Prestashop module for product features - php

Question

I'm working on a prestashop custom module.
This module will have to show some product features, let the admin modify its values and then save them in a custom tab.
So, showing features in a custom tab haven't been so difficult:
public function hookDisplayAdminProductsExtra($params) {
$id_lang = $this->context->language->id;
$features = FeatureCore::getFeatures($id_lang);
$values = [];
foreach ($features as $feature) {
array_push($values, $feature['value']);
}
$this->context->smarty->assign(array(
'features' => $features,
'values' => $values
));
if(!empty($sampleObj) && isset($sampleObj->id)){
$this->context->smarty->assign(array(
'custom_text_area' => $sampleObj->textarea
));
}
return $this->display(__FILE__, 'views/admin/sample.tpl');
}
The only thing which I can't get is features' default values in order to put them into a select.
After that what I want to do is to be able to save every value on product save.
So I wrote this hook:
public function hookActionProductUpdate($params) {
$id_product = $params['id_product'];
$product = new Product($id_product);
var_dump($product);
$all_tpl_vars = $smarty->getTemplateVars();
print_r($all_tpl_vars);
die("hello");
}
but when I press the save button nothing happens, and nothing will be shown on screen.
This is my first prestashop module, all the hooks has been registered in the module constructor.
Thanks to all.