了解工厂设计模式

Question

I was learning about factory design pattern in php. From what i understand, this pattern is useful in cases where we have a bunch of classes, lets say, class_1, class_2, class_3 etc. If the particular class which has to be instantiated is known only at runtime, then instead of using the new operator to create the objects for these classes we create a factory class which will do the job for us.

The factory class will look somewhat like this:

class Factory
{
// $type will have values 1, 2, 3 etc.
public function create_obj($type)
{
  $class_name = "class_".$type;
  if(class_exists($class_name))
  {
        return new $class_name();
  } 
}
}

My question is what is the advantage in using a factory class here? why not just use a simple function instead of a class which is going to complicate things?

Answer 1

The method in your code snippet is not a factory method, but merely a helper method which does a well-known reflective task: instantiates a class based on its name. This is exactly the opposite of what a Factory pattern is for: creating objects (products) without specifying the exact class of object that will be created.

As explained in Wikipedia:

The essence of this pattern is to "Define an interface for creating an object, but let the classes that implement the interface decide which class to instantiate."

You are probably confused by the last PHP example in the Wikipedia article on Factory pattern, and yes, it is a bad example. Check the Java example just above that for a meaningful example (whoever tried to convert that to PHP missed the whole point). The Java example returns a file reader based on its extension, and that is exactly the use case for a factory pattern. Creating your own personal "rule" that certain classes need to have a certain name prefix is most likely a bad design decision.