Dynamic Form Events

Alright, here's the issue and it is super technical. If we change the Location from "Near a Star" to "Solar System", even if we "hack" the specificLocationName field so that it submits the value "Earth", it doesn't work! It fails validation!

This is a real problem, because, in a few minutes, we're going to add JavaScript to the page so that when we change location to "The Solar System", it will dynamically update the specificLocationName dropdown down to be the list of planets. But for that to work, our form system needs to be smart enough to realize - at the moment we're submitting - that the location has changed. And then, before it validates the ChoiceType, it needs to change the choices to be the list of planets.

Don't worry if this doesn't make complete sense yet - let's see some code!

Adding an Event Listener

There's one piece of the form system that we haven't talked about yet: it has an event system, which we can use to hook into the form loading & submitting process.

At the end of the form, add $builder->get('location')->addEventListener() and pass this FormEvents::POST_SUBMIT. This FormEvents class holds a constant for each "event" that we can hook into for the form system. Pass a callback as a second argument: Symfony will pass that a FormEvent object.

Let's dd() the $event so we can see what it looks like.

But before we check it out, two important things. First, when you build a form, it's actually a big form tree. We've seen this inside of the form profiler. There's a Form object on top and then each individual field below is itself a full Form object. The same is true with the "form builder": we normally just interact with the top-level $builder by adding fields to it. When we call $builder->add(), that creates another "form builder" object for that field, and you can fetch it later by saying $builder->get().

Second, we're attaching the event to only the location field - not the entire form. So, when the form submits, Symfony will call this function, but the $event object will only have information about the location field - not the entire form.

Let's actually see this! Refresh to re-submit the form. There it is! The FormEvent contains the raw, submitted data - the solar_system string - and the entire Form object for this one field.

Dynamically Updating the Field

This gives us the hook we need: we can use the submitted data to dynamically change the specificLocationName field to use the correct choices, right before validation occurs. Actually, this hook happens after validation - but we'll use a trick where we remove and re-add the field, to get around this.

To start, create a new private function called setupSpecificLocationNameField(). The job of this function will be to dynamically add the specificLocationName field with the correct choices. It will accept a FormInterface - we'll talk about that in a minute - and a ?string $location, the ? part so this can be null.

Inside, first check if $location is null. If it is, take the $form object and actually ->remove() the specificLocationName field and return. Here's the idea: if when I originally rendered the form there was a location set, then, thanks to our logic in buildForm(), there will be a specificLocationName field. But if we changed it to "Choose a location", meaning we are not selecting a location, then we want to remove the specificLocationName field before we do any validation. We're kind of trying to do the same thing in here that our future JavaScript will do instantly on the frontend: when we change to "Choose a location" - we will want the field to disappear.

Next, get the $choices by using $this->getLocationNameChoices() and pass that $location. Then, similar to above, if (null === $choices) remove the field and return. This is needed for when the user selects "Interstellar Space": that doesn't have any specific location name choices, and so we don't want that field at all.

Finally, we do want the specificLocationName field, but we want to use our new choices. Scroll up and copy the $builder->add() section for this field, paste down here, and change $builder to $form - these two objects have an identical add() method. For choices pass $choices.

Nice! We created this new function so that we can call it from inside of our listener callback. Start with $form = $event->getForm(): that gives us the actual Form object for this one field. Now call $this->setupSpecificLocationNameField() and, for the first argument, pass it $form->getParent().

This is tricky. The $form variable is the Form object that represents just the location field. But we want to pass the top level Form object into the function so that the specificLocationName field can be added or removed from it.

The second argument is the location itself, which will be $form->getData(), or $event->getData().

Okay guys, I know this is craziness, but we're ready to try it! Refresh to resubmit the form. It saves. Now change the Location to "Near a Star". In a few minutes, our JavaScript will reload the specificLocationName field with the new options. To fake that, inspect the element. Let's go copy a real star name - how about Sirius. Change the selected option's value to that string.

Hit update! Yes! It saved! We were able to change both the location and specificLocationName fields at the same time.

And that means that we're ready to swap out the field dynamically with JavaScript. But first, we're going to leverage another form event to remove some duplication from our form class.