Flavour - The module to avoid ifs when your model is State driven behaviour

Flavour.rb

module Flavour
  
  module ClassMethods
    
    def watch(&block)
      @watchable = block
    end
    
    def watchable
      @watchable
    end
    
    def state(state_name, options = {} , &block)
      @state_behavior = @state_behavior || {}
      @state_behavior[state_name] = (Class.new &block).new
    end
    
    def state_behavior
      @state_behavior
    end
    
  end
  
  module InstanceMethods
    
    def method_missing(method, *args, &block)
      instance_state = self.instance_eval &self.class.watchable 
      if self.class.state_behavior[instance_state.to_sym].respond_to? method.to_sym
             self.class.state_behavior[instance_state.to_sym].send(method.to_sym, *args, &block)
           else
             super
           end
    end
    
  end
    
  def self.included(base)
    base.send(:include, InstanceMethods)
    base.extend ClassMethods
  end
    
end

class Invoice
  include Flavour
  attr_accessor :invoice_state
  
  watch { self.invoice_state } 

   state :draft do
     
     def calculate
       self.total
     end

     def signature
       "Invoice Not Finalized"
     end

   end

   state :final do
     
     def calculate
       self.total = self.total + self.total * 0.10
     end

     def signature
       "2178 - Proccessed by Computer AT/192"
     end

   end
   
  def initialize(initial_state = :draft)
    self.invoice_state = initial_state
  end
     
  def total
    100
  end

end

invoice = Invoice.new
invoice.invoice_state = :final
puts invoice.signature