factory.md

Factory Pattern

Problem

We need to create objects without having to specify the exact class of the object that will be created.

Solution

The Factory pattern is a specialization of the Template pattern. We start by creating a generic base class where we don't make the "which class" decision. Instead, whenever it needs to create a new object, it calls a method that is defined in a subclass. So, depending on the subclass we use (factory), we create objects of one class or another (products).

Example

Imagine that you are asked to build a simulation of life in a pond, which has plenty of ducks:

class Pond
  def initialize(number_ducks)
    @ducks = number_ducks.times.inject([]) do |ducks, i|
      ducks << Duck.new("Duck#{i}")
      ducks
    end
  end

  def simulate_one_day
    @ducks.each {|duck| duck.speak}
    @ducks.each {|duck| duck.eat}
    @ducks.each {|duck| duck.sleep}
  end
end

pond = Pond.new(3)
pond.simulate_one_day

But, how would we model our Pond if we wanted to have frogs instead of ducks? In the implementation above, we are specifying in the Pond's initializer that it should be filled up with ducks. So, we'll refactor it so that the decision of creating one type of animal or another is made in a subclass:

class Pond
  def initialize(number_animals)
    @animals = number_animals.times.inject([]) do |animals, i|
      animals << new_animal("Animal#{i}")
      animals
    end
  end

  def simulate_one_day
    @animals.each {|animal| animal.speak}
    @animals.each {|animal| animal.eat}
    @animals.each {|animal| animal.sleep}
  end
end

class FrogPond < Pond
  def new_animal(name)
    Frog.new(name)
  end
end

pond = FrogPond.new(3)
pond.simulate_one_day