Magento Fundamentals for Developers

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by Allan MacGregor | June 2013 | e-Commerce Open Source PHP

In this article by Allan Macgregor, author of Magento PHP Developer’s Guide, we will cover the fundamental concepts for working with Magento. We will learn how Magento is structured, and we will go over the source of Magento's flexibility, that is, its modular architecture.

Magento is a flexible and powerful system. Unfortunately, this adds some level of complexity too. Currently, a clean installation of Magento has around 30,000 files and over 1.2 million lines of code.

With all that power and complexity, Magento can be daunting for new developers; but don't worry. This article is designed to teach new developers all the fundamental concepts and tools they will need to use and extend Magento.

(For more resources related to this topic, see here.)

Zend Framework – the base of Magento

As you probably know, Magento is the most powerful e-commerce platform in the market; what you might not know about Magento is that it is also an object-oriented (OO) PHP framework developed on top of Zend Framework.

Zend's official site describes the framework as:

Zend Framework 2 is an open source framework for developing web applications and services using PHP 5.3+. Zend Framework 2 uses 100% object-oriented code and utilises most of the new features of PHP 5.3, namely namespaces, late static binding, lambda functions and closures.

The component structure of Zend Framework 2 is unique; each component is designed with few dependencies on other components. ZF2 follows the SOLID object oriented design principle. This loosely coupled architecture allows developers to use whichever components they want. We call this a "use-at-will" design.

But what is Zend Framework exactly? Zend Framework is an OO framework developed on PHP that implements the Model-View-Controller (MVC) paradigm. When Varien, now Magento Inc., started developing Magento it decided to do it on top of Zend because of the following components:

  • Zend_Cache

  • Zend_Acl

  • Zend_DB

  • Zend_Pdf

  • Zend_Currency

  • Zend_Date

  • Zend_Soap

  • Zend_Http

In total, Magento uses around 15 different Zend components. The Varien library directly extends several of the Zend components mentioned previously, for example Varien_Cache_Core extends from Zend_Cache_Core.

Using Zend Framework, Magento was built with the following principles in mind:

  • Maintainability: It occurs using code pools to keep the core code separate from local customizations and third-party modules

  • Upgradability: Magento modularity allows extensions and third-party modules to be updated independently from the rest of the system

  • Flexibility: Allows seamless customization and simplifies the development of new features

Although having used Zend Framework or even understanding it are not the requirements for developing with Magento, having at least a basic understanding of the Zend components, usage, and interaction can be invaluable information when we start digging deeper into the core of Magento.

You can learn more about Zend Framework at http://framework.zend.com/

Magento folder structure

Magento folder structure is slightly different from other MVC applications; let's take a look at the directory tree, and each directory and its functions:

  • app: This folder is the core of Magento and is subdivided into three importing directories:

    • code: This contains all our application code divided into three code pools such as core, community, and local

    • design: This contains all the templates and layouts for our application

    • locale: This contains all the translation and e-mail template files used for the store

  • js: This contains all the JavaScript libraries that are used in Magento

  • media: This contains all the images and media files for our products and CMS pages as well as the product image cache

  • lib: This contains all the third-party libraries used in Magento such as Zend and PEAR, as well as the custom libraries developed by Magento, which reside under the Varien and Mage directories

  • skin: This contains all CSS code, images, and JavaScript files used by the corresponding theme

  • var: This contains our temporary data such as cache files, index lock files, sessions, import/export files, and in the case of the Enterprise edition the full page cache folders

Magento is a modular system. This means that the application, including the core, is divided into smaller modules. For this reason, the folder structure plays a key role in the organization of each module core; a typical Magento module folder structure would look something like the following figure:

Let's review each folder in more detail:

  • Block: This folder contains blocks in Magento that form an additional layer of logic between the controllers and views

  • controllers: controllers folders are formed by actions that process web server requests

  • Controller: The classes in this folder are meant to be abstract classes and extended by the controller class under the the controllers folder

  • etc: Here we can find the module-specific configuration in the form of XML files such as config.xml and system.xml

  • Helper: This folder contains auxiliary classes that encapsulate a common-module functionality and make it available to a class of the same module and to other modules' classes as well

  • Model: This folder contains models that support the controllers in the module for interacting with data

  • sql: This folder contains the installation and upgrade files for each specific module

As we will see later on in this article, Magento makes heavy use of factory names and factory methods. This is why the folder structure is so important.

