Joomla! Documentation™

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.Framework
 * @subpackage Application
 * @since 1.5
 */
class JPlugin extends JObserver
{
	/**
	 * Constructor
	 * 
	 * For php4 compatability we must not use the __constructor as a constructor for plugins
	 * because func_get_args ( void ) returns a copy of all passed arguments NOT references.
	 * This causes problems with cross-referencing necessary for the observer design pattern.
	 * 
	 * @param object $subject The object to observe
	 * @since 1.5
	 */
	function JPlugin(& $subject)
	{
		parent::__construct($subject);
	}

	/**
	 * Method to map events to handler methods
	 * 
	 * @access public
	 * @param array Arguments
	 * @return mixed Routine return value
	 * @since 1.1
	 */
	function update( &$args )
	{
		/*
		 * First lets get the event from the argument array.  Next we will unset the 
		 * event argument as it has no bearing on the method to handle the event.
		 */
		$event = $args['event'];
		unset($args['event']);
		
		/*
		 * If the method to handle an event exists, call it and return its return
		 * value.  If it does not exist, return a boolean true.
		 */ 
		if (method_exists($this, $event)) {
			return call_user_func_array(array($this, $event), $args);
		} else {
			return true;
		}
	}
}

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 JObserver. 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. 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
{
	/**
	 * Constructor
	 * 
	 * @param object $subject The object to observe
	 * @since 1.1
	 */
	function ExamplePlugin( &$subject ) {
		parent::__construct( $subject );
	}

	/**
	 * This method handles the onIncrement event.  It takes an integer input and 
	 * increments its value.
	 * 
	 * @access public
	 * @param int $input An integer to increment
	 * @return int Incremented integer
	 * @since 1.1
	 */
	function onIncrement($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.