Welcome new hook_module_implements_alter

Introduction

Whilst trying to debug something in Drupal 7, I came across a nice new hook a while back. hook_module_implements_alter allows you to hook into any calls to module_implements. You get to find out which hook is being called, and as a bonus to you get to see which modules are returning implementations for the hook and block them if you so wish.

Why is this useful?

I have to admit that this doesn't appear immediately useful, however Jamie has already described very well how you can use this hook. In very quick summary if a module is doing something in a hook that you don't want it to do, you can intervene, you can step in block it and if necessary provide your own implementation, the post I have linked to provides a good explanation of this.

There are other ways in which this hook may be useful, brutally simple to start with is just to quickly find out which hooks are being called for a page request and which modules are returning implementations for them.

function {mymodule}_module_implements_alter(&$implementations, $hook) {  
  drupal_set_message("<pre>" . print_r($hook,true) . "</pre>");
  drupal_set_message("<pre>" . print_r($implementations,true) . "</pre>");
}

This will naturally print a long list, which can be useful in emergencies, this hook could be used for more sophisticated debugging.