Question

I’m trying to understand the factory design pattern.

I don’t understand why it’s good to have a middleman between the client and the product (object that the client wants).

example with no factory:

$mac = new Mac();

example with a factory:

$appleStore = new AppleStore();
$mac = $appleStore->getProduct('mac');

How does the factory pattern decouple the client from the product?

Could someone give an example of a future code change that will impact on example 1 negative, but positive in example 2 so I understand the importance of decoupling?

Thanks.

Answer 1

I think it has to do with the resources needed to construct some types of objects.

Informally, if you told someone to build a Mac, it would be a painstaking process that would take years of design, development, manufacturing, and testing, and it might not be done right. This process would have to be repeated for every single Mac. However, if you introduce a factory, all the hard work can be done just once, then Macs can be produced more cheaply.

Now consider Joomla’s factory.php. From what I can tell, the main purpose of JFactory is to pool objects and make sure objects that should be the same aren’t copied. For instance, JFactory::getUser() will return a reference to one and only one object. If something gets changed in that user object, it will appear everywhere. Also, note that JFactory::getUser() returns a reference, not a new object. That is something you simply cannot do with a constructor.

Often, you need local context when constructing an object, and that context may persist and possibly take on many forms. For instance, there might be a MySQL database holding users. If User objects are created with a constructor, you’ll need to pass a Database object to the constructor (or have it rely on a global variable). If you decide to switch your application to PostgreSQL, the semantics of the Database object may change, causing all uses of the constructor to need review. Global variables let us hide those details, and so do factories. Thus, a User factory would decouple the details of constructing User objects from places where User objects are needed.

When are factories helpful? When constructing an object involves background details. When are constructors better? When global variables suffice.