Editing Manual:Developing extensions

==Overview==
Conceptually each extension consists of three parts:
* setup
* execution
* internationalization

==Internal organization of an extension==
The internal organization of extensions has changed over time.  Initially extensions were simply single files named after the extension and you may still be able to find some examples of this organization.  As MediaWiki has matured this organization has been deprecated.  Instead, each extension is placed in a directory containing one or more files corresponding to the three parts: setup, execution, and internationalization.

* '''''myextension''/''myextension''.php''' - stores setup instructions.  (Note: Some extensions name this file <code>''myextension''/''myextension''.setup.php</code>)

* '''''myextension''/''myextension''.body.php''' - stores the execution code for the extension for simple extensions.  For complex extensions requiring multiple php files, the implementation code may instead be placed in a subdirectory, <code>''myextension''/includes</code>.  For an example, please see [http://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/SemanticMediaWiki/ Semantic MediaWiki]

* '''''myextension''/''myextension''.i18n.php''' - stores internationalization information for the extension.

===Writing Setup Instructions===
Your goal in writing the setup portion is to consolidate set up so that users installing your extension need do nothing more than include the setup file in their [[Manual:LocalSettings.php|LocalSettings.php]] file, like this (replace XXX with ''myextension''):

<source lang="php">
require_once( "$IP/extensions/XXX/XXX.php" );
</source>

If you want to make your extension user configurable, you need to define and document some configuration parameters and your users setup should look something like this (replace XXX with ''myextension''):

<source lang="php">
require_once( "$IP/extensions/XXX/XXX.php" );
$wgXXXConfigThis = 1;
$wgXXXConfigThat = false;
</source>

To reach this simplicity, your setup file will need to accomplish a number of tasks:
* define and/or validate any configuration variables you have defined for your extension.
* prepare the classes used by your extension for autoloading
* determine what parts of your setup should be done immediately and what needs to be deferred until the MediaWiki core has been initialized and configured
* register any [[Manual:Special pages|special pages]], [[Manual:Tag extensions|custom XML tags]], [[Manual:Parser functions|parser functions]], and [[Manual:Variable|variables]] used by your extension.
* define any additional [[Manual:Hooks|hooks]] needed by your extension
* setup internationalization and localization for your extension

===Making your extension user configurable===
If you want your user to be able to configure your extension, you'll need to provide one or more configuration variables.  It is a good idea to give those variables a unique name.  They should also follow MediaWiki naming conventions (e.g. global variables should begin with $wg).  

For example, if your extension is named "Very silly extension that does nothing", you might want to name all your configuration variables to begin <code>$wgVsetdn</code> or <code>$wgVSETDN</code>. It doesn't really matter what you choose so long as <em>none</em> of the MediaWiki core begins its variables this way and you have done a reasonable job of checking to see that none of the published extensions begin their variables this way.  Users won't take kindly to having to choose between your extension and some other extensions because you chose overlapping variable names.

It is also a good idea to include extensive documentation of any configuration variables in your installation notes.

===Preparing classes for autoloading===
If you choose to use classes to implement your extension, MediaWiki provides a simplified mechanism for helping php find the source file where your class is located.  In most cases this should eliminate the need to write your own <code>__autoload($classname)</code> method. 

To use MediaWiki's autoloading mechanism, you add entries to the variable [[Manual:$wgAutoloadClasses|$wgAutoloadClasses]].  The key of each entry is the class name; the value is the file that stores the definition of the class.  For a simple one class extension, the class is usually given the same name as the extension, so your autoloading section might look like this (extension is named Foobar): 

<source lang="php">
$wgAutoloadClasses['Foobar'] = dirname(__FILE__) . '/Foobar.body.php';
</source>.

For complex extensions with multiple classes, your autoloading section might look like this:
<source lang="php">
$wgFoobarIncludes = dirname(__FILE__) . '/includes/';
$wgAutoloadClasses['SpecialFoobar'] 
  = $wgFoobarIncludes . 'SpecialFoobar.php'; #implements special page Foobar
$wgAutoloadClasses['FoobarTag']
  = $wgFoobarIncludes . 'FoobarTag.php';   #implements tag <foobar>
</source>

