event.rb

module Listener
  def on(*types, &blk)
    types.each do |type|
      (self.listeners[type] ||= []) << blk
    end
  end

  def trigger(event)
    return unless self.listeners[event.type]

    self.listeners[event.type].each do |blk|
      blk.call(event)
    end
  end

  private
  def listeners
    @listeners ||= {}
  end
end

class Event
  attr_reader :type

  def initialize(type)
    @type = type;
  end
end

class Foo
  include Listener
end

foo = Foo.new
foo.on :bar do |event|
  puts 'inside :bar event!'
end

foo.trigger(Event.new(:bar))