A very simple pub sub implementation.

pub_sub.rb

class Hub
  class << self
    NAMESPACE_SEPARATOR = ':'

    def publish(event, payload = {})
      handlers[event].each do |handler|
        handler.call(payload)
      end
    end

    def subscribe(event, &block)
      handlers[event] << block
    end

    def namespaces
      names = handlers.keys
      names.map! { |k| k.split(NAMESPACE_SEPARATOR).first }
      names.uniq
    end

  private

    def handlers
      @handlers ||= Hash.new do |hash, key|
        hash[key] = []
      end
    end
  end
end

run.rb

require_relative('pub_sub.rb')

Hub.subscribe('user:register') do |payload|
  puts "User with and ID of #{payload[:id]} registered."
end

Hub.subscribe('post:created') do |payload|
  puts "Post with and ID of #{payload[:id]} created."
end

Hub.subscribe('test') do |payload|
  puts "Hi there #{payload[:name]}!"
end

Hub.publish('user:register', id: 1)
Hub.publish('post:created', id: 2)
Hub.publish('test', name: 'Bob')

puts "Current namespaces:"
puts Hub.namespaces