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If you've ever create an event listener or event subscriber in Symfony, you're creating a "listener" for an event that's dispatched through a service called the "event dispatcher". The purpose of the event dispatcher is to allow one piece of code to "notify" the app that something happened and for anyone else to "listen" to that event and run some code.
Which... huh... is the exact same purpose of dispatching an event into Messenger. What the heck? If I want to dispatch an event in my code, should I use the EventDispatcher or Messenger? Are animated image files pronounced "jif" or "gif"? Should toilet paper hang "over" the roll or "under"? Ahh!
First, there is a practical difference between dispatching an event to the EventDispatcher versus Messenger: Messenger allows your handlers to be called asynchronously, whereas listeners to events from the EventDispatcher are always synchronous.
And this leads to a nice rule of thumb. Whenever you dispatch an event, if you want listeners to that event to be able to communicate back to you, so you can then do something based on their feedback, use the EventDispatcher. But if you simply want to say "this thing happened" and you don't need any feedback from possible listeners or handlers, use Messenger.
For example, in
AddPonkaToImageHandler, suppose we wanted to dispatch an event here so that other parts of the system could tell us exactly which Ponka image should be added to this photo. In that case, we need those listeners to be able to communicate back to us. To do that we would create an Event class that holds the
ImagePost object and has a setter on it that listeners can call - maybe
setPonkaImageToUse(). We would then use the
EventDispatcher and dispatch the message before actually adding Ponka to the image. Once all the listeners were called, we could see if any of them called that
But what if we simply wanted to say:
Hey! We just added Ponka to an image!
... add didn't need any information back from possible handlers? In that case we would create a similar event class, leave off the
setPonkaImageToUse() method and dispatch it with Messenger. Messenger is perfect if you don't need any info back from your handlers because... those handlers might end up being called asynchronously!
If it's still not clear to you, just use whichever you want. Why? Because if you end up wanting your code to run asynchronously, you'll end up choosing Messenger. And if you want your listeners to be able to communicate back to the code that dispatches the messages, you'll use EventDispatcher. Otherwise, either will work.
Next, let's use some service configuration tricks to tighten up how we've organized our commands, command handlers, events and event handlers.