how to access collection member labels in collection extension - symfony

Question

I have extended the collection form field type with CollectionTableExtension. I am trying to set a variable, FormView::$vars['headers'] so that collection can be output with default headers when is_table is true and no user-defined headers are available. The default headers should be the same as the labels which would normally be applied to collection members.
e.g.
if FooType has 3 fields, foo, bar and fibble, its labels will be Foo, Bar and Fibble (after humanizing with Twig template) or the values stored in the label attribute of each property.
So if my DoofusType has a collection of FooTypes,
$builder->add('foos', 'collection', array('type'=>'acc_foo', 'is_table'=>true));
should result in a view where headings for the collection will be Foo, Bar and Fibble or the values of the label attributes of the FooType if they are set.
Here is my collection extension:
<?php
namespace ACC\MainBundle\Form;
use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\Form\FormView;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\PropertyAccess\PropertyAccess;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
class CollectionTableExtension extends AbstractTypeExtension
{
public function setDefaultOptions(OptionsResolverInterface $resolver)
{
$resolver->setOptional(array('is_table'));
$resolver->setOptional(array('headers'));
$resolver->setOptional(array('caption'));
}
public function getExtendedType()
{
return 'collection';
}
public function buildView(FormView $view, FormInterface $form, array $options)
{
if (array_key_exists('is_table', $options)) {
$view->vars['is_table'] = $options['is_table'];
if (array_key_exists('caption', $options)) $view->vars['caption'] = $options['caption'];
if (array_key_exists('headers', $options)) {
$view->vars['headers'] = $options['headers'];
}else{
//harvest labels from collection members, but HOW?
}
}
}
}
The issue is that I don't know how to access the properties of the collection element type from inside the collection extension. I could access the first element in the collection like this:
if($form->has('0')) {
foreach($form->get('0')->all() as $item)
$view->vars['headers'][] = $item->getName();
}
but that doesn't help if the collection is empty. And it doesn't help if the collection element type has defined labels. Any ideas?

Answer 1

After much slogging through Symfony source code, I've found a way to do it. Essentially, in ::buildView a dummy collection element of the correct type must be instantiated so that its labels or properties can be extracted. Here's what I did:
public function buildView(FormView $view, FormInterface $form, array $options)
{
if (array_key_exists('is_table', $options)) {
$view->vars['is_table'] = $options['is_table'];
if (array_key_exists('caption', $options)) $view->vars['caption'] = $options['caption'];
if (array_key_exists('headers', $options)) {
if($options['headers']===false){ //no headers
unset($view->vars['headers']);
}else if (is_array($options['headers'])){ //headers passed in
$view->vars['headers'] = $options['headers'];
}
}else { //harvest labels from collection elements
$elementtype = $form->getConfig()->getOption('type');
//should be a guard clause here so types that won't supply good headers (e.g. a collection of text fields) will skip the rest
$element = $form->getConfig()->getFormFactory()->create($elementtype); //get dummy instance of collection element
$fields = $element->all();
$headers = array();
foreach($fields as $field){
$label= $field->getConfig()->getOption('label');
$headers[] = empty($label) ? $field->getName() : $label;
}
$view->vars['headers'] = $headers;
}
}
}
If anyone has a cleaner method, I'm all ears.