hookz.rb

class SomeClass

  class << self
    def hooks
      @hooks ||= Hash.new([])
    end
    
    def add_hook(kind, &block)
      self.hooks[kind] << block
    end
  end
  
  def hook(kind)
    self.class.hooks[kind].each { |block| instance_eval(&block) }
  end

  def foo
    puts "Monkey"
    hook :foo
  end
  
  def baz
    puts "Ape"
  end

end

s = SomeClass.new

s.foo # =>  Monkey

SomeClass.add_hook :foo do
  puts "Donkey"
  baz
end

s.foo # =>  Monkey
      #     Donkey
      #     Ape