Modular architecture

Rather than being a large application, Magento is built by smaller modules, each adding specific functionality to Magento.

One of the advantages of this approach is the ability to enable and disable specific module functionality with ease, as well as add new functionality by adding new modules.

Autoloader

Magento is a huge framework, composed of close to 30,000 files. Requiring every single file when the application starts would make it incredibly slow and heavy. For this reason, Magento makes use of an autoloader class to find the required files each time a factory method is called.

So, what exactly is an autoloader? PHP5 includes a function called __autoload().When instantiating a class, the __autoload() function is automatically called; inside this function, custom logic is defined to parse the class name and the required file.

Let's take a closer look at the Magento bootstrap code located at app/Mage.php:


Mage::register('original_include_path', get_include_path());
if (defined('COMPILER_INCLUDE_PATH')) {
$appPath = COMPILER_INCLUDE_PATH;
set_include_path($appPath . PS .
Mage::registry('original_include_path'));
include_once "Mage_Core_functions.php";
include_once "Varien_Autoload.php";
} else {
/**
* Set include path
*/
$paths[] = BP . DS . 'app' . DS . 'code' . DS . 'local';
$paths[] = BP . DS . 'app' . DS . 'code' . DS . 'community';
$paths[] = BP . DS . 'app' . DS . 'code' . DS . 'core';
$paths[] = BP . DS . 'lib';
$appPath = implode(PS, $paths);
set_include_path($appPath . PS .
Mage::registry('original_include_path'));
include_once "Mage/Core/functions.php";
include_once "Varien/Autoload.php";
}
Varien_Autoload::register();

The bootstrap file takes care of defining the include paths and initializing the Varien autoloader, which will in turn define its own autoload function as the default function to call. Let's take a look under the hood and see what the Varien autoload function is doing:

/**
* Load class source code
*
* @param string $class
*/
public function autoload($class)
{
if ($this->_collectClasses) {
$this->_arrLoadedClasses[self::$_scope][] = $class;
}
if ($this->_isIncludePathDefined) {
$classFile = COMPILER_INCLUDE_PATH .
DIRECTORY_SEPARATOR . $class;
} else {
$classFile = str_replace(' ', DIRECTORY_SEPARATOR,
ucwords(str_replace('_', ' ', $class)));
}
$classFile.= '.php';
//echo $classFile;die();
return include $classFile;
}

The autoload class takes a single parameter called $class, which is an alias provided by the factory method. This alias is processed to generate a matching class name that is then included.

As we mentioned before, Magento's directory structure is important due to the fact that Magento derives its class names from the directory structure. This convention is the core principle behind factory methods that we will be reviewing later on in this article.

Code pools

As we mentioned before, inside our app/code folder we have our application code divided into three different directories known as code pools. They are as follows:

  • core: This is where the Magento core modules that provide the base functionality reside. The golden rule among Magento developers is that you should never, by any circumstance, modify any files under the core code pool.

  • community: This is the location where third-party modules are placed. They are either provided by third parties or installed through Magento Connect.

  • local: This is where all the modules and code developed specifically for this instance of Magento reside.

The code pools identify where the module came from and on which order they should be loaded. If we take another look at the Mage.php bootstrap file, we can see the order on which code pools are loaded:

$paths[] = BP . DS . 'app' . DS . 'code' . DS . 'local';
$paths[] = BP . DS . 'app' . DS . 'code' . DS . 'community';
$paths[] = BP . DS . 'app' . DS . 'code' . DS . 'core';
$paths[] = BP . DS . 'lib';

This means that for each class request, Magento will look in local, then community, then core, and finally inside the lib folder.

This also leads to an interesting behavior that can easily be used for overriding core and community classes, by just copying the directory structure and matching the class name.

Needless to say that this is a terrible practice, but it is still useful to know about just in case you someday have to take care of a project that exploits this behavior.

Routing and request flow

Before going into more detail about the different components that form a part of Magento, it is important that we understand how these components interact together and how Magento processes requests coming from the web server.

As with any other PHP application, we have a single file as an entry point for every request; in the case of Magento this file is index.php, which is in charge of loading the Mage.php bootstrap class and starting the request cycle. It then goes through the following steps:

  1. The web server receives the request and Magento is instantiated by calling the bootstrap file, Mage.php.

  2. The frontend controller is instantiated and initialized; during this controller initialization Magento searches for the web routes and instantiates them.

