wait... oh, I forgot to stop the worker... let's do that... then try again. This time... it works... but in the worker log... CRITICAL error!

Exception occurred while handling ImagePostDeletedEvent: no handler for message.

By default, Messenger requires each message to have at least one handler. That's to help us avoid silly mistakes. But... for an event bus... we do want to allow zero handlers. Again... this is more of a philosophical problem than a real one: it's unlikely you'll decide to dispatch events that have no handlers. But, let's see how to fix it!

In messenger.yaml, take the ~ off of event.bus and add a new option below: default_middleware: allow_no_handlers. The default_middleware option defaults to true and its main purpose is to allow you to set it to false if, for some reason, you wanted to completely remove the default middleware - the middleware that handle & send the messages, among other things. But you can also set it to allow_no_handlers if you want to keep the normal middleware, but hint to the HandleMessageMiddleware that it should not panic if there are zero handlers.

35 lines | config/packages/messenger.yaml
// ... lines 3 - 4
// ... lines 6 - 9
default_middleware: allow_no_handlers
// ... lines 12 - 35

Go back and restart the worker. Then, delete another image... come back here and... cool! It says "No handler for message" but it doesn't freak out and cause a failure.

So now our command bus and event bus do have a small difference... though they're still almost identical... and we could really still get away with sending both commands and events through the same bus. Put the MessageHandlerInterface back on the class... and restart our worker one more time.

// ... lines 1 - 8
class RemoveFileWhenImagePostDeleted implements MessageHandlerInterface
// ... lines 11 - 21

Now that we're feeling good about events... I have a question: what's the difference between dispatching an event into Messenger versus dispatching an event into Symfony's EventDispatcher?

Let's talk about that next.