test.rb

class Event
  def initialize
    @callbacks = {}
  end
  
  def on(event, &block)
    @callbacks[event] ||= []
    @callbacks[event] << block
  end
  
  def fire(event)
    (@callbacks[event] || []).collect { |callback|
      callback.call
      callback
    }
  end
end

event = Event.new
event.on 'ping' do
  puts 'pong'
end

event.on 'ping' do
  puts 'kong'
end

puts 'firing ping'
event.fire('ping').each do |callback|
  puts "calling callback again"
  callback.call
end

puts 'firing poo'
event.fire('poo').each do |callback|
  puts "calling callback again"
  callback.call
end