  3. Magento then iterates through each of the routers and calls the match. The match method is responsible for processing the URL and generating the corresponding controller and action.

  4. Magento then instantiates the matching controller and takes the corresponding action.

Routers are especially important in this process. The Router objects are used by the frontend controller to match a requested URL (route) to a module controller and action. By default, Magento comes with the following routers:

  • Mage_Core_Controller_Varien_Router_Admin

  • Mage_Core_Controller_Varien_Router_Standard

  • Mage_Core_Controller_Varien_Router_Default

The action controller will then load and render the layout, which in turn will load the corresponding blocks, models, and templates.

Let's analyze how Magento will handle a request to a category page; we will use http:// localhost/catalog/category/view/id/10 as an example. Magento URIs are comprised of three parts – /FrontName/ControllerName/ActionName.

This means that for our example URL, the breakdown would be as follows:

  • FrontName: catalog

  • ControllerName: category

  • ActionName: view

If I take a look at the Magento router class, I can see the Mage_Core_Controller_ Varien_Router_Standard match function:

public function match(Zend_Controller_Request_Http $request)
{

$path = trim($request->getPathInfo(), '/');
if ($path) {
$p = explode('/', $path);
} else {
$p = explode('/', $this->_getDefaultPath());
}

}

From the preceding code, we can see that the first thing the router tries to do is to parse the URI into an array. Based on our example URL, the corresponding array would be something like the following code snippet:

$p = Array
(
[0] => catalog
[1] => category
[2] => view
)

The next part of the function will first try to check if the request has the module name specified; if not, then it tries to determine the module name based on the first element of our array. And if a module name can't be provided, then the function will return false. Let's take a look at that part of the code:

// get module name
if ($request->getModuleName()) {
$module = $request->getModuleName();
} else {
if (!empty($p[0])) {
$module = $p[0];
} else {
$module = $this->getFront()->getDefault('module');
$request->setAlias(Mage_Core_Model_Url_Rewrite::
REWRITE_REQUEST_PATH_ALIAS, '');
}
}
if (!$module) {
if (Mage::app()->getStore()->isAdmin()) {
$module = 'admin';
} else {
return false;
}
}

Next, the match function will iterate through each of the available modules and try to match the controller and action, using the following code:


foreach ($modules as $realModule) {
$request->setRouteName
($this->getRouteByFrontName($module));
// get controller name
if ($request->getControllerName()) {
$controller = $request->getControllerName();
} else {
if (!empty($p[1])) {
$controller = $p[1];
} else {
$controller =
$front->getDefault('controller');
$request->setAlias(
Mage_Core_Model_Url_Rewrite::REWRITE_REQUEST_
PATH_ALIAS,
ltrim($request->
getOriginalPathInfo(), '/')
);
}
}
// get action name
if (empty($action)) {
if ($request->getActionName()) {
$action = $request->getActionName();
} else {
$action = !empty($p[2]) ? $p[2] :
$front->getDefault('action');
}
}
//checking if this place should be secure
$this->_checkShouldBeSecure($request,
'/'.$module.'/'.$controller.'/'.$action);
$controllerClassName = $this->_validate
ControllerClassName($realModule, $controller);
if (!$controllerClassName) {
continue;
}
// instantiate controller class
$controllerInstance = Mage::getControllerInstance
($controllerClassName,
$request, $front->getResponse());
if (!$controllerInstance->hasAction($action)) {
continue;
}
$found = true;
break;
}
...

Now that looks like an awful lot of code, so let's break it down even further. The first part of the loop will check if the request has a controller name; if it is not set, it will check our parameter array's ($p) second value and try to determine the controller name, and then it will try to do the same for the action name.

If we got this far in the loop, we should have a module name, a controller name, and an action name, which Magento will now use to try and get a matching controller class name by calling the following function:

$controllerClassName = $this->_validateControllerClassName($realModu
le, $controller);

This function will not only generate a matching class name but it will also validate its existence; in our example case this function should return Mage_Catalog_ CategoryController.

Since we now have a valid class name, we can proceed to instantiate our controller object; if you were paying attention up to this point, you have probably noticed that we haven't done anything with our action yet, and that's precisely the next step in our loop.

Our new instantiated controller comes with a very handy function called hasAction(); in essence, what this function does is to call a PHP function called is_callable(), which will check if our current controller has a public function matching the action name; in our case this will be viewAction().

The reason behind this elaborate matching process and the use of a foreach loop is that it is possible for several modules to use the same FrontName.

