One of the neat features of the Magento application is code generation. Code generation, as implied by its name, generates nonexistent classes. These classes are generated in Magento's var/generation directory.

The directory structure within var/generation is somewhat similar to that of the core vendor/magento/module-* and app/code directories. To be more precise, it follows the module structure. The code is generated for something that is called Factory, Proxy, and Interceptor classes.

The Factory class creates an instance of a type. For example, a var/generation/Magento/Catalog/Model/ProductFactory.php file with a MagentoCatalogModelProductFactory class has been created because somewhere within the vendor/magento directory and its code, there is a call to the MagentoCatalogModelProductFactory class, which originally does not exist in Magento. During runtime, when {someClassName}Factory is called in the code, Magento creates a Factory class under the var/generation directory if it does not exist. The following code is an example of the (partial) ProductFactory class:

namespace MagentoCatalogModel;

/**
* Factory class for @see MagentoCatalogModelProduct
*/
class ProductFactory
{
    //...

    /**
    * Create class instance with specified parameters
    *
    * @param array $data
    * @return MagentoCatalogModelProduct
    */
    public function create(array $data = array())
    {
        return $this->_objectManager->create($this->_instanceName, $data);
    }
}

Note the create method that creates and returns the Product type instance. Also, note how the generated code is type safe providing @return annotation for integrated development environments (IDEs) to support the autocomplete functionality.

Factories are used to isolate an object manager from the business code. Factories can be dependent on the object manager, unlike business objects.

The Proxy class is a wrapper for some base class. Proxy classes provide better performance than the base classes because they can be instantiated without instantiating a base class. A base class is instantiated only when one of its methods is called. This is highly convenient for cases where the base class is used as a dependency, but it takes a lot of time to instantiate, and its methods are used only during some paths of execution.

Like Factory, the Proxy classes are also generated under the var/generation directory.

If we were to take a look at the var/generation/Magento/Catalog/Model/Session/Proxy.php file that contains the MagentoCatalogModelSessionProxy class, we would see that it actually extends MagentoCatalogModelSession. The wrapping Proxy class implements several magical methods along the way, such as __sleep, __wakeup, __clone, and __call.

Interceptor is yet another class type that gets autogenerated by Magento. It is related to the plugins feature, which will be discussed in detail later in Chapter 6, Plugins.

In order to trigger code regeneration, we can use the code compiler that is available on the console. We can run either the single-tenant compiler or the multi-tenant compiler.

The single-tenant implies one website and store, and it is executed by using the following command:

magento setup:di:compile

The multi-tenant implies more than one independent Magento application, and it is executed by using following command.

magento setup:di:compile-multi-tenant

Code compilation generates factories, proxies, interceptors, and several other classes, as listed in the setup/src/Magento/Setup/Module/Di/App/Task/Operation/ directory.