Symfony2 Datalist Extension

DataListExtension.php

<?php

namespace AppBundle\Form\Extension;

use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormView;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

/**
 * @author David Badura <d.a.badura@gmail.com>
 */
class DataListExtension extends AbstractTypeExtension
{
    /**
     * @param FormView $view
     * @param FormInterface $form
     * @param array $options
     */
    public function buildView(FormView $view, FormInterface $form, array $options)
    {
        if (!$options['datalist']) {
            return;
        }

        $dataListId = $view->vars['id'] . '_datalist';
        $view->vars['attr']['list'] = $dataListId;

        $view->vars['datalist_id'] = $dataListId;
        $view->vars['datalist'] = $options['datalist'];
    }

    /**
     * @param OptionsResolverInterface $resolver
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(['datalist' => []]);
    }

    /**
     * Returns the name of the type being extended.
     *
     * @return string The name of the type being extended
     */
    public function getExtendedType()
    {
        return 'text';
    }
}

form.html.twig

{% block form_widget_simple -%}
    {{ parent() }}

    {% if datalist is defined %}
        <datalist id="{{ datalist_id }}">
            {% for item in datalist %}
            <option value="{{ item }}">
            {% endfor %}
        </datalist>
    {% endif %}
{%- endblock %}

services.xml

<container xmlns="http://symfony.com/schema/dic/services"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>  
        <service id="form.typ_extension.data_list" class="AppBundle\Form\Extension\DataListExtension">
            <tag name="form.type_extension" alias="text"/>
        </service>
        
    </services>

</container>

Update for Symfony 4.4:

namespace App\Form\Extension;

use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormView;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\OptionsResolver\OptionsResolver;

/**
 * https://gist.github.com/DavidBadura/94e8636f8fec5d9a2a03
 * @author David Badura <d.a.badura@gmail.com>
 */
class DatalistExtension extends AbstractTypeExtension
{
    /**
     * @param FormView $view
     * @param FormInterface $form
     * @param array $options
     */
    public function finishView(FormView $view, FormInterface $form, array $options): void
    {
        if (!$options['datalist']) {
            return;
        }

        $datalistId = $view->vars['id'] . '_datalist';
        $view->vars['attr']['list'] = $datalistId;

        $view->vars['datalist_id'] = $datalistId;
        $view->vars['datalist'] = $options['datalist'];
    }

    public function configureOptions(OptionsResolver $resolver): void
    {
        $resolver->setDefaults(['datalist' => []]);
    }

    public static function getExtendedTypes(): iterable
    {
        return [TextType::class];
    }
}

And the configuration in services.xml (or services.yaml) is not necessary any more.