Now, http://localhost/catalog/category/view/id/10 is not a very user-friendly URL; fortunately, Magento has its own URL rewrite system that allows us to use http: //localhost/books.html.

Let's dig a little deeper into the URL rewrite system and see how Magento gets the controller and action names from our URL alias. Inside our Varien/Front.php controller dispatch function, Magento will call:

Mage::getModel('core/url_rewrite')->rewrite();

Before actually looking into the inner workings of the rewrite function, let's take a look at the structure of the core/url_rewrite model:

Array (
["url_rewrite_id"] => "10"
["store_id"] => "1"
["category_id"] => "10"
["product_id"] => NULL
["id_path"] => "category/10"
["request_path"] => "books.html"
["target_path"] => "catalog/category/view/id/10"
["is_system"] => "1"
["options"] => NULL
["description"] => NULL
)

As we can see, the rewrite module is comprised of several properties, but only two of them are of particular interest to use – request_path and target_path. Simply put, the job of the rewrite module is to modify the request object path information with the matching values of target_path.

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Magento version of MVC

If you are familiar with the traditional MVC implementations such as CakePHP or Symfony, you may know that the most common implementation is called a convention-based MVC. With a convention-based MVC, to add a new model or let's say a controller, you only need to create the file/class (following the framework conventions) and the system will pick it up automatically.

Magento, on the other hand, uses a configuration-based MVC pattern, meaning that creating our file/class is not enough; we explicitly have to tell Magento that we added a new class.

Each Magento module has a config.xml file, which is located under the module etc/ directory and contains all the relevant module configuration. For example, if we want to add a new module that includes a new model, we would need to define a node in the configuration file that tells Magento where to find our model, such as:

<global>

<models>
<group_classname>
<class>Namespace_Modulename_Model</class>
<group_classname>
</models>
...
</global>

Although this might look like additional work, it also gives us a huge amount of flexibility and power. For example, we can rewrite another class by using the rewrite node:

<global>

<models>
<group_classname>
<rewrite>
<modulename>Namespace_Modulename_Model</modulename>
</rewrite>
<group_classname>
</models>
...
</global>

Magento will then load all the config.xml files and merge them at runtime, creating a single configuration tree.

Additionally, modules can also have a system.xml file, which is used to specify configuration options in the Magento backend, which in turn can be used by end users to configure the module functionality. A snippet of a system.xml file would look like the following code:

<config>
<sections>
<section_name translate="label">
<label>Section Description</label>
<tab>general</tab>
<frontend_type>text</frontend_type>
<sort_order>1000</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
<groups>
<group_name translate="label">
<label>Demo Of Config Fields</label>
<frontend_type>text</frontend_type>
<sort_order>1</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
<fields>
<field_name translate="label comment">
<label>Enabled</label>
<comment>
<![CDATA[Comments can contain
<strong>HTML</strong>]]>
</comment>
<frontend_type>select</frontend_type>
<source_model>adminhtml/system_config_source_yesno</
source_model>
<sort_order>10</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</field_name>
</fields>
</group_name>
</groups>
</section_name>
</sections>
</config>

Let's break down each node function:

  • section_name: This is just an arbitrary name that we use to identify our configuration section; inside this node we will specify all the fields and groups for the configuration section.

  • group: Groups, as the name implies, are used to group configuration options and display them inside an accordion section.

  • label: This defines the title or label to be used on the field/section/group.

  • tab: This defines the tab on which the section should be displayed.

  • frontend_type: This node allows us to specify which render to use for our custom option field. Some of the available options are:

    • button

    • checkboxes

    • checkbox

    • date

    • file

    • hidden

    • image

    • label

    • link

    • multiline

    • multiselect

    • password

    • radio

    • radios

    • select

    • submit

    • textarea

    • text

    • time

  • sort_order: It specifies the position of the field, group, or section.

  • source_model: Certain type of fields such as a select field can take options from a source model. Magento already provides several useful classes under Mage/Adminhtml/Model/System/Config/Source. Some of the classes we can find are:

    • YesNo

    • Country

    • Currency

    • AllRegions

    • Category

    • Language

By just using XML, we can build complex configuration options for our modules right on the Magento backend, without having to worry about setting up templates for populating fields or validating data.

