Manual:Developing extensions



Each extension consists of three parts:


 * 1)  Setup
 * 2)  Execution
 * 3)  Localisation

A minimal extension will consist of the following structure:


 * MyExtension/extension.json: Stores the setup instructions. The file name must be extension.json.  (Prior to MediaWiki 1.25 the setup instructions were in a  file named after the extension.  Many extensions still have backwards-compatibility shims in this PHP file.)
 * MyExtension/includes/: Stores the PHP execution code for the extension.
 * MyExtension/resources/: Stores the client-side resources such as JavaScript, CSS and LESS for the extension.
 * MyExtension/i18n/*.json: Stores localisation information for the extension.

When you develop an extension, replace MyExtension above with the name of your extension. Use UpperCamelCase names for its directory and PHP file(s); this is the general file naming convention. (The is a good starting point for your extension.)

Setup
Your goal in writing the setup portion is to make installing the extension as easy as possible, so users only have to add this line to LocalSettings.php:

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:

To reach this simplicity, your setup file needs to accomplish a number of tasks (described in detail in the following sections):


 * register any media handler, parser function, special page, custom XML tag, and variable used by your extension.


 * 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


 * define any additional hooks needed by your extension


 * create or check any new database tables required by your extension.


 * set up localisation for your extension

Registering features with MediaWiki
MediaWiki lists all the extensions that have been installed on its  page. For example, you can see all the extensions installed on this wiki at Special:Version.

To do this, add the extension details to extension.json. The entry will look something like this:

Many of the fields are optional, but it's still good practice to fill them out. The  refers to the version of the schema the  file is written against. The available versions are 1 and 2. See here for the documentation on this feature. Unless you need to support an older version of MediaWiki, pick the latest version.

In addition to the above registration, you must also "hook" your feature into MediaWiki. The above only sets up the Special:Version page. The way you do this depends on the type of your extension. For details, please see the documentation for each type of 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 "MyExtension", you might want to name all your configuration variables to begin with. It is important that none 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.

Here is an example boiler plate that can be used to get started:

Note that after calling  the global variable   does not exist. If you set the variable, e.g. in  then the default value given in extension.json will not be used.

For more details on how to use global variable inside custom extensions, please refer to.

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 method.

To use MediaWiki's autoloading mechanism, you add entries to the  field. 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 MyExtension):

The filename is relative to the directory the extension.json file is in.

For more complex extensions, namespaces should be considered. See Manual:Extension.json/Schema#AutoloadNamespaces for details.

Defining additional hooks
See.

Adding database tables
Make sure the extension doesn't modify the core database tables. Instead, extension should create new tables with foreign keys to the relevant MW tables.

If your extension needs to add its own database tables, use the  hook. See the manual page for more information on usage.

Set up localisation
See:
 * Localisation (summary)

Add logs
On MediaWiki, all actions by users on wiki are tracked for transparency and collaboration. See for how to do it.

Handling dependencies
Assume that an extension requires the presence of another extension, for example because functionalities or database tables are to be used and error messages are to be avoided in case of non-existence.

For example the extension requires the presence of the extension  for certain functions. Some ideas to figure this out:

This should work at least from 1.23 up to 1.35.

Localisation
If you want your extension to be used on wikis that have a multi-lingual readership, you will need to add localisation support to your extension.

Store messages in &lt;language-key>.json
Store message definitions in a localisation JSON file, one for each language key your extension is translated in. The messages are saved with a message key and the message itself using standard JSON format. Each message id should be lowercase and may not contain spaces. Each key should begin with the lowercased extension name. An example you can find in the [ https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/i18n/de.json MobileFrontend extension]. Here is an example of a minimal JSON file (in this case en.json:

en.json

Store message documentation in qqq.json
The documentation for message keys can be stored in the JSON file for the pseudo language with code qqq. A documentation of the example above can be:

qqq.json:

Load the localisation file
In your extension.json, define the location of your messages files (e.g. in directory i18n/):

Use wfMessage in PHP
In your setup and implementation code, replace each literal use of the message with a call to . In classes that implement  (as well as some others such as subclasses of SpecialPage), you can use instead. Example:

Use mw.message in JavaScript
It's possible to use i18n functions in JavaScript too. Look at for details.

Extension types
Extensions can be categorized based on the programming techniques used to achieve their effect. Most complex 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:
 *  – Subclasses of the  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.
 *  – Skins change the look and feel of MediaWiki by altering the code that outputs pages by subclassing the MediaWiki class .
 *  – 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.
 *  – 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 HTML that embeds Google objects, data entry forms, RSS feeds, excerpts from selected wiki articles.
 *  – A technique for mapping a variety of wiki text string to a single id that is associated with a function. Both variables and 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 the array $magicWords.  The interpretation of the id is a somewhat complex process – see  for more information.
 *  – 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  or  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.
 *  – .  Similar to tag extensions, parser functions process arguments and returns a value.  Unlike tag extensions, the result of parser functions is wikitext.
 *  – you can add custom modules to MediaWiki's action API, that can be invoked by JavaScript, bots or third-party clients.
 *  – If you need to store data in formats other than wikitext, JSON, etc. then you can create a new.

Support other core versions
There are two widespread conventions for supporting older versions of MediaWiki core:


 * Master: the master branch of the extension is compatible with as many old versions of core as possible. This results in a maintenance burden (backwards-compatibility hacks need to be kept around for a long time, and changes to the extension need to be tested with several versions of MediaWiki), but sites running old MediaWiki versions benefit from functionality recently added to the extension.


 * Release branches: release branches of the extension are compatible with matching branches of core, e.g. sites using MediaWiki need to use the  branch of the extension. (For extensions hosted on gerrit, these branches are automatically created when new versions of MediaWiki are released.) This results in cleaner code and faster development but users on old core versions do not benefit from bugfixes and new features unless they are backported manually.

Extension maintainers should declare with the  parameter of the Extension template which convention they follow.

Publishing
To autocategorize and standardize the documentation of your existing extension, please see. To add your new extension to this Wiki:

Deploying and registering
If you intend to have your extension deployed on Wikimedia sites (including possibly Wikipedia), additional scrutiny is warranted in terms of performance and security. Consult.

If your extension adds namespaces, you may wish to register its default namespaces; likewise, if it adds database tables or fields, you may want to register those at.

Please be aware that review and deployment of new extensions on Wikimedia sites can be extremely slow, and in some cases has taken more than two years.

Help documentation
You should provide public domain help documentation for features provided by your extension. is a good example. You should give users a link to the documentation via the  function.

Providing support / collaboration
Extension developers should open an account on Wikimedia's { {ll|Phabricator}}, and request a new project for the extension. This provides a public venue where users can submit issues and suggestions, and you can collaborate with users and other developers to triage bugs and plan features of your extension.