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/**
 * {@inheritdoc}
 */
public function buildForm(FormBuilderInterface $builder, array $options)
{
    $app = $this->app;

    $builder
        ->addEventListener(FormEvents::PRE_SET_DATA, function ($event) use ($app) {
            /** @var \Eccube\Entity\Shipping $data */
            $data = $event->getData();
            /** @var \Symfony\Component\Form\Form $form */
            $form = $event->getForm();
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name: SampleForm
code: SampleForm
version: 0.0.1

form:
    onPreSetData:
        SampleForm

namespace Plugin\SampleForm;
use Symfony\Component\Form\FormEvent;
class SampleForm
{
    public function onPreSetData(FormEvent $event)
    {
        $form = $event->getForm();
        if ('contact' === $form->getName()) {
            $form->add('sample_form', 'text');
        }
    }
}