Magento is also kind enough to provide a comprehensive amount of form field validation models, which we can use with the <validate> tag. Among the following field validators we have:

  • validate-email

  • validate-length

  • validate-url

  • validate-select

  • validate-password

As with any other part of Magento we can extend the source_model, frontend_ type, and validator functions and even create new ones. We will be tackling this task in a later article where we will create a new type of each. But for now, we will explore the concepts of models, views, file layouts, and controllers.

Models

Magento makes use of the ORM approach; although we can still use Zend_Db to access the database directly, we will be using models to access our data most of the time. For this type of task, Magento provides the following two types of models:

  • Simple models: This model implementations are a simple mapping of one object to one table, meaning our object attributes match each field and our table structure

  • Entity Attribute Value (EAV) models: This type of models are used to describe entities with a dynamic number of attributes

Magento splits the model layer up into two parts: a model handling the business logic and a resource handling the database interaction. This design decision allows Magento to eventually support multiple database platforms without having to change any of the logic inside the models.

Magento ORM uses one of PHP's magic class methods to provide dynamic access to object properties. In the next article we will look into models, the Magento ORM, and the data collections in more detail.

Magento models don't necessarily have to be related to any type of table in the database or an EAV entity. Observers, who we will be reviewing later, are perfect examples of this type of Magento models.

Views

The view layer is one of the areas where Magento truly sets itself apart from other MVC applications. Unlike traditional MVC systems, Magento's view layer is divided into the following three different components:

  • Layouts: Layouts are XML files that define the block structure and properties such as name and the template file we can use. Each Magento module has its own set of layout files.

  • Blocks: Blocks are used in Magento to reduce the burden on the controller by moving most of the logic into blocks.

  • Templates: Templates are PHTML files that contain the required HTML code and PHP tags.

Layouts give the Magento frontend an amazing amount of flexibility. Each module has its own layout XML files, which tell Magento what to include and render on each page request. Through the use of the layouts, we can move, add, or remove blocks from our store without worrying about changing anything else other than our XML files.

Dissecting a layout file

Let's examine one of the core layout files of Magento, in this case catalog.xml:

<layout version="0.1.0">
<default>
<reference name="left">
<block type="core/template" name="left.permanent.callout"
template="callouts/left_col.phtml">
<action method="setImgSrc">
<src>images/media/col_left_callout.jpg</src></action>
<action method="setImgAlt" translate="alt"
module="catalog"><alt>
Our customer service is available 24/7.
Call us at (555) 555-0123.</alt></action>
<action method="setLinkUrl">
<url>checkout/cart</url></action>
</block>
</reference>
<reference name="right">
<block type="catalog/product_compare_sidebar"
before="cart_sidebar" name="catalog.compare.sidebar"
template="catalog/product/compare/sidebar.phtml"/>
<block type="core/template" name="right.permanent.callout"
template="callouts/right_col.phtml">
<action method="setImgSrc">
<src>images/media/col_right_callout.jpg</src></action>
<action method="setImgAlt" translate="alt"
module="catalog"><alt>
Visit our site and save A LOT!</alt></action>
</block>
</reference>
<reference name="footer_links">
<action method="addLink" translate="label title"
module="catalog" ifconfig="catalog/seo/site_map">
<label>Site Map</label><url
helper="catalog/map/getCategoryUrl" />
<title>Site Map</title></action>
</reference>
<block type="catalog/product_price_template"
name="catalog_product_price_template" />
</default>

Layout blocks are comprised of three main XML nodes, as follows:

  • handle: Each page request will have several unique handles; the layout uses these handles to tell Magento which blocks to load and render on a per page basis. The most commonly used handles are default and [frontname]_[controller]_[action].

    The default handle is especially useful for setting global blocks, for example adding a CSS or JavaScript to all pages on the header block.

  • reference: A <reference> node is used to make references to a block. It is useful for specifying nested blocks or modifying an already existing block. In our example we can see a new children block being specified inside <reference name="left">.

  • block: The <block> node is used to load our actual blocks. Each block node can have the following properties:

    • type: This is the identifier for the actual block class. For example, catalog/product_list makes reference to the Mage_Catalog_ Block_Product_List.

    • name: This is the name used by other blocks to make reference to this block.

    • before/after: These properties can be used to position the blocks relative to other blocks' position. Both these properties can use a hyphen as a value to specify if the module should appear at the very top or the very bottom.

    • template: This property determines the .phtml template file, which will be used for rendering the block.

    • action: Each block type has specific actions that affect the frontend functionality. For instance, the page/html_head block, which has actions for adding CSS and JavaScript (addJs and addCss).

