So maybe you are looking into how to use Symfony in a more Event Driven way. Or maybe you want to speed up your apps response time to users. Or maybe the article title sounds interesting so you are reading this.
Every time a page is loaded Symfony executes many event listeners/subscribers. You can see a list of these inside your profiler by clicking pretty much any of the options in the profiler bar. Click the one that tells you the load time for example or even the 200 response. Click the “Events” tag to see the events.
You will see something like this :
You can listen to any of these events and perform an action based on them. The information is in the documentation link above. This allows you to create an event or subscribe to one that happens on every request, whether ajax or page loading.
The messenger component on the other hand, allows you to create custom events or send messages to handlers. This is handy for when handling a request requires multiple steps, or reaching out to another website/service or heavy processing like image and video uploads.
The messenger component is useful for asynchronous activities and processing. This is really useful for uploads in your application. Do you really need the user to sit there and watch a spinny wheel for many seconds while you process their image? Or would it be better to take the upload and return to the user as quickly as possible while the system performs the operations in the background? You always want the experience to be the best for the user.
In the end it is up to you to review both methods and figure out which system woks best for you particular use case.
So a use case for the EventDispatcher is more like, store the page name the user is on before processing for analytics or redirecting. Or catch errors/exceptions and handle them in some specific way. Or just do something when x happens. EventDispatcher is ONLY Synchronous so anything you do here adds to the time it takes to finish the request/response cycle for the user.
If you want an Excellent explanation and deep dive into how to use the EventDispatcher, I suggest reading this SymfonyCast on how the Http Request/Response cycle and events work.
Messenger on the other hand is good for things like handling an image upload since you may be using something like S3 object storage and that may have network issues. Messenger can be either synchronous or asynchronous. The messenger component helps you build an asynchronous processing system so that your users experience is as quick as possible for slow processing events.
In general the Messenger Component is good for building an Event driven system to make your app feel quicker to the user. User experience is #1.
If you want a deep dive into the Messenger Component, how it works and how to use it to the fullest. I suggest you read this SymfonyCast it goes deep into the subject and will answer all of the questions you have after reading this article and the documentation about the component.