Plugin Developer Overview

Version Note: While this document pertains to Joomla! 2.5 and 3.x, JEventDispatcher does not exist in Joomla 2.5. In that case, change JEventDispatcher to JDispatcher. Using JDispatcher is possible in Joomla 3.x but will generate a deprecated notice.

Joomla! 1.5 introduced the JPlugin class. In the effort to move Joomla! toward a more efficient object-oriented framework, a new plugin system has been developed which follows the Observer pattern. Plugins are observer classes that attach to a global event dispatcher object in the Joomla! core. What does this mean in English? It means that either the Joomla! core or a third party component or module can trigger an event which causes one or more plugins to execute some code.

Implementation

The implementation of the plugin system is that of an observer pattern. It has two parts, an observer class, JPlugin, and an observable class, JEventDispatcher.

/**
 * JPlugin Class
 *
 * @package     Joomla.Platform
 * @subpackage  Plugin
 * @since       11.1
 */
class JPlugin extends JEvent
{
	/**
	 * Constructor
	 *
	 * @param   object  &$subject  The object to observe
	 * @param   array   $config    An optional associative array of configuration settings.
	 *                             Recognized key values include 'name', 'group', 'params', 'language'
	 *                             (this list is not meant to be comprehensive).
	 *
	 * @since   11.1
	 */
	public function __construct(&$subject, $config = array())
	{
		// Get the parameters.
		if (isset($config['params']))
		{
			if ($config['params'] instanceof JRegistry)
			{
				$this->params = $config['params'];
			}
			else
			{
				$this->params = new JRegistry;
				$this->params->loadString($config['params']);
			}
		}

		// Get the plugin name.
		if (isset($config['name']))
		{
			$this->_name = $config['name'];
		}

		// Get the plugin type.
		if (isset($config['type']))
		{
			$this->_type = $config['type'];
		}

		// Load the language files if needed. Note whilst this method is in the
		// JPlugin class it has been left out of the docs for code clarity.
		if ($this->autoloadLanguage)
		{
			$this->loadLanguage();
		}

		parent::__construct($subject);
	}
}

There are two important things that makes this class work.

One is the constructor which actually gets executed by the parent class of this class JEvent. The following is what happens in the constructor:

// Register the observer ($this) so we can be notified
$subject->attach($this);

// Set the subject to observe
$this->_subject = &$subject;

This attaches the JPlugin to an observable object. In the case of plugins, they observe the JEventDispatcher object.

The second important thing to note is the update method in the JEvent class. The update method is passed an array from its trigger. The array contains two elements - the event and the arguments. Once the update method receives this array it extracts the event and removes it from the arguments. It then calls a method of name ‘event’ (passing the arguments array) and returns its response.

Third Party Usage

<?php
/**
 * @version $Id: $
 * @package 
 * @subpackage 
 * @copyright 
 * @license 
 */

jimport('joomla.plugin');


/**
 * Example Plugin
 *
 * @author 
 * @package 
 * @subpackage 
 * @since 
 */
class ExamplePlugin extends JPlugin
{
	/**
	 * This method handles the onIncrement fictional event.  It takes an integer input and 
	 * increments its value.
	 *
	 * @param  integer  $input An integer to increment
	 *
	 * @return integer  Incremented integer
	 *
	 * @since 3.x
	 * @access public
	 */
	function onIncrement($input)
	{	
		return $input++;
	}
}
?>

As you can see, it is quite simple to create a JPlugin. It is truly as simple as creating a class that extends JPlugin and writing a method for each event you want the plugin to handle.

Example

The administrator module mod_quickicon is a good example of implementing the Observer Pattern. The module Quickicons shows several buttons in administrator:

Thanks to the plugin system any programmer can add buttons to this module without modifying/hacking the module. How?

because mod_quickicons does this:

// Include buttons defined by published quickicon plugins
JPluginHelper::importPlugin('quickicon');
$app = JFactory::getApplication();
$arrays = (array) $app->triggerEvent('onGetIcons', array($context));

see: https://github.com/joomla/joomla-cms/blob/staging/administrator/modules/mod_quickicon/helper.php#L149-L153

That means that the plugins in this folders are called:

https://github.com/joomla/joomla-cms/tree/staging/plugins/quickicon

And if they have a method "onGetIcons" they can inject their own icons. See how Quickicon/extensionupdate do it:

https://github.com/joomla/joomla-cms/blob/staging/plugins/quickicon/extensionupdate/extensionupdate.php#L40

We can all agree that this is actually a lot of code for just adding a button, however think on the flexibility that this solution provides: now Extension developers can attach buttons to that menu without hacking Joomla.