    • as: This is used to specify the unique identifier that we will be using for calling the block from the template, for example calling a child block by using getChildHtml('block_name').

Blocks are a new concept that Magento implements in order to reduce the controller load. They are basically data resources that communicate directly with the models, which manipulate the data, if needed, and then pass it to the views.

Finally, we have our PHTML files; the templates contain the html and php tags and are in charge of formatting and displaying the data from our models. Let's take a look at a snippet from the product view template:

<div class="product-view">
...
<div class="product-name">
<h1><?php echo $_helper->productAttribute
($_product, $_product->getName(), 'name') ?></h1>
</div>
...
<?php echo $this->getReviewsSummaryHtml
($_product, false, true)?>
<?php echo $this->getChildHtml('alert_urls') ?>
<?php echo $this->getChildHtml('product_type_data') ?>
<?php echo $this->getTierPriceHtml() ?>
<?php echo $this->getChildHtml('extrahint') ?>
...
<?php if ($_product->getShortDescription()):?>
<div class="short-description">
<h2><?php echo $this->__('Quick Overview') ?></h2>
<div class="std"><?php echo $_helper->
productAttribute($_product, nl2br($_product->
getShortDescription()), 'short_description') ?></div>
</div>
<?php endif;?>
...
</div>

The following is the block diagram of MVC:

Controllers

In Magento, MVC controllers are designed to be thin controllers; thin controllers have little business logic and are mostly used for driving the application requests. A basic Magento controller action would just load and render the layout:

public function viewAction()
{
$this->loadLayout();
$this->renderLayout();
}

From here it is the job of the blocks to handle the display logic, get the data from our models, prepare the data, and send it to the views.

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Websites and store scopes

One of the core features of Magento is the ability to handle multiple websites and stores with a single Magento installation; internally, Magento refers to each of these instances as scopes.

Values for certain elements such as products, categories, attributes, and configurations are scope specific and can differ on different scopes; this gives Magento tremendous flexibility, for example, a product can be set up on two different websites with different prices but can still share the rest of the attribute configuration.

As developers, one of the areas where we will be using scopes the most is when working with configuration. The different configuration scopes available in Magento are:

  • Global: As the name implies, this applies across all scopes.

  • Website: These are defined by a domain name and are composed by one or more stores. Websites can be set up to share customer data or be completely isolated.

  • Store: Stores are used to manage products and categories, and to group store views. Stores also have a root category that allows us to have separated catalogs per store.

  • Store view: By using store views we can set up multiple languages on our store frontend.

Configuration options in Magento can store values on three scopes (global, website, and store view); by default, all the values are set on the global scope. By using system.xml on our modules, we can specify the scopes on which the configuration options can be set; let's revisit our previous system.xml:


<field_name translate="label comment">
<label>Enabled</label>
<comment>
<![CDATA[Comments can contain <strong>HTML</strong>]]>
</comment>
<frontend_type>select</frontend_type>
<source_model>adminhtml/system_config_source_yesno</source_model>
<sort_order>10</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</field_name>

Factory names and functions

Magento makes use of factory methods to instantiate Model, Helper, and Block classes. A factory method is a design pattern that allows us to instantiate an object without using the exact class name and using a class alias instead.

Magento implements several factory methods, as follows:

  • Mage::getModel()

  • Mage::getResourceModel()

  • Mage::helper()

  • Mage::getSingleton()

  • Mage::getResourceSingleton()

  • Mage::getResourceHelper()

Each of these methods takes a class alias that is used to determine the real class name of the object that we are trying to instantiate; for example, if we wanted to instantiate a product object, we can do so by calling the getModel() method:

$product = Mage::getModel('catalog/product');

Notice that we are passing a factory name composed of group_classname/model_name; Magento will resolve this to the actual class name of Mage_Catalog_Model_Product. Let's take a closer look at the inner workings of getModel():

public static function getModel($modelClass = '', $arguments =
array())
{
return self::getConfig()->getModelInstance
($modelClass, $arguments);
}
getModel calls the getModelInstance from the Mage_Core_Model_Config
class.
public function getModelInstance($modelClass='',
$constructArguments=array())
{
$className = $this->getModelClassName($modelClass);
if (class_exists($className)) {
Varien_Profiler::start('CORE::create_object_of::'.$className);
$obj = new $className($constructArguments);
Varien_Profiler::stop('CORE::create_object_of::'.$className);
return $obj;
} else {
return false;
}
}

getModelInstance()in return calls the getModelClassName() method, which takes our class alias as a parameter. Then it tries to validate the existence of the returned class, and if the class exists, it will create a new instance of that class and return it to our getModel() method:

public function getModelClassName($modelClass)
{
$modelClass = trim($modelClass);
if (strpos($modelClass, '/')===false) {
return $modelClass;
}
return $this->getGroupedClassName('model', $modelClass);
}

getModelClassName() calls the getGroupedClassName() method, which is actually in charge of returning the real class name of our model.

