observer-pattern

appliance.rb

class Appliance
  attr_accessor :name, :state , :switch
  def initialize(name, switch = nil)
    @name = name
    @state = switch.nil? ? :off : switch.state
    @switch = switch
  end
 
  def update(changed_state)
    self.state = changed_state
  end
 
  def on?
    self.state == :on
  end
 
  def off?
    self.state == :off
  end
end

observer.rb

module Observer
  attr_reader :observers
  def initialize
    @observers = []
  end

  def add_observer(observer)
    @observers << observer
  end

  def delete_observer(observer)
    @observers.delete(observer)
  end

  def notify_observers(method = :update)
    self.observers.map do |observer|
      if observer.respond_to?(method)
        observer.send(method, self.state)
      else
        raise NoMethodError, "undefinded method #{method.to_s}"
      end
    end
  end
end

run.rb

switch_box = SwitchBox.new(argf.gets.chomp)
switch1 = Switch.new(argf.gets.chomp, :off)

switch_box.switches << switch1

appliance = Appliance.new(argf.gets.chomp)
appliance.switch = switch1
switch1.add_observer(appliance)

puts "Switch named '#{switch1.name}' is currently #{switch1.state.to_s.upcase}"
puts "Appliance '#{appliance.name}' is currently #{appliance.state.to_s.upcase}"
puts '-'*10
puts 'Changing switch state!!'
switch1.state = :on
puts 'Switch state has been changed!!'
puts '-'*10
puts "Switch named '#{switch1.name}' is now #{switch1.state.to_s.upcase}"
puts "Appliance '#{appliance.name}' is now #{appliance.state.to_s.upcase}"

switch_box.rb

class SwitchBox
  attr_accessor :name, :switches
  def initialize(name)
    @name = name
    @switches = []
  end
end