Extending Joomla! with Plugins

Apr 26, 2012

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Joomla! Programming

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How Do Plugins Work?

Plugins work in three steps, as follows:

One or more plugin files are included into the current script, usually with the JPluginHelper::importPlugin() method. Because plugins are normally class declarations, no code is executed at this point.
An event is triggered, usually with the $dispatcher->trigger() method (where $dispatcher is a JDispatcher object). Each event has a name, such as onBeforeInitialise or onContentBeforeSave.
The event processing code looks for any enabled plugins that are currently loaded that have a method that matches the event name. If any matching methods are found, they are executed.

Loading and Executing PHP Scripts that Declare Classes

Most plugins in Joomla contain only a class declaration. This means that they declare a class and define the methods for the class, but they don’t include any lines of code outside the class declaration.

When this type of file is loaded (for example, with the PHP command require_once), a new class is created in working memory, with all its methods. However, none of the methods in the class actually get executed. A method will only get executed when a line of code calls that method.

This is why the JPluginHelper::importPlugin() method gets the plugin ready to execute, but it normally doesn’t actually execute any code. The plugin methods are only executed when the event is triggered, for example with the $dispatcher-> trigger() method.

Naming Conventions for Plugins

For plugins to be found by the JPluginHelper::importPlugin() method, we need to follow the correct naming conventions for the plugin file and class names.

Plugin folder and file names are created as follows:

plugins/<plugin type>/<plugin name>/<plugin name>.php

So, for example, the SEF file is plugins/system/sef/sef.php. Plugins have an XML file with the same name (for example, plugins/system/sef/sef.xml). We discuss the XML file later in the chapter when we talk about packaging a plugin extension.

The class name of the plugin is based on the naming convention

"plg" + <plugin type> + <plugin file name>

So, for example, the class name of the SEF plugin is plgSystemSEF.

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