getGroupedClassName() takes two parameters – $groupType and $classId; $groupType refers to the type of object that we are trying to instantiate (currently only models, blocks, and helpers are supported) and $classId, which we are trying to instantiate.

public function getGroupedClassName($groupType, $classId,
$groupRootNode=null)
{
if (empty($groupRootNode)) {
$groupRootNode = 'global/'.$groupType.'s';
}
$classArr = explode('/', trim($classId));
$group = $classArr[0];
$class = !empty($classArr[1]) ? $classArr[1] : null;
if (isset($this->_classNameCache
[$groupRootNode][$group][$class])) {
return $this->_classNameCache
[$groupRootNode][$group][$class];
}
$config = $this->_xml->global->{$groupType.'s'}->{$group};
$className = null;
if (isset($config->rewrite->$class)) {
$className = (string)$config->rewrite->$class;
} else {
if ($config->deprecatedNode) {
$deprecatedNode = $config->deprecatedNode;
$configOld = $this->_xml->global->
{$groupType.'s'}->$deprecatedNode;
if (isset($configOld->rewrite->$class)) {
$className = (string) $configOld->rewrite->$class;
}
}
}
if (empty($className)) {
if (!empty($config)) {
$className = $config->getClassName();
}
if (empty($className)) {
$className = 'mage_'.$group.'_'.$groupType;
}
if (!empty($class)) {
$className .= '_'.$class;
}
$className = uc_words($className);
}
$this->_classNameCache
[$groupRootNode][$group][$class] = $className;
return $className;
}

As we can see, getGroupedClassName() is actually doing all the work; it grabs our class alias catalog/product and creates an array by exploding the string on the slash character.

Then, it loads an instance of VarienSimplexml_Element and passes the first value in our array (group_classname). It will also check if the class has been rewritten, and if it has, we will use the corresponding group name.

Magento also uses a custom version of the uc_words() function, which will capitalize the first letters and convert separators of the class alias if needed.

Finally, the function will return the real class name to the getModelInstance() function; in our example case it will return Mage_Catalog_Model_Product.

Events and observers

The event and observer pattern is probably one of Magento's more interesting features, since it allows developers to extend Magento in critical parts of the application flow.

In order to provide more flexibility and facilitate the interaction between the different modules, Magento implements an event/observer pattern; this pattern allows for modules to be loosely coupled.

There are two parts of this system – an event dispatch with the object and event information, and an observer listening to a particular event.

Event dispatch

Events are created or dispatched using the Mage::dispatchEvent() function. The core team has already created several events on critical parts of the core. For example, the model abstract class Mage_Core_Model_Abstract calls two protected functions every time a model is saved – _beforeSave() and _afterSave(); on each of these methods two events are fired:

protected function _beforeSave()
{
if (!$this->getId()) {
$this->isObjectNew(true);
}
Mage::dispatchEvent('model_save_before',
array('object'=>$this));
Mage::dispatchEvent($this->_eventPrefix.'_save_before',
$this->_getEventData());
return $this;
}
protected function _afterSave()
{
$this->cleanModelCache();
Mage::dispatchEvent('model_save_after',
array('object'=>$this));
Mage::dispatchEvent($this->_eventPrefix.'_save_after',
$this->_getEventData());
return $this;

Each function fires a generic mode_save_after event, and then a dynamic version based on the type of object being saved. This gives us a wide range of possibilities for manipulating objects through observers.

The Mage::dispatchEvent() method takes two parameters: the first is the event name and the second is an array of data that is received by the observer. We can pass values or objects in this array. This comes in handy if we want to manipulate the objects.

