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Custom Fields with Events

It's possible to add some custom fields (we call them dynamic fields) to every form.

Info: It's always a good idea to use presets if you're using custom fields.

  1. Register a Listener:
services:
    App\EventListener\FormListener:
        autowire: true
        tags:
            - { name: kernel.event_subscriber }
  1. To add custom fields, you need to listen to the form pre set data event.

Important! Do not add fields to the symfony form directly! Form Builder will add additional data to your field, like template, order, email label and so on.

<?php

namespace App\EventListener;

use FormBuilderBundle\Event\Form\PreSetDataEvent;
use FormBuilderBundle\FormBuilderEvents;
use FormBuilderBundle\Form\Data\FormDataInterface;
use FormBuilderBundle\Model\FormDefinitionInterface;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Form\Extension\Core\Type\EmailType;
use Symfony\Component\Validator\Constraints\NotBlank;

class FormListener implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
        {
            return [
                FormBuilderEvents::FORM_PRE_SET_DATA => 'formPreSetData'
            ];
        }
        
    public function formPreSetData(PreSetDataEvent $event)
    {
        $formEvent = $event->getFormEvent();

        /** @var FormDataInterface $formData */
        $formData = $formEvent->getData();
        /** @var FormDefinitionInterface $formDefinition */
        $formDefinition = $formData->getFormDefinition();

        // get the form id/name from backend
        $formId = $formDefinition->getId();
        $formName = $formDefinition->getName();

        // get form options like the selected form preset
        $formOptions = $event->getFormOptions();

        //add dynamic fields depending on custom presets, is available.
        var_dump($formOptions['form_preset']);

        // add your fields
        $formData->getFormDefinition()->addDynamicField(
            
            //field name
            'your_dynamic_field_name',
            
            //field type
            EmailType::class,
            
            //all the field options
            [
                //no need to add translations here, fb will do this for you.
                'label'       => 'Your Field Label',
                'help_text'   => 'Help Text for your Custom Field',
                'constraints' => [
                    new NotBlank(),
                ]
            ],
            
            //optional options
            [
                //add a template
                'template' => 'col-xs-6',
                
                //set the order of your field
                'order' => 10,
                
                //no need to add translations here, fb will do this for you.
                'email_label' => 'Your Field Email Label',

                // form builder usually tries to render the value for the email by itself.
                
                //optional: add a output transformer (only affects this element)
                'output_transformer' => 'my_output_transformer',
            ]
        );
    }

    public function checkValue(FormInterface $field, $value, $locale)
    {
        return 'YOUR_TRANSFORMED_VALUE';
    }
}

Info: Checkout the dynamic fields with ajax example to learn how to combine dynamic fields with ajax forms.