How to pass custom parameters / attributes from twig template to custom form field?

Question

I am working with Symfony 3.4 and have created a custom FormType. I would like to pass some parameters to a form field of this type from a twig template.

According to the Symfony Docs this should be possible:

{# render a widget, but add a "foo" class to it #}
{{ form_widget(form.name, { 'attr': {'class': 'foo'} }) }} 

While this does work when using a build in form type such as TextType I could not find out how to achieve the same for my custom types:

// Form Type class
class MyCustomType extends AbstractType {

    public function getBlockPrefix() {
        return 'my_custom_type';
    }

    public function getParent() {
        return ChoiceType::class;
    }

    public function configureOptions(OptionsResolver $resolver) {
        $resolver->setDefaults(array(
            'translation_domain' => 'app',
        ));
    }
}


// Widget Template
{% block my_custom_type_widget %}
    {{ dump(form) }}
    {% if form.vars.attr.class is defined %} 
        {{ form.vars.attr.class }}
    {% endif %}
    ...
{% endblock %}


// Form template
...
{{ form_start(form) }}
    {{ form_widget(form.myCustom, {'attr': {'class': 'customClass'}}) }}
    ...
{{ form_end(form) }}

The customClass which is added in the form template is not passed to the widget template. The {% if form.vars.attr.class is defined %} test works, but only the default class formName-myCustom is available.

So, how do I pass a custom attribute from twig to my custom form type? What do I have to change on my custom type to let it work just like the TextType where passing a class in Twig is no problem?

Answer 1

Try to merge default class with the new one like in https://github.com/symfony/symfony/blob/5.x/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_layout.html.twig#L29