In order to understand the details of the event system, let's take a look at the dispatchEvent() method:

public static function dispatchEvent($name, array $data = array())
{
$result = self::app()->dispatchEvent($name, $data);
return $result;
}

This function is actually an alias to the dispatchEvent() function inside the app core class located in Mage_Core_Model_App:

public function dispatchEvent($eventName, $args)
{
foreach ($this->_events as $area=>$events) {
if (!isset($events[$eventName])) {
$eventConfig = $this->getConfig()->
getEventConfig($area, $eventName);
if (!$eventConfig) {
$this->_events[$area][$eventName] = false;
continue;
}
$observers = array();
foreach ($eventConfig->observers->
children() as $obsName=>$obsConfig) {
$observers[$obsName] = array(
'type' => (string)$obsConfig->type,
'model' => $obsConfig->class ?
(string)$obsConfig->
class : $obsConfig->getClassName(),
'method'=> (string)$obsConfig->method,
'args' => (array)$obsConfig->args,
);
}
$events[$eventName]['observers'] = $observers;
$this->_events
[$area][$eventName]['observers'] = $observers;
}
if (false===$events[$eventName]) {
continue;
} else {
$event = new Varien_Event($args);
$event->setName($eventName);
$observer = new Varien_Event_Observer();
}
foreach ($events[$eventName]
['observers'] as $obsName=>$obs) {
$observer->setData(array('event'=>$event));
Varien_Profiler::start('OBSERVER: '.$obsName);
switch ($obs['type']) {
case 'disabled':
break;
case 'object':
case 'model':
$method = $obs['method'];
$observer->addData($args);
$object = Mage::getModel($obs['model']);
$this->_callObserverMethod
($object, $method, $observer);
break;
default:
$method = $obs['method'];
$observer->addData($args);
$object = Mage::getSingleton($obs['model']);
$this->_callObserverMethod
($object, $method, $observer);
break;
}
Varien_Profiler::stop('OBSERVER: '.$obsName);
}
}
return $this;
}

The dispatchEvent() method is actually doing all the work on the event/observer model:

  1. It gets the Magento configuration object.

  2. It walks through the observer's node children, checking if the defined observer is listening to the current event.

  3. For each of the available observers, the dispatch event will try to instantiate the observer object.

  4. Lastly, Magento will try to call the corresponding observer function mapped to this particular event.

Observer bindings

Now, dispatching an event is the only part of the equation. We also need to tell Magento which observer is listening to each event. Not to our surprise, observers are specified through config.xml. As we saw before, the dispatchEvent() function queries the configuration object for available observers. Let's take a look at an example config.xml file:

<events>
<event_name>
<observers>
<observer_identifier>
<class>module_name/observer</class>
<method>function_name</method>
</observer_identifier>
</observers>
</event_name>
</events>

The event node can be specified in each of the configuration sections (admin, global, frontend, and so on) and we can specify multiple event_name children nodes; the event_name has to match the event name used in the dispatchEvent() function.

Inside each event_name node, we have a single observer node that can contain multiple observers, each with a unique identifier.

Observer nodes have two properties such as <class>, which points to our observer model class and <method>, which in turn points to the actual method inside the observer class. Let's analyze an example observer class definition:

class Namespace_Modulename_Model_Observer
{
public function methodName(Varien_Event_Observer $observer)
{
//some code
}
}

One interesting thing about observer models is that they don't extend any other Magento class.

Summary

In this article, we covered many important and fundamental topics about Magento such as its architecture, folder structure, routing system, MVC patterns, events and observers, and configuration scopes.

And while this might seem overwhelming at first sight, it is just the tip of the iceberg. There is a lot more to learn about each of these topics and Magento. The purpose of this article is to make developers aware of all the important components of the platform from the configuration object up to the way the event/object pattern is implemented.

Magento is a powerful and flexible system, and it is much more than an e-commerce platform. The core team has put a lot of effort in making Magento a powerful framework.

Resources for Article :


Further resources on this subject:


About the Author :


Allan MacGregor

Allan MacGregor is a Magento Certified Developer Plus with four years of Magento experience. He also has a certification in Linux System Administration by IBM.

He started working with Magento as a freelance looking for a better framework to build e-commerce solutions, and he is now the Magento Lead Developer at Demac Media (www.demacmedia.com). He's very passionate about software development in general. He is constantly working with new technologies and frameworks.

At Demac Media, he has participated in building core solutions for a wide range of clients; this has given him the experience and knowledge to solve many Magento challenges.

As part of an internal project at Demac Media, he worked on Triplecheck.io (www.triplecheck.io), a unique service for monitoring and auditing the code health of a Magento store. You can also follow him on Twitter at http://www.twitter.com/allanmacgregor.

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