Eavesdrop : Declaritively decouple your Ruby

eavesdrop.gemspec

Gem::Specification.new do |s|
  s.name        = 'eavesdrop'
  s.version     = '0.1.0'
  s.platform    = Gem::Platform::RUBY
  s.author      = 'Arne Brasseur'
  s.email       = '[email protected]'
  s.summary     = 'Event listener DSL'
  s.description = 'Decouple your code declaritively.'

  s.files         = ['eavesdrop.rb']
  s.test_file     = 'eavesdrop_spec.rb'
  s.require_path  = '.'

  s.add_development_dependency('rspec', ["~> 2.0"])
end

eavesdrop.rb

class Eavesdropper
  attr_accessor :event_source

  def self.support( prefix = nil)
    listener_class = self
    ::Module.new do
      listeners = [prefix, 'listeners'].compact.join('_')
      fire_event = ['fire', prefix, 'event'].compact.join('_')
      eval %<
        def #{listeners}
          @#{listeners} ||= []
        end

        def #{fire_event}(name, *args)
          #{listeners}.each do |l|
            l.event_source = self
            l.send(name, *args) if l.respond_to? name
          end
        end
      >

      listener_class.protocol.each do |event|
        fire_method = ['fire', prefix, event].compact.join('_')
        define_method( fire_method ) do |*args|
          fire_event( event, *args )
        end
      end

    end
  end

  def self.inherited( base )
    base.extend( DSL )
  end

  module DSL
    def protocol ; @protocol ||= [] ; end

    def listen_to( name )
      protocol << name
    end
  end
end

eavesdrop_spec.rb

require 'rspec'
require File.dirname(__FILE__) + '/eavesdrop.rb'

class KettleListener < Eavesdropper
  listen_to :boiling
  listen_to :full
end

class Kettle
  include KettleListener.support

  def initialize( capacity = 20 )
    @capacity = capacity
    @content = 0
    @temperature = 0
  end

  def heat
    @temperature = [@temperature + 10, 100].min
    fire_boiling if @temperature == 100
  end

  def fill( amount = 5 )
    @content = [ @content + amount, @capacity ].min
    fire_full if @content == @capacity
  end
end

class Waiter < KettleListener
  def full
    puts 'kettle is full'
  end

  def boiling
    puts 'kettle is boiling'
  end
end

describe 'eavesdropper' do
  let ( :kettle ) { Kettle.new } # !> (...) interpreted as grouped expression
  let ( :listener ) { Waiter.new } # !> (...) interpreted as grouped expression

  before { kettle.listeners << listener }

  it "should receive events" do
    listener.should_receive(:full).once
    kettle.fill( 20 )
  end
end

This has been turned into a full gem. See http://github.com/arnebrasseur/eavesdrop