Understand Registry and Loader Objects in OpenCart

Introduction

OpenCart has became a very useful eCommerce framework for small to medium level online stores. Although it provides comprehensive features in its stack, it maintains a simple framework, with a nice modular architecture that can be extended. In this article, we'll focus on certain elements of the bootstrapping process.

Although there are lots of components which are involved in the typical bootstrapping process, we'll focus on the "Registry" and "Loader" objects for the course of this tutorial. The code snippets explained in this article belong to OpenCart version 2.0.x. Although the "Registry" class code is the same in versions 1.5.x and 2.0.x, the "Loader" class code has changed a lot. So we'll focus on the 2.0.x version of OpenCart.

The Registry Object

As the name suggests, the "Registry" object is used to store elements, from simple variables to complex objects, when the "set" method is called. It stores all the elements using "key", so later on they can be accessed easily when the "get" method is called. 

Let's have a closer look at the class file itself. Open the file located at "system/engine/registry.php" in your favorite text editor!

<?php
final class Registry {
  private $data = array();

  public function get($key) {
    return (isset($this->data[$key]) ? $this->data[$key] : null);
  }

  public function set($key, $value) {
    $this->data[$key] = $value;
  }

  public function has($key) {
    return isset($this->data[$key]);
  }
}

As you can see, the class definition is fairly simple to understand. It stores everything in the "data" property of the object, which is declared as an array, and the scope is private. In the "get" method, it checks whether "value" is available for the desired "key", and it returns the value if it's available, and "null" otherwise. In the "set" method, it inserts the new element into the "data" array using the arguments passed to the method. Finally, it provides the "has" method to check if a certain "key" is already set into the "data" array.

Now, let's see how the OpenCart framework uses the registry object during the initial phase of the page execution. Open the "index.php" file in the document root of OpenCart. You'll see the $registry object is created very early in the script execution.

// Registry
$registry = new Registry();

After the creation of the $registry object, it stores several other objects using the "set" method. Let's see a couple of examples.

<?php
// Loader
$loader = new Loader($registry);
$registry-­>set('load', $loader);

// Config
$config = new Config();
$registry-­>set('config', $config);

// Database
$db = new DB(DB_DRIVER, DB_HOSTNAME, DB_USERNAME, DB_PASSWORD, DB_DATABASE);
$registry-­>set('db', $db);

// Request
$request = new Request();
$registry-­>set('request', $request);

// Session
$session = new Session();
$registry-­>set('session', $session);

I've listed here some example usages of the $registry object. As you may have noticed, frequently used objects are created and stored in the registry. The reason is that you don't have to instantiate common class objects multiple times, so you could simply call the "get" method of the $registry object to use the desired object. The approach is somewhat similar to the "Singleton Pattern", in which you're forced to keep a single instance of the class.

Now, the $registry object is populated with useful stuff, but how is it used? Let's see how the $db object stored in the $registry is used in the "Activity" model of the "Account" module. Open the file located at "catalog/model/account/activity.php". You can see that in the "addActivity" method, an insert query is fired.

$this->db->query("INSERT INTO `" . DB_PREFIX . "customer_activity`
SET `customer_id` = '" . (int)$customer_id . "', `key` = '" .
$this->db->escape($key) . "', `data` = '" . $this->db->escape(serialize($data)) . 
"', `ip` = '" . $this->db->escape($this->request->server['REMOTE_ADDR']) . 
"', `date_added`= NOW()");

You may wonder about the way it's called, as there is no "db" method or property defined in the "ModelAccountActivity" class. You can go to the parent model class "Model" to see if it's defined there or not. Yeah, you won't find a "db" method or property in that class either. But if you look closely at the "Model" class, you'll see that it implements the magic methods, specifically the "__get" method in this case.

public function __get($key) {
  return $this->registry->get($key);
}

For now, let's assume that the $registry object is stored in the protected "registry" property of the "Model" class. We'll see how it's stored when "Model" is instantiated in the "Loader" class. 

The __get method is called when you call any method which is not defined in the class. In this method, "db" is passed as an argument as we are trying to call $this-­>db in the "activity.php" file. And as discussed earlier, $registry has all the utility objects already stored during the bootstrapping process. So we just need to fetch the "db" object using the key by calling the "get" method of the "Registry" object!

In the same way, $this­->load works from the controller files as well. So overall, "Registry" is a really useful component of the OpenCart framework which stores commonly used variables and objects, which are used throughout the script execution.

The Loader Object

The "Loader" object is used to load the different components of OpenCart as required, like model, controller, language, view, library, etc. It's important to note here that when the "Loader" object is created, it is stored in the $registry object with "load" as an array key. So you can access the $loader object by using a $this­->load call as explained in the above section.

// Loader instantiation
$loader = new Loader($registry);
$registry-­>set('load', $loader);

Now, let's see how different components are loaded using the "Loader". Open "system/engine/loader.php" to see the definition of a "Loader" class. We'll start with the "controller" method to understand how it works.

// load controller
$this->load->controller('common/column_left');

It's a code snippet which loads the "common/column_left.php" controller and calls the "index" method as well. We make this call to get the XHTML output of the "Left Column" of the OpenCart page. The $this­->load part works similar to the $this­->db example which I explained earlier! Thus, it returns the $loader object stored in the $registry, and finally it'll call the "controller" method of the "Loader" class!

In the same way, the following snippets work to load different components.

// load Model
$this-­>load-­>model('catalog/category');

// load View
$this­->load-­>view('default/template/product/category.tpl', $data);

// load Library
$this­->load-­>library('user');

// load Helper
$this­->load-­>helper('json');

// load Language
$this­->load-­>language('product/category');

Looking at the method definitions in the "Loader" class, you'll see that it's not that complicated to understand exactly how it works. First, it prepares the "file path" for the corresponding component, and it's included using the "include_once" function.

Overall, "Registry" and "Loader" are two very important components in the OpenCart framework which make things a lot easier for module developers.

Conclusion

So today, we looked at the "Loader" and "Registry" components of the OpenCart framework. I hope you've learned something useful in this tutorial. And hopefully, I'll come up with something more on the same topic! Submit your queries and suggestions using the feed below!