Ruby pub-sub implementation example

pubsub.rb

class Pubsub
  class EventBus
    attr_reader :topics

    def initialize
      @topics = {}
    end

    def publish(topic, message)
      topics[topic.to_s].to_a.each do |subscriber|
        subscriber.send(topic, message)
      end
    end

    def subscribe(topic, obj)
      topics[topic.to_s] ||= []
      return if topics[topic.to_s].include?(obj)
      topics[topic.to_s] << obj
    end

    def unsubscribe(topic, obj)
      topics[topic.to_s] ||= []
      topics[topic.to_s].delete(obj)
    end
  end

  module Publisher
    def publish(topic, message = nil)
      Pubsub.event_bus.publish(topic, message = nil)
    end
  end

  module Subscriber
    def subscribe(topic)
      Pubsub.event_bus.subscribe(topic, self)
    end

    def unsubscribe(topic)
      Pubsub.event_bus.unsubscribe(topic, self)
    end
  end

  def self.event_bus
    @event_bus ||= EventBus.new
  end
end

pubsub_example.rb

require_relative 'pubsub'

class P
  include Pubsub::Publisher
end

class S
  include Pubsub::Subscriber

  def echo(_message)
    puts "echo"
  end

  def test(_message)
    puts "test"
  end
end

p = P.new
s = S.new

puts '- S is not subscribed'

p.publish('test')
p.publish('echo')

puts '- S subscribed to echo'
s.subscribe('echo')

p.publish('test')
p.publish('echo')

puts '- S subscribed to test'
s.subscribe('test')

p.publish('test')
p.publish('echo')

puts '- S resubscribed to echo'
s.subscribe('echo')

p.publish('test')
p.publish('echo')

puts '- S unsubscribed from echo'
s.unsubscribe('echo')

p.publish('test')
p.publish('echo')

puts '- S unsubscribed from test'
s.unsubscribe('test')

p.publish('test')
p.publish('echo')

# Outputs the following:
# - S is not subscribed
# - S subscribed to echo
# echo
# - S subscribed to test
# test
# echo
# - S resubscribed to echo
# test
# echo
# - S unsubscribed from echo
# test
# - S unsubscribed from test