Ruby Decorator Design Pattern

README.md

Ruby Decorator Design Pattern

While styding the Decorator Design Pattern i read there was a drawback on implementing the Decorator pattern using the module + extend + super implementation.

The drawbacks of this implementation are

1. can not use the same decorator more than once on the same object

2. difficult to tell which decorator added the functionality

Thinking about that i made the maesters example.

A maester is kind of scholars in the universe of Game of Thrones. They get rings of different metals on a chain the hang around their necks symbolizing their mastery. The maester.rb examples is a work around those drawback.

It works like this. The maester starts without any knowledge, any ring in his chain. Every time the maester study(extend) a Tome he increases his knowledge about the subject and get the correspondent ring on his chain. If he study more that subject he will increase his knowledge but he already got the ring so his rings will remain unchanged.

To implement that behavior using the chosen design pattern the Tome module defines the hook self.included to add another hook on the classes that include it. This method is the self.extended method. This second hook sets increases the knowledge of the maester (base class of the extended method) about the tome that is being extended.

maesters.rb

class Maester 
  def initialize
    @tomes = {}
    @rings = []
  end

  def show_rings
    @rings
  end

  def tomes
    @tomes
  end
end

module Tome
  def self.included(base)
    base.instance_eval do
      def self.extended(base)
        base.tomes[self.name.to_s] ||= 0
        base.tomes[self.name.to_s] += 1
      end
    end
  end
end

module MoneyTome
  include Tome
  def show_rings
    super + ['gold']
  end
end

module WarcraftTome
  include Tome
  def show_rings
    super + ['iron']
  end
end

maester = Maester.new
maester.extend MoneyTome
maester.extend MoneyTome
maester.extend WarcraftTome
p maester.show_rings # => ["gold", "iron"]
p maester.tomes      # => {"MoneyTome"=>2, "WarcraftTome"=>1}