watchers.rb

module Watchable
  attr_reader :watchers
  def watchers
    @watchers ||= Hash.new{|hsh,k| hsh[k]=[]}
  end

  def watch(&blk)
    b=WatcherBuilder.new self
    blk[b] if block_given?
    b
  end

  def fireEvent(name, *args)
    watchers[name].each {|w| w.call(*args)}
  end

  class WatcherBuilder
    def initialize(watchable)
      @watchable = watchable
    end

    def method_missing(name,&blk)
      @watchable.watchers[name] << blk
    end
  end
end