MediaWikiDoc:Hooks.doc

Techniques: Software: MediaWiki: Developer Documents: Deferred.doc

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 would handle this as follows (note: not real code, here):

    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', array(&$article))) {
	
	    # code to actually show the article goes here
	    
	    wfRunHooks('ArticleShowComplete', array(&$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 little title-reversing if-blocks 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 load-up performance 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 normally 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 $wgUser;
	    if (wfRunHooks('ArticleProtect', array(&$this, &$wgUser))) {
	        # protect the article
		wfRunHooks('ArticleProtectComplete', array(&$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.

Note that hook parameters are passed in an array; this is a necessary inconvenience to make it possible to pass reference values (that can be changed) into the hook code. Also note that earlier versions of wfRunHooks took a variable number of arguments; the array() calling protocol came about after MediaWiki 1.4rc1.

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.

'ArticleDelete': before an article is deleted
$article: the article (object) being deleted

$user: the user (object) deleting the article

$reason: the reason (string) the article is being deleted

'ArticleDeleteComplete': after an article is deleted
$article: the article that was deleted

$user: the user that deleted the article

$reason: the reason the article was deleted

'ArticleProtect': before an article is protected
$article: the article being protected

$user: the user doing the protection

$protect: boolean whether this is a protect or an unprotect

$reason: Reason for protect

$moveonly: boolean whether this is for move only or not

'ArticleProtectComplete': after an article is protected
$article: the article that was protected

$user: the user who did the protection

$protect: boolean whether it was a protect or an unprotect

$reason: Reason for protect

$moveonly: boolean whether it was for move only or not

'ArticleSave': before an article is saved
$article: the article (object) being saved

$user: the user (object) saving the article

$text: the new article text

$summary: the article summary (comment)

$isminor: minor flag

$iswatch: watch flag

$section: section #

'ArticleSaveComplete': after an article is saved
$article: the article (object) saved

$user: the user (object) who saved the article

$text: the new article text

$summary: the article summary (comment)

$isminor: minor flag

$iswatch: watch flag

$section: section #

'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)

'EmailUser': before sending email from one user to another
$to: address of receiving user

$from: address of sending user

$subject: subject of the mail

$text: text of the mail

'EmailUserComplete': after sending email from one user to another
$to: address of receiving user

$from: address of sending user

$subject: subject of the mail

$text: text of the mail

'TitleMoveComplete': after moving an article (title)
$old: old title

$nt: new title

$user: user who did the move

$oldid: old article database ID

$newid: new article database ID

'UnknownAction': An unknown "action" has occured (useful for defining your own actions)
$action: action name

$article: article "acted on"

'UnwatchArticle': before a watch is removed from an article
$user: user watching

$article: article object to be removed

'UnwatchArticle': after a watch is removed from an article
$user: user that was watching

$article: article object removed

'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.)

'WatchArticle': before a watch is added to an article
$user: user that will watch

$article: article object to be watched

'WatchArticleComplete': after a watch is added to an article
$user: user that watched

$article: article object watched

Edit Log

2005-06-13 Transcribed from docs for MediaWiki version 1.4.5

MediaWikiDoc:Hooks.doc

Contents

Contents

Glossary

Rationale

Writing hooks

Using hooks

Events and parameters

Edit Log

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