Quick example on using a local hash to hold event callbacks.

event_queue_callbacks.rb

class Foo
  def start
    # Do something important here
    perform :start
  end

  def finish
    perform :finish
    # Clean up stuff here
  end

  # Add a new callback to the event queue
  def on event
    queue[event] << Proc.new
  end

  private

  def queue
    @queue ||= Hash.new { |h, k| h[k] = [] unless h.key? k }
  end

  def perform event, *args
    queue[event].each do |p|
      p.call args if p.respond_to? :call
    end
  end
end

# Create a new object
foo = Foo.new

# Register new callbacks on events
foo.on :start do
  puts 'Starting!'
end
foo.on :finish do
  puts 'So done!'
end

# Use the object as you normally would
foo.start
foo.finish

##########################################
# And we can have fun with state

class Bar
  attr_accessor :name
  def log
    puts "Logging #{@name}"
  end
end

bar = Bar.new
bar.name = 'FOO'

foo.on :finish do
  bar.log
end

foo.finish

# Change the objects name and the callback uses it
bar.name = 'bar'
foo.finish