wiki.techinc.nl/docs/hooks.doc

HOOKS.DOC

This document describes how event hooks work in MediaWiki; how to add
hooks for an event; and how to run hooks for an event.

==Glossary==

event
     Something that happens with the wiki. For example: a user logs
     in. A wiki page is saved. A wiki page is deleted. Often there are
     two events associated with a single action: one before the code
     is run to make the event happen, and one after. Each event has a
     name, preferably in CamelCase. For example, 'UserLogin',
     'ArticleSave', 'ArticleSaveComplete', 'ArticleDelete'.

hook
     A clump of code and data that should be run when an event
     happens. This can be either a function and a chunk of data, or an
     object and a method.
     
hook function
     The function part of a hook.
     
==Rationale==

Hooks allow us to decouple optionally-run code from code that is run
for everyone. It allows MediaWiki hackers, third-party developers and
local administrators to define code that will be run at certain points
in the mainline code, and to modify the data run by that mainline
code. Hooks can keep mainline code simple, and make it easier to
write extensions. Hooks are a principled alternative to local patches.

Consider, for example, two options in MediaWiki. One reverses the
order of a title before displaying the article; the other converts the
title to all uppercase letters. Currently, in MediaWiki code, we
handle this as follows:

    function showAnArticle($article) {
        global $wgReverseTitle, $wgCapitalizeTitle;
	
	if ($wgReverseTitle) {
	    wfReverseTitle($article);
        }
	
	if ($wgCapitalizeTitle) {
	   wfCapitalizeTitle($article);
        }
	
	# code to actually show the article goes here
    }
    
An extension writer, or a local admin, will often add custom code to
the function -- with or without a global variable. For example,
someone wanting email notification when an article is shown may add:

    function showAnArticle($article) {
        global $wgReverseTitle, $wgCapitalizeTitle;
	
	if ($wgReverseTitle) {
	    wfReverseTitle($article);
        }
	
	if ($wgCapitalizeTitle) {
	   wfCapitalizeTitle($article);
        }

	# code to actually show the article goes here
	
	if ($wgNotifyArticle) {
	   wfNotifyArticleShow($article));
        }
    }

Using a hook-running strategy, we can avoid having all this
option-specific stuff in our mainline code. Using hooks, the function
becomes:

    function showAnArticle($article) {
    	if (wfRunHooks('ArticleShow', $article)) {
	    # code to actually show the article goes here
	    wfRunHooks('ArticleShowComplete', $article);
	}
    }

We've cleaned up the code here by removing clumps of weird,
infrequently used code and moving them off somewhere else. It's much
easier for someone working with this code to see what's _really_ going
on, and make changes or fix bugs.

In addition, we can take all the code that deals with the little-used
title-reversing options (say) and put it in one place. Instead of
having a little title-reversing if-block spread all over the codebase
in showAnArticle, deleteAnArticle, exportArticle, etc., we can
concentrate it all in an extension file:

    function reverseArticleTitle($article) {
        # ...
    }

    function reverseForExport($article) {
        # ...
    }

The setup function for the extension just has to add its hook
functions to the appropriate events:

    setupTitleReversingExtension() {
        global $wgHooks;
	
	$wgHooks['ArticleShow'][] = reverseArticleTitle;
	$wgHooks['ArticleDelete'][] = reverseArticleTitle;
	$wgHooks['ArticleExport'][] = reverseForExport;
    }

Having all this code related to the title-reversion option in one
place means that it's easier to read and understand; you don't have to
do a grep-find to see where the $wgReverseTitle variable is used, say.

If the code is well enough isolated, it can even be excluded when not
used -- making for some slight savings in memory and time at runtime.
Admins who want to have all the reversed titles can add:

    require_once('extensions/ReverseTitle.php');
    
...to their LocalSettings.php file; those of us who don't want or need
it can just leave it out.

The extensions don't even have to be shipped with MediaWiki; they
could be provided by a third-party developer or written by the admin
him/herself.

==Writing hooks==

A hook is a chunk of code run at some particular event. It consists of:

  * a function with some optional accompanying data, or
  * an object with a method and some optional accompanying data.

Hooks are registered by adding them to the global $wgHooks array for a
given event. All the following are valid ways to define hooks:

      $wgHooks['EventName'][] = someFunction; # function, no data
      $wgHooks['EventName'][] = array(someFunction, $someData);
      $wgHooks['EventName'][] = array(someFunction); # weird, but OK
      
      $wgHooks['EventName'][] = $object; # object only
      $wgHooks['EventName'][] = array($object, 'someMethod');
      $wgHooks['EventName'][] = array($object, 'someMethod', $someData);
      $wgHooks['EventName'][] = array($object); # weird but OK

When an event occurs, the function (or object method) will be called
with the optional data provided as well as event-specific parameters.
The above examples would result in the following code being executed
when 'EventName' happened:

      # function, no data
      someFunction($param1, $param2)
      # function with data
      someFunction($someData, $param1, $param2)

      # object only
      $object->onEventName($param1, $param2)
      # object with method
      $object->someMethod($param1, $param2)
      # object with method and data
      $object->someMethod($someData, $param1, $param2)
      
Note that when an object is the hook, and there's no specified method,
the default method called is 'onEventName'. For different events this
would be different: 'onArticleSave', 'onUserLogin', etc.

The extra data is useful if we want to use the same function or object
for different purposes. For example:

      $wgHooks['ArticleSaveComplete'][] = array(ircNotify, 'TimStarling');
      $wgHooks['ArticleSaveComplete'][] = array(ircNotify, 'brion');

This code would result in ircNotify being run twice when an article is
saved: once for 'TimStarling', and once for 'brion'.

Hooks can return three possible values:
  * true: the hook has operated successfully
  * "some string": an error occurred; processing should
    	  stop and the error should be shown to the user
  * false: the hook has successfully done the work
    	   necessary and the calling function should skip
	
The last result would be for cases where the hook function replaces
the main functionality. For example, if you wanted to authenticate
users to a custom system (LDAP, another PHP program, whatever), you
could do:

    $wgHooks['UserLogin'][] = array(ldapLogin, $ldapServer);
			        
    function ldapLogin($username, $password) {
        # log user into LDAP
	return false;
    }

Returning false makes less sense for events where the action is
complete, and will probably be ignored.

==Using hooks==

A calling function or method uses the wfRunHooks() function to run
the hooks related to a particular event, like so:

    class Article { 
        # ...
	function protect() {
	    global $wgUser;
	    if (wfRunHooks('ArticleProtect', $this, $wgUser)) {
	        # protect the article
		wfRunHooks('ArticleProtectComplete', $this, $wgUser);
	    }
	}
						    
wfRunHooks() returns true if the calling function should continue
processing (the hooks ran OK, or there are no hooks to run), or false
if it shouldn't (an error occurred, or one of the hooks handled the
action already). Checking the return value matters more for "before"
hooks than for "complete" hooks.

==Events and parameters==

This is a list of known events and parameters; please add to it if
you're going to add events to the MediaWiki code.

'BlockIp': before an IP address or user is blocked
$block: the Block object about to be saved
$user: the user _doing_ the block (not the one being blocked)

'BlockIpComplete': after an IP address or user is blocked
$block: the Block object that was saved
$user: the user who did the block (not the one being blocked)

'UserLoginComplete': after a user has logged in
$user: the user object that was created on login
		    
'UserLogout': before a user logs out
$user: the user object that is about to be logged out
       
'UserLogoutComplete': after a user has logged out
$user: the user object _after_ logout (won't have name, ID, etc.)