Difference between hook_node_view() and template_preprocess_node() with seeing messages

Question

When I add a form to a node's render tree using hook_node_view(), eg

<?php

/**
* Implements hook_node_view().
*/
function MODULE_node_view($node, $view_mode, $langcode){
  if ($node->nid == 9 && module_exists('contact')) {
    module_load_include('inc', 'contact', 'contact.pages');
    $cForm = drupal_get_form('contact_site_form');
    //$cForm['#action'] .= '?destination=' .    request_path();
    $node->content['contact_form'] = array(
    '#markup' => drupal_render($cForm),
    '#weight' => 20,
    );
  }
}

then I get my messages displayed right after the submission.

I do the same thing with template_preprocess_node(), the messages are displayed one request after the submission.

I suspect this is related to the execution order. Can someone shed some light on the difference here?

Answer 1

It is indeed related to execution order and how messages are handled by the theme layer.

The $messages variable (which contains system messages) is processed and output at the page level (page.tpl.php) which occurs before node (node.tpl.php). So by the time you invoke drupal_get_form() from your node preprocessor, $messages has already been set up.

So, although $_SESSION['messages'] has been populated with the validation errors via drupal_get_form(), these won't appear until a page refresh.

hook_node_view(), on the other hand, is outside the theme layer and is invoked even before page processing begins, which is why it works. But if you want to make the change at the theme level, a workaround is to set up the messages manually after you call drupal_get_form() in template_preprocess_node():

$contact_form = drupal_get_form('contact_site_form');
$contact_form['#prefix'] = theme('set_messages');

Thanks to rémy for originally pointing out the problem on our site.

Answer 2

It is the execution order. hook_node_view() acts on the node object as it is being assembled. When template_preprocess_node() is called, the node is ready to be rendered. For instance you may use template_preprocess_node() to change the date format.