Experimenting with observer pattern (publish/subscribe pattern) in Ruby

car.rb

# Simple Car class. Nothing special here...
class Car
  attr_accessor :brand
  attr_accessor :model
  attr_accessor :year
  
  def initialize(brand, model, year=2011)
    @brand, @model, @year = brand, model, year
  end
  
  def to_s; "#{@brand}/#{@model} - #{@year}"; end
  
end

car_shop.rb

require "rubygems"
# require "pp"
# require "ruby-debug"

require "./car"
require "./observable"
require "./observer"

# Simple list of cars that can trigger events
class CarsList < Observable
  
  attr_accessor :cars
  
  # Initialize cars list
  def initialize()
    @cars = []
  end
  
  # Add another car to the list and trigger car_added event
  def add(car)
    @cars << car  
    trigger :car_added, car
  end
  
  # Trigger car_sold event and remove car from list
  def sell(index)
    trigger :car_sold, @cars[index]
    
    @cars.delete_at index
  end
  
  def each &blk
    @cars.send :each, &blk
  end
  
end

# Following observer is happy when u sell car
class AccountingObserver < Observer
  def car_sold(sender, car)
    puts "Yey. We sold another car! " + car.to_s
  end
end

# This observer detects when the car is added to list
class CarServisObserver < Observer
  def car_added(sender, car)
    puts "CarServisObserver received car : " + car.to_s
  end
end

### ### ### ### ### ### ### ### ### ### ### ### ### 

# Using pattern

my_cars = CarsList.new
my_cars.add_observer AccountingObserver.new   # Add accounting observer
my_cars.add_observer CarServisObserver.new    # Add car servis observer


# Add some cars
my_cars.add Car.new "Audi", "Q3", 2010
my_cars.add Car.new "BMW", "X6"
my_cars.add Car.new "BMW", "Z4", 2010
my_cars.add Car.new "Audi", "R8 Coupe 5.2 FSI quattro"

# Sell 2nd car on list
my_cars.sell 1

# Sell another car
my_cars.sell 1

# Add another car to the list
my_cars.add Car.new "Audi", "R8 Coupe 5.2 FSI quattro"


# See all the cars out there
puts "\nList of my cars: "

my_cars.each do |car|
  puts "# #{car}"
end

observable.rb

# This class is used when you want to observe object
class Observable
  
  # Create emptry list of observers and add observer to it
  def add_observer(observer)
    if @observers.nil?
      @observers = []    
    end
    
    @observers << observer
  end
  
  # Returning list of observers
  def observers; @observers; end
  
  # Trigger event with "any name u want" and attach sender and sending object
  def trigger(event, object=nil)
    @observers.each do |observer|
      observer.send event, self, object
    end
  end
  
end

observer.rb

# This class represents observer
class Observer
  
  # Call method/event if exists
  def method_missing(method, *args, &block)
    if self.respond_to? method
      self.send(method, *args, &block)
    end
  end

end