Decoupled way of modifying forms in Symfony 3/4 - php

Question

I'd like to be able to alter a form using the Symfony Event Dispatcher. The Symfony documentation talks about dynamic form modification, however, in all examples the event listener or subscriber is created in the form class. I'd like to keep this logic decoupled from my form class.
How can I modify a Symfony form without having to specify which event listeners are going to be called in the form class?

Answer 1

Probably what you need is a form type extension, it allow you to modify any existing form types across the entire system. There, you can add event listeners/subscribers or what you want to any specific or generic form type.
However, this task tends to get tedious if it's a very frequent case. So doing something like this can provide you with a perfect fit:
class FoobarFormSubscriber implements EventSubscriberInterface, FormEventSubscriberInterface
{
public static function getSubscribedEvents()
{
return array(FormEvents::PRE_SET_DATA => 'preSetData');
}
public static function getFormClass()
{
return FoobarType::class;
}
public function preSetData(FormEvent $event)
{
$form = $event->getForm();
$form->add('custom', null, array('mapped' => false));
}
}
But obviously this isn't a feature implemented by Symfony. Here I leave you a recipe to achieve it:
First, create a new form type extension to add the subscriber to the form builder according to configuration:
class FormEventTypeExtension extends AbstractTypeExtension
{
private $subscribers;
public function __construct(array $subscribers = array())
{
$this->subscribers = $subscribers;
}
public function buildForm(FormBuilderInterface $builder, array $options)
{
$formClass = get_class($builder->getType()->getInnerType());
if (isset($this->subscribers[$formClass])) {
foreach ($this->subscribers[$formClass] as $subscriber) {
$builder->addEventSubscriber($subscriber);
}
}
}
public function getExtendedType()
{
return FormType::class;
}
}
Create a new interface to configure the form class to listen to:
interface FormEventSubscriberInterface
{
public static function getFormClass();
}
Finally, into a new compiler pass, injects to the extension service all registered kernel.event_subscriber that implement the previous interface:
public function process(ContainerBuilder $container)
{
$subscribers = array();
foreach ($container->findTaggedServiceIds('kernel.event_subscriber') as $serviceId => $tags) {
$subscriberClass = $container->getDefinition($serviceId)->getClass();
if (is_subclass_of($subscriberClass, FormEventSubscriberInterface::class, true)) {
$subscribers[$subscriberClass::getFormClass()][] = new Reference($serviceId);
}
}
$extensionDef = $container->getDefinition(FormEventTypeExtension::class);
$extensionDef->setArgument(0, $subscribers);
}
Then, your custom subscribers are decoupled and ready to work as is, just make sure to implement both interfaces (EventSubscriberInterface, FormEventSubscriberInterface) and register the event subscriber as service.