<span id="Credits"></span><!-- former title: prevent broken links -->

===Registering features with MediaWiki===
MediaWiki lists all the extensions that have been installed on its <code><nowiki>Special:Version</nowiki></code> page.  For example, you can see all the extensions installed on this wiki at [[Special:Version]]. It is good form to make sure that your extension is also listed on this page. To do this, you will need to add an entry to [[Manual:$wgExtensionCredits|$wgExtensionCredits]] for each [[Manual:Special pages|special page]], [[Manual:Tag extensions|custom XML tag]], [[Manual:Parser functions|parser function]], and [[Manual:Variable|variable]] used by your extension.  The entry will look something like this:

<source lang="php">
   $wgExtensionCredits['validextensionclass'][] = array(
       'name' => 'Example',
       'author' =>'John Doe', 
       'url' => 'http://www.mediawiki.org/wiki/User:JDoe', 
       'description' => 'This extension is an example and performs no discernable function'
       );
</source>

''validextensionclass'' must be one of <tt>specialpage, parserhook, variable, other</tt>, as specified on [[Manual:$wgExtensionCredits]].


In addition to the above registration, you must also "hook" your feature into MediaWiki. The above only sets up the Special:Version page.  For details, please see the documentation for each type of extension:

* [[Manual:Special pages]]
* [[Manual:Variable]]
* [[Manual:Parser functions]]
* [[Manual:Tag extensions]]

===Deferring setup===
<!-- TODO: List some setup actions that *must* be deferred -->
[[Manual:LocalSettings.php|LocalSettings.php]] runs early in the MediaWiki setup process and a lot of things are not fully configured at that point.  This can cause problems for certain setup activities.  To work around this problem, MediaWiki gives you a choice of when to run set up actions. You can either run them immediately by inserting the commands in your setup file -or- you can run them later, after MediaWiki has finished configuring its core software.

To defer setup actions, your setup file must contain two bits of code:
* the definition of a setup function
* the assignment of that function to the [[Manual:$wgExtensionFunctions|$wgExtensionFunctions]] array.

The PHP code should look something like this:
<source lang="php">
$wgExtensionFunctions[] = 'efFoobarSetup';

function efFoobarSetup() {
   #do stuff that needs to be done after setup
}
</source>

===Adding database tables===
If your extension needs to add its own database tables, use the [[Manual:Hooks/LoadExtensionSchemaUpdates|LoadExtensionSchemaUpdates]] hook. See the manual page for more information on usage.

==Writing the execution portion==
The technique for writing the implementation portion depends upon the part of MediaWiki system you wish to extend:

* '''Wiki markup:''' Extensions that extend wiki markup will typically contain code that defines and implements [[Manual:Tag extensions|custom XML tags]], [[Manual:Parser functions|parser functions]] and [[Manual:Variable|variables]].  You can click on any of the links in the preceding sentence to get full details on how to implement these features in your extension.
* '''Reporting and administration:''' Extensions that add reporting and administrative capabilities usually do so by adding [[Manual:Special pages|special pages]].  For more information see [[Manual:Special pages]].
* '''Article automation and integrity:''' Extensions that improve the integration between MediaWiki and its backing database or check articles for integrity features, will typically add functions to one of the many hooks that affect the process of creating, editing, renaming, and deleting articles.  For more information about these hooks and how to attach your code to them, please see [[Manual:Hooks]].
* '''Look and feel:'''  Extensions that provide a new look and feel to mediaWiki are bundled into [[Manual:Skins|skins]].  For more information about how to write your own skins, see [[Manual:Skin]] and [[Manual:Skinning]].
* '''Security:'''  Extensions that limit their use to certain users should integrate with MediaWiki's own permissions system.  To learn more about that system, please see [[Manual:Preventing access]]. Some extensions also let MediaWiki make use of external authentication mechanisms. For more information, please see [[AuthPlugin]].  In addition, if your extension tries to limit readership of certain articles, please check out the gotchas discussed in [[Security issues with authorization extensions]].

<!-- TODO: stuff about AjaxDispatcher, custom JS code, etc... -->

See also the '''[[Extensions FAQ]]''', [[Developer hub]]

==Internationalizing your extension==
If you want your extension to be used on wikis that have a multi-lingual readership, you will need to add internationalization support to your extension.  Fortunately this is relatively easy to do.

<ol>
<li>For any text string displayed to the user, define a message. MediaWiki supports parameterized messages and that feature should be used when a message is dependent on information generated at runtime.  Assign each message a <em>lowercase</em> message id.</li>

<li>In your setup and implementation code, replace each literal use of the message with a call to <code>wfMsg( $msgID, $param1, $param2, ... )</code>.

