Symfony2 twig forms - forms

Question

Symfony2 uses some twig templates when rendering forms.
In particular, in order to render the choice form field collapsed, symfony2 uses the following snippet of code (from form_div_layout.html.twig):
{%- block choice_widget_collapsed -%}
{%- if required and placeholder is none and not placeholder_in_choices and not multiple -%}
{% set required = false %}
{%- endif -%}
<select {{ block('widget_attributes') }}{% if multiple %} multiple="multiple"{% endif %}>
{%- if placeholder is not none -%}
<option value=""{% if required and value is empty %} selected="selected"{% endif %}>{{ placeholder != '' ? placeholder|trans({}, translation_domain) }}</option>
{%- endif -%}
{%- if preferred_choices|length > 0 -%}
{% set options = preferred_choices %}
{{- block('choice_widget_options') -}}
{%- if choices|length > 0 and separator is not none -%}
<option disabled="disabled">{{ separator }}</option>
{%- endif -%}
{%- endif -%}
{%- set options = choices -%}
{{- block('choice_widget_options') -}}
</select>
{%- endblock choice_widget_collapsed -%}
In my application, I want to create a new form type, that extends the default collapsed choice type, but where choice list has custom formatting.
In other words, for my form field type, named places_widget, extending choice_widget_collapsed block but redefining choice_widget_options.
I already tried:
/* My first attempt */
{%- block places_widget -%}
{% block choice_widget_options %}
{{ block('choice_places_widget_options') }}
{% endblock choice_widget_options %}
{{ block('choice_widget_collapsed') }}
{%- endblock places_widget -%}
/* My other attempt */
{%- block places_widget -%}
{{ block('choice_widget_collapsed', { 'choice_widget_options' => choice_places_widget_options }) }}
{%- endblock places_widget -%}
/* together with */
{%- block choice_places_widget_options %}
// code here
{%- endblock choice_places_widget_options -%}
None of the two solutions is working, and actually I have the feeling I am doing things in the wrong way.
Does anyone have any idea on how to achieve the goal?
Thank you in advance

Answer 1

Forms can be customized using themes (see the doc here: http://symfony.com/doc/current/cookbook/form/form_customization.html#form-theming-in-twig)
In your case, it seems you need to load your own theme with:
{% form_theme form _self %}
Note that you could externalize these blocks in another file (refer to http://symfony.com/doc/current/cookbook/form/form_customization.html#method-2-inside-a-separate-template to see how to do that)