Creating new event in a module - drupal

Question

I'm trying to execute some code right before content is deleted. The Rules module has events for
After updating existing content
Before saving content
After saving new content
After deleting content
However, none of these execute my code at the right time.
I discovered a simple module called Predelete, which provides hooks for executing code before deletion. This seemed like an excellent candidate to call the Rules event from.
So, I created a very simple module based on the "predelete_field" example module contained within Predelete. The folder is called "predelete_field", is in the "modules" folder, and contains the following files:
1: predelete_field.info
core = "7.x"
dependencies[] = "rules"
dependencies[] = "list"
dependencies[] = "predelete"
description = "Example for the predelete module with a content type and a node"
name = "Predelete Field"
package = Other
project = "predelete_field"
version = "7.x-1.0"
; Information added by drupal.org packaging script on 2011-07-11
version = "7.x-1.1"
core = "7.x"
project = "predelete"
datestamp = "1310360219"
2: predelete_field.module
<?php
/**
* Implements hook_predelete_node().
*/
function predelete_field_predelete_node($node) {
drupal_set_message( "PREDELETE HOOK CALLED", 'warning' );
rules_invoke_event('predelete_field', $node);
$deletable = TRUE;
$reason = t('Deletable by default.');
return array('result' => $deletable, 'reason' => $reason);
}
3: predelete_field.rules.inc
<?php
/**
* Implements hook_rules_event_info() on behalf of the predelete_field module.
*/
function rules_predelete_field_event_info() {
$items = array(
'predelete_field_predelete' => array(
'label' => t('Before deleting content'),
'group' => t('Node'),
),
);
return $items;
}
Unfortunately, this does not appear to work: The event does not show up in the event list, even after clearing cache and disabling and re-enabling the module. Likewise, the drupal_set_message function does not appear to fire.
Is anyone able to spot any errors I may have made, or provide a solution?

Answer 1

The Predelete module just seems to hook into the confirmation form for multiple node deletions, it won't ever be fired using Rules as Rules doesn't invoke the form but uses the node API instead.
If you look at the node_delete_multiple() function though you'll see that several hooks are called before any content is actually deleted from the database (namely hook_node_delete and hook_entity_delete). One of these is the hook you'll want to implement in your custom module, like so:
function mymodule_node_delete($node) {
// Perform some action based on values in $the node object.
// Nothing has been deleted from the database at this point.
}
You'll need to clear Drupal's caches again when you've added that hook but according to the documentation it should work.