Core Listeners & Accessing the "Resource" Objects

I gotta admit... I kinda cheated with the last example. The logic in the listener was really easy because the only object that we needed to operate on was the currently-authenticated User. That made it super easy to grab that one object and set some data on it.

But... what if the object we need to set the data on is not the current User object... it's a CheeseListing or something else. For example, if the user goes to /api/cheeses/1.jsonld, how can we get access to the CheeseListing object with id 1 so that we can set data on it? Or even more interesting, if the user goes to /api/cheeses.jsonld, how could we get access to all of the CheeseListing objects that are about to be displayed so that we can set custom data on all of them?

The Core API Platform Listeners

To answer that question, search for "API Platform events". We haven't talked much about the API Platform event system yet. But in reality, almost everything that API Platform does is actually done by a listener behind the scenes.

For example, this ReadListener is what's responsible for calling the data providers, which fetch the objects from the database. Then, the DeserializeListener is what deserializes the JSON data on POST, PUT and PATCH requests to update or create an object. Later, ValidateListener executes validation and WriteListener calls the data persisters. There are a few other listeners, but these do the majority of the work.

Now check this out: both ReadListener and DeserializeListener listen to the kernel.request event - the same event that we are listening to! And these have a priority of 4 and 2.

When you create a subscriber, you can specify a priority in the getSubscribedEvents() method:

Since we haven't, our priority is zero. That means that both ReadListener and DeserializeListener are called before us.

That's important because after calling the data providers, ReadListener stores that information on a request attribute. And if DeserializeListener creates a new object, it does the same thing!

Grabbing the Request "data" Attribute

Check this out, in our listener - as an experiment - add dd($event->getRequest()->attributes->get('data')):

That's the special key where API platform puts the "data" for the current API request. When we spin over now and refresh the collections endpoint for users... awesome! It's our Paginator object! We could loop over that to get access to every User object that is about to be serialized.

And when we go to /api/users/1.jsonld, this dumps the individual User object.

So... this is awesome! At any point, we can grab the data key off of the request attributes to get access to the item or items for the current API request. This is how you could set a custom field for any entity inside a listener.

Remove that dd():

I really like the listener solution! Though, it does have two downsides, which may or may not be important. First, the event system isn't used in API Platform's GraphQL support... so this won't work if you're using GraphQL. And second, if you're writing some custom console commands, the isMe field will never be set there... because there's no request and so no RequestEvent!

For the isMe field... that's probably fine... because nobody is the currently-authenticated user in a console command anyways. But if you did want a custom field to be populated everywhere - even in a console command - we have one more solution: a Doctrine postLoad listener. Let's check that out next!