Example : <code>wfMsg( 'addition', '1', '2', '3' )</code></li>

<li>Store the message definition in your internationalization file (''myextension''.i18n.php) .  This is normally done by setting up an array that maps language and message id to each string.  Each message id should be lowercase and they may '''not''' contain spaces.  A minimal file might look like this:
<source lang="php">
<?php
$messages = array();
$messages['en'] = array(
    'sillysentence' => 'This sentence says nothing',
    'answertoeverything' => 'Forty-two',
    'addition' => '$1 plus $2 equals $3', //sentence with params
);
$messages['fr'] = array(
    'sillysentence' => 'Une phrase absurde',
    'answertoeverything' => 'quarante-deux',
    'addition' => '$1 et $2 font $3', //phrase avec paramètres
);

</source>
</li>
<li>In your setup routine, load the internalization file :
<source lang="php">
$wgExtensionMessagesFiles['myextension'] = dirname( __FILE__ ) . '/myextension.i18n.php';
</source>
</li>
</ol>

For more information, please see:
* [[Internationalisation]] - discusses the MediaWiki internationalization engine, in particular, there is a list of features that can be localized and some review of the MediaWiki source code classes involved in localization.
* [[Localization checks]] - discusses common problems with localized messages

==Publishing your extension==
To autocategorize and standardize the documentation of your existing extension, please see [[Template:Extension]].  To add your new extension to this Wiki:
{{Template:Extension/CreateExtensionInputBox}}

==Extension techniques==
Extensions can be categorized based on the programming techniques used to achieve their effect.  Most extensions will use more than one of these techniques:

* '''Subclassing:''' MediaWiki expects certain kinds of extensions to be implemented as subclasses of a MediaWiki provided base class:
**'''[[Manual:Special pages|Special pages]]''' - Subclasses of the [[SpecialPage.php|SpecialPage]] class are used to build pages whose content is dynamically generated using a combination of the current system state, user input parameters, and database queries.  Both reports and data entry forms can be generated.  They are used for both reporting and administration purposes.
**'''[[Manual:Skins|Skins]]''' - Skins change the look and feel of MediaWiki by altering the code that outputs pages by subclassing the MediaWiki class [[SkinTemplate]].
*'''[[Manual:Hooks|Hooks]]:''' A technique for injecting custom php code at key points within MediaWiki processing.  They are widely used by MediaWiki's parser, its localization engine, its extension management system, and its page maintenance system.
*'''[[Manual:Tag extensions|Tag-function associations]]''' - [[w:XML|XML]] style tags that are associated with a php function that outputs HTML code.  You do not need to limit yourself to formatting the text inside the tags.  You don't even need to display it.  Many tag extensions use the text as parameters that guide the generation of [[w:HTML|HTML]] that embeds google objects, data entry forms, RSS feeds, excerpts from selected wiki articles.
*'''[[Manual:Magic words|Magic words]]:''' A technique for mapping a variety of wiki text string to a single id that is associated with a function.  Both [[Manual:Variable|variables]] and [[Manual:Parser functions|parser functions]] use this technique.   All text mapped to that id will be replaced with the return value of the function.  The mapping between the text strings and the id is stored in an array passed to each function attached to the [[Manual:Hooks/LanguageGetMagic|LanguageGetMagic]] hook.  The interpretation of the id is a somewhat complex process - see [[Manual:Magic words]] for more information.
** '''[[Manual:Variable|Variables]]''' - Variables are something of a misnomer.  They are bits of wikitext that look like templates but have no parameters and have been assigned hard-coded values. Standard wiki markup such as [[Help:Variables|<code><nowiki>{{</nowiki>PAGENAME}}</code>]] or [[Help:Variables|<code><nowiki>{{SITENAME}}</nowiki></code>]] are examples of variables.  They get their name from the source of their value: a php variable or something that could be assigned to a variable, e.g. a string, a number, an expression, or a function return value.
** '''[[Manual:Parser functions|Parser functions]]''' - <nowiki>{{</nowiki>functionname: argument 1 | argument 2 | argument 3...}}.  Similar to tag extensions, parser functions process arguments and returns a value.  Unlike tag extensions, the result of parser functions is [[w:wikitext|wikitext]].
* '''[[Manual:Ajax|Ajax]]''' - you can use [[:en:Ajax (programming)|AJAX]] in your extension to let your JavaScript code interact with your server side extension code, without the need to reload the page.