At what stage does wp_ajax hooks gets applied during Wordpress request?

Question

At some point between muplugins_loaded and shutdown

wp_ajax_*,

wp_ajax_nopriv_* are executed, but where/when exactly?

I have found that this is working:

add_action(        'wp_ajax_actionname', 'callbackname');
add_action( 'wp_ajax_nopriv_actionname', 'callbackname);

as well as:

add_action('wp_loaded', function(){
  add_action(        'wp_ajax_actionname', 'callbackname');
  add_action( 'wp_ajax_nopriv_actionname', 'callbackname);
});

So it is after wp_loaded, but when exactly?

This isn't pure curiosity, the alternative wording of this question would be "can we safely assume that wp_ajax hooks aren't executed when Wordpress loading, including it's plugins, is incomplete"?

Answer 1

They're not triggered in the normal WordPress lifecycle. They're only triggered when you make a request to admin-ajax.php, and they're triggered as the last action before shutdown. You can see this in the file itself: https://github.com/WordPress/WordPress/blob/master/wp-admin/admin-ajax.php#L188