Twig - Why can't I access a variable I've set? - symfony

Question

For some reason, a variable I'm setting in one form template bloc is not available in a child form block.
I have an 'entity' field type to present a selection of checkboxes to allow the user to select related items...
$builder
->add( 'title' )
->add(
'apps',
'entity',
[
'class' => 'OurAdminBundle:App',
'choices' => $apps,
'property' => 'title',
'expanded' => true,
'multiple' => true
]
)
And here's the template that renders the form
// Effectively imported using the MopaBootstrapBundle
// {% form_theme form 'OurAdminBundle:Form:fields.html.twig %}
// Further in page theming
{% form_theme form _self %}
// Set variable when on the apps field, so it should be available to all child
// forms
{% block _gallery_apps_widget %}
{% set custom_checkboxes = 1 %}
{{ block('choice_widget') }}
{% endblock %}
// Attempt to retrieve the variable on the checkboxes within the apps entity
/ field
{% block checkbox_widget %}
{{ dump(custom_checkboxes|default(0) }} // Displays 0
{% endblock checkbox_widget %}
Here's the code from the fields.html.twig file (with minor debugging additions...
{% block choice_widget_expanded %}
{{ dump(custom_checkboxes|default(0)) }}
{% set custom_checkboxes = custom_checkboxes|default(0) %}
{{ dump(custom_checkboxes|default(0)) }}
{% spaceless %}
{% set label_attr = label_attr|merge({'class': (label_attr.class|default(''))}) %}
{% set label_attr = label_attr|merge({'class': (label_attr.class ~ ' ' ~ (widget_type != '' ? (multiple ? 'checkbox' : 'radio') ~ '-' ~ widget_type : ''))}) %}
{% if expanded %}
{% set attr = attr|merge({'class': attr.class|default(horizontal_input_wrapper_class)}) %}
{% endif %}
{% for child in form %}
{% if widget_type != 'inline' %}
<div class="{{ multiple ? 'checkbox' : 'radio' }}">
{% endif %}
<label{% for attrname, attrvalue in label_attr %} {{ attrname }}="{{ attrvalue }}"{% endfor %}>
{{ form_widget(child, {'horizontal_label_class': horizontal_label_class, 'horizontal_input_wrapper_class': horizontal_input_wrapper_class, 'attr': {'class': attr.widget_class|default('')}}) }}
{{ child.vars.label|trans({}, translation_domain) }}
</label>
{% if widget_type != 'inline' %}
</div>
{% endif %}
{% endfor %}
{% endspaceless %}
{% endblock choice_widget_expanded %}
... which successfully displays '1' on both counts.
I've racked my brains over this one, but can't for the life of me understand why I can't access the variable in the checkbox_widget block. Please help.

Answer 1

This is due to how Symfony renders form fields when calling form_widget() or any other form* family of functions.
Symfony creates a new separate scope which do not share the scope of the parent (in order to prevent scope polluting while rendering fields).
If you which to pass a variable to the checkbox widget, edit the form_widget call in the choice_widget_expanded to pass on the custom_checkboxes as so (added tabbing for clarity only):
{{ form_widget(child, {
'horizontal_label_class': horizontal_label_class,
'horizontal_input_wrapper_class': horizontal_input_wrapper_class,
'attr': {'class': attr.widget_class|default('')},
'custom_checkboxes': custom_checkboxes
}) }}