Pass custom options to a symfony2 form

The solution is simple, if you want your custom option to be available also in Twig template, you must use
$builder->setAttribute() in buildForm method
and
$view->set() method in buildView() method, too.

<?php

namespace Acme\DemoBundle\Form\Type;

use Symfony\Component\Form\AbstractType as FormAbstractType;
use Symfony\Component\Form\FormBuilder;
use Symfony\Component\Form\FormView;
use Symfony\Component\Form\FormInterface;

// For Symfony 2.1 and higher:
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

/**
 * ImagePreviewType
 *
 */
class ImagePreviewType extends FormAbstractType
{

    /**
     * {@inheritDoc}
     * For Symfony 2.0
     */
    //public function getDefaultOptions(array $options)
    //{
    //    $options = parent::getDefaultOptions($options);
    //    $options['base_path'] = 'path/to/default/dir/';
    //
    //    return $options;
    //}

    /**
     * {@inheritDoc}
     * For Symfony 2.1 and higher
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'base_path'         => '',
        ));
    }

    /**
     * {@inheritDoc}
     */
    public function buildView(FormView $view, FormInterface $form, array $options)
    {
        // For Symfony 2.0:
        // $view->set('base_path', $form->getAttribute('base_path'));

        // For Symfony 2.1 and higher:
        $view->vars['base_path'] = $options['base_path'];
    }

    /**
     * {@inheritDoc}
     */
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->setAttribute('base_path', $options['base_path'])
        ;
    }

    /**
     * {@inheritDoc}
     */
    public function getName()
    {
        return 'image_preview';
    }

    public function getParent(array $options)
    {
        // for Symfony 2.0:
        // return 'field';

        // for Symfony 2.1 and higher:
        return 'form';
    }
}

Template for custom form type (file …Acme/DemoBundle/Resources/views/Form/fields.html.twig):

{% block image_preview_widget %}
{% spaceless %}
<img src="https://stackoverflow.com/questions/10920006/{{ base_path ~ value }}" alt=""  {{ block('widget_container_attributes') }} />
{% endspaceless %}
{% endblock %}

Register your template for custom form types in app/config/config.yml

twig:
    debug:            %kernel.debug%
    strict_variables: %kernel.debug%
    form:
        resources:
            - 'AcmeDemoAdminBundle:Form:fields.html.twig'

Usage: Display preview of user’s image while editing his profile:

// src/Acme/DemoBundle/Form/Type/UserType.php
namespace Acme\DemoBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilder;

class UserType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder->add('user_profile_image_file_name', new ImagePreviewType(), array(
            'base_path' => 'some/other/dir',
        ));
    }
}

2014-08-18: Updated for Symfony 2.1 or higher