Manual:Developing extensions/en

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Each extension consists of three parts:


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

A minimal extension will consist of three files, one for each part:


 * 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 MyExtension/MyExtension.php file named after the extension. Many extensions still have backwards-compatibility shims in this PHP file.)
 * MyExtension/MyExtension_body.php: Stores the execution code for the extension. The file name MyExtension_body.php is conventional but not required. If your extension is complex and involves multiple PHP files, you should follow the convention to put its implementation code in a subdirectory named MyExtension/includes (although the Example and BoilerPlate extensions do not follow this convention). For example, see the extension.
 * MyExtension/i18n/*.json: Stores localisation information for the extension.

Originally, extensions were single files, and you may still find some examples of this deprecated style.

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. Also check out the cookiecutter template for MediaWiki extensions on GitHub.)

The three parts of an extension, setup, execution, and, localisation as well as extension types and licensing and publishing your extension are described in the following sections of this page. While developing, you may want to disable caching by setting = CACHE_NONE and  = false, otherwise system messages and other changes may not show up.

Setup
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 file, like this:

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. 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 for each media handler, parser function, special page, custom XML tag, and variable used by your extension. The entry will look something like this:

See for full details on what these fields do. 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. As of writing (1.26alpha), the only supported version is 1.

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 "Very silly extension that does nothing", you might want to name all your configuration variables to begin  or. It doesn't really matter what you choose so long as 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:

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.

Defining additional hooks
See Manual:Hooks.

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

Set up localisation
See:
 * Localisation (summary) or
 * Localisation (detailed)

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

Execution
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 custom XML tags, parser functions and variables.
 * Reporting and administration: Extensions that add reporting and administrative capabilities usually do so by adding 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 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.

See also the Extensions FAQ, Developer hub

Localisation
(Note: While developing, you may want to disable both cache by setting $wgMainCacheType = CACHE_NONE and $wgCacheDirectory = false, otherwise your system message changes may not show up). 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 .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.  An example you can find e.g. in extension MobileFrontend. 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:

Define messages

 * Assign each message a unique, lowercase, no space message id; e.g., uploadwizard-desc
 * 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. Parameter placeholders are specified with $n, where n represents the index of the placeholder; e.g.

Define message documentation

 * Each message you define needs to have an associated message documentation entry Message_documentation; in qqq.json e.g.

Load the localisation file

 * In your setup routine, 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 IContextSource (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 Manual:Messages API 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:
 * Special pages – Subclasses of the 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.
 * Skins – Skins change the look and feel of MediaWiki by altering the code that outputs pages by subclassing the MediaWiki class SkinTemplate.
 * 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.
 * Tag-function associations – 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.
 * Magic words – 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 Manual:Magic words for more information.
 * 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   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.
 * Parser functions – .  Similar to tag extensions, parser functions process arguments and returns a value.  Unlike tag extensions, the result of parser functions is wikitext.
 * API modules – you can add custom modules to MediaWiki's "action" web API, that can be invoked by JavaScript, bots or third-party clients.

Support other core versions
You can visit the extension support portal to keep on top of changes in future versions of MediaWiki and also add support for older versions that are still popular.

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

Deploying and registering
Consult Writing an extension for deployment. 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 database table and field registration.

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

Providing support / collaboration
Extension developers should open an account on Wikimedia's 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.