CiviCRM Community Forums (archive)

a) It does not - in fact, I suspect the order is non-deterministic, with a slight bias towards installation-order (ie the extension which was installed first would tend to run first). I came to this belief from tracing CRM_Utils_Hook::commonInvoke() => commonBuildModuleList() => requireCiviModules() ...etc... finally arriving at CRM_Extension_Mapper::getActiveModuleFiles(), which queries the DB and does not have an ORDER BY.

b) There's an extra element worth noting: when Civi fires an event, it actually fires ~2 times (once with the CMS's hook/event system, and once with the extensions' hook/event system). Weights in the two systems would not be able to mingle. Chalk that up to "edge-cases from having two systems".

c) I've posted a rant arguing to consolidate the event subsystems - https://forum.civicrm.org/index.php?topic=36086.0 . Just to pitch this a little more... if we consolidated on Symfony's EventDispatcher, then it would possible to define weights for each listener.

For a little follow-up/recap on that thread: we've been making baby steps with EventDispatcher in core. For example, in Civi\Core\Container::createDispatcher, we register some core listeners for hook_civicrm_post.

When PR #6297 gets merged (esp. https://github.com/totten/civicrm-core/commit/8ba6fb8c9f2052656cae06547eae91cdc932ec6c ), I believe that any type of package will be able to work with EventDispatcher (eg Civi extensions/Drupal modules/Joomla extensions/WordPress plugins). The basic technique is to build on hook_civicrm_container like so:

function mymodule_civicrm_container(ContainerBuilder $c) {
  $c->findDefinition('dispatcher')->addMethodCall('addListener', array(
    'hook_civicrm_post',
    'my_custom_callback_func',
    1234 /* the desired weight; omit for default */
   ));
}

That nominally does what you're asking about, although it doesn't give full flexibility (because we can't have weights that cut across fundamentally different event systems), and it's only one step on a longer path toward consolidated events.