Alternative implementation to LightService example https://github.com/adomokos/light-service

example.rb

class TaxController < ApplicationController
  def update
    update_tax.call(order)
    redirect_to checkout_shipping_path(order), :notice => "Tax was calculated successfully"
  
  rescue BusinessException => e
    render :action => :edit, :error => e.message
  end

  private

  def update_tax
    UpdateTaxFactory.build
  end

  def order
    @order ||= Order.find(params[:id])
  end
end

class UpdateTaxFactory
  def self.build
    UpdateTax.new(tax_calculator) do |service|
      service.add_listener FreeShippingListener.new
    end
  end

  private

  def self.tax_calculator
    TaxCalculator.new
  end
end

class TaxCalculator
  def initialize
    @tax_ranges = TaxRange
  end

  def calculate(order)
    tax_ranges = @tax_ranges.for_region(order.region)
    raise BusinessException.new("The tax ranges were not found") if tax_ranges.nil?

    tax_percentage = tax_ranges.for_total(order.total)
    raise BusinessException.new("The tax percentage  was not found") if tax_percentage.nil?

    order.total * (tax_percentage/100).round(2)
  end
end

class UpdateTax
  include Annoucer

  def initialize(tax_calculator)
    @tax_calculator = tax_calculator
    yield self if block_given?
  end

  def call(order)
    tax_total = @tax_calculator.calculate(order)
    order.update_attribute(:tax, tax_total)

    annouce :tax_updated, order
  end
end

class FreeShippingListener
  def tax_updated(order)
    assign_free_shipping(order)
  end
  
  private
  
  def assign_free_shipping(order)
    unless FreeShippingPolicy.applicable?(order)

    order.provide_free_shipping!
  end
end

class FreeShippingPolicy
  def self.applicable?(order)
    order.total_with_tax > 200
  end
end

# Framework code
#

require 'set'

module Announcer
  def announce(event, *args)
    listeners.each { listener.send(event, *args) if listener.respond_to?(event) }
  end

  def add_listener(listener)
    listeners.add(listener)
  end

  def listeners
    @listeners ||= Set.new
  end
end