Triggers in Drupal

To make a Drupal module that provides triggers you need to

Use module_invoke_all at the point your code triggers an event.

<?php
module_invoke_all('mymodule', $op, $arg1, $arg2, ....);
?>

This basically defines a hook that your module provides - it has to be called after the name of your module because of a bug

Define your modules implementation of this hook to run any actions registered for it.

<?php

function mymodule_mymodule($op, $arg1, $arg2...){
  $aids = _trigger_get_hook_aids('mymodule', $op);
  $context = array(
    'hook' => 'mymodule',
    'op' => $op,
    'arg2' => $arg2,
     (arg3 etc)
  );
  actions_do(array_keys($aids), $arg1, $context);
}

?>

You can wrap up as many arguments as you like in the context array, $op can be used to define related actions like the way the user module has op for insert, delete, update etc.

$arg1 is passed by reference as the $object parameter to actions.

Tell Drupal about your triggers by implementing hook_hook_info

This adds the triggers on the triggers admin page

<?php
function mymodule_hook_info() {
  return array(
      'mymodule' => array(
        'mymodule' => array(
          'some_op' => array(
            'runs when' => t('Text to explain when the trigger runs'),
           ),
           'some_other_op' => array(
             'runs when' => t('more user text'),
           ),
         ),
       ),
   );
}

?>

Now you can use the admin interface to run actions when your triggers fire.

See Writing triggers (Drupal 6.x) for more details - I wrote this up as I found it helpful to approach the issue in reverse order to that page.