i needed a form type that determines based on the bound data if it should be required or not (to make file upload required if we create a new entity, but optional when editing the entity with the file. as this happens in a embedded sonata, i have no way of knowing the actual data when creating the form in the form builder. this is what i came up…

ImageType.php

<?php

class ImageType extends AbstractType
{
 
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        if ($options['required_auto'] && ! $options['required']) {
            $builder->addEventListener(\Symfony\Component\Form\FormEvents::PRE_SET_DATA, array($this, 'determineRequired'));
        }
    }
 
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array('required_auto' => true, 'required' => false));
    }
 
    public function determineRequired(FormEvent $event)
    {
        $data = $event->getData();
        $form = $event->getForm();
 
        if (! $data->isNew()) { // my model data knows if it is new or not
            $config = $form->getConfig();
            $reflection = new \ReflectionClass($config);
            $reflection = $reflection->getParentClass();
            $prop = $reflection->getProperty('required');
            $prop->setAccessible(true);
            $prop->setValue($config, true);
        }
    }
}

This question have a answer on: https://knpuniversity.com/screencast/question-answer-day/symfony2-conditionally-required-field#play