originally posted at
http://www.jlleblanc.com/index.php?option=com_content&task=view&id=11&Itemid=52
Please note: although the described use of parameters and module
positions here are specific to Mambo 4.5, the concepts behind
self-contained templates, modules, and components still apply for newer
versions. -Joseph L. LeBlanc, 11/3/04
Quite
often throughout the message boards on forums.opensourcematters.org, new Joomla users
display confusion over modules and components. However, once this
differentiation is made, the architecture of Joomla becomes very clear. So, what
is the difference between a component and a module?
Here's the short answer: Modules can appear on
any page and are usually very simple. Components load as their own page and can
be quite complex.
This
should be a satisfying answer if you are simply curious or don't have a lot of
time to read about the wonders of the Joomla architecture. For those
interested in more, there is quite a bit to discover. Let's start with the way
a Joomla page actually loads…
When
somebody visits your site, the index.php file is called from the root
directory. This file loads many different classes and other pieces of code that
make Joomla work. After all of this code loads, Joomla looks for the directory of
the current template and attempts to load the index.php file located there. So,
both the root directory and the template directory have their own index.php
file, with the root index loading the template index for the layout.
The
index.php file in the template directory looks mostly like a typical HTML file.
Within this file, you should find the code
<?php include_once('mainbody.php'); ?>. This tells PHP to stop
writing lines of HTML and include the code from mainbody.php, which loads
exactly one component. This means that
every
page in Joomla loads a component. (OK, well, you could remove the include for
mainbody.php and no components would ever load, but this would severely limit
the functionality of your site.) By default, Joomla will load the component
corresponding to the first item in the menu. Typically this is the 'Home' item
which points to the 'frontpage' component. Frontpage displays the introductions
to news articles written by site authors. When you click to on a link for an
article, it tells Mambo to load the 'content' component, along with the
specific article to load from the database. Other links can tell Joomla to load
other components.
In
addition to including mainbody.php, the template will have many
mosLoadModules() function calls, ie… mosLoadModules('left');. This code
tells Joomla to load all of the modules in the 'left' position that are
attached to
the current component (default positions are left, right, top, bottom,
inset,
user1, and user2). When a module is configured in the backend, a
selection list
appears allowing the administrator to attach the module to certain
components
(or all of them, which is very typically the case). This allows the
administrator to set, say, the login form on the front page, but not on
subsequent pages. Templates are not required to load the modules for
all the
positions (or any at all!), but it is typically helpful to load at
least one
module so that there is a place for the menu, unless you wish to code
your own
navigation.
When
visitors load your site, they probably won't know or care about the difference
between modules and components. However, anyone using the administrator
interface to modify the site will want to learn how to manage these elements.
Modules can be installed from the install/uninstall option under the modules
menu. This will provide a list of the modules installed from files, with a form
for uploading more modules at the bottom.
When
the 'Manage Modules' option is clicked on the menu, another list is presented
with options for publishing/unpublishing, positioning, and ordering. This list
may be longer than the first list, as custom content modules can be created.
These modules are not fed from code in the modules folder, but are fed from
content in the database as articles are. They appear just as other modules
would.
All modules allow for the inclusion of parameters.
Parameters allow module developers to give site administrators a way of
changing the module's options without creating a complex interface. For
instance, if a module developer wanted to give administrators control over the
background color, a parameter could be defined in the module management.
Administrators would click on the title of this module, then type
backgroundcolor = green (or whichever color desired) on a single line in
the 'Parameters' box.
Designing
component administration requires much more effort, but results in easy to use
interfaces that allow administrators to manage complex functionality. When a
component's administrative interface is loaded, the file admin.[nameofthecomponent].php is executed from the
/administrator/components/com_[nameofthecomponent]
folder. This file then loads the appropriate interface for the current task or
action. More about creating administrative interfaces can be discovered through
the Daily Message Component Tutorial.
Once
you understand the difference between components and modules, the general
structure of Joomla as a whole becomes very clear. If you are comfortable with
your PHP programming skills, you should not hesitate in creating your own
components and modules. However, it is advisable to check mosforge.net first to
search for open-source solutions already under development. Chances are that
somebody has created the functionality you desire, or has released something
very similar.
