ruby cookbook -- 使分配程