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Factory Method

Imagine a Creator object that creates a Product object for latter use. According to Open/closed principle it is usually important to facilitate subclassing (or other extension technique) of Creator class as well as Product class. This pattern suggests encapsulate the creation of Product object in a factory method. Thus Creator::getProduct returns Product object and any SubCreator::getProduct returns appropriate SubProduct.

See https://en.wikipedia.org/wiki/Factory_method_pattern for more information.

Notes

  1. Use this pattern when Creator should not or cannot be aware of concrete of concrete Product class.
  2. Factory Method creates a point of extension that permits to easily introducing a new class to a system.
  3. The pattern facilitates creating of abstract layer of model. This in turn enables to interchange different implementations (e.g. an application that uses different database types or drivers).
  4. When appropriate Creator can provide a default implementation of Facroty Method to return default Product class. Otherwise the Creator class must be subclassed.
  5. Take care not to call abstract Factory Method in Creator constructor:
abstract class AbstractCreator 
{
    public function __construct() 
    {
        $product = createProduct();
    }
    
    public abstract function createProduct();
}

class Creator extends AbstractCreator
{
    public function __construct() 
    {
        parent::__construct();
    }
    
    public function createProduct()
    {
        return "Created";
    }
}

$c = new Creator;

Predictibly, an error is thrown:

Fatal error:  Uncaught Error: Call to undefined function createProduct() in [...][...]:5

Implementation

Consider a draft of web framework that parses an URL string and invokes a corresponding controller.

Factory method pattern class diagram

In the base level There are three abstractions:

  • Request (Product) - parses URL string, stores the current URL,
  • Router (Product) - matches a controller against current URL,
  • Application (Creator) - controls the request-response flow. This class delegates URL parsing to Request class and controller resolving to Router class.

Application has two abstract methods createRouter and createRequest to create Request and Router respectively. These methods are in fact factory methods that must be defined in subclass of Application.

The handle method of Application calls createRouter and createRequest and can work with any subclass of Request and Router.

Two variants of Application are implemented:

  1. SimpleApplication:

  2. ParameterApplication:

Check out SimpleApplicationTest and ParameterApplicationTest the see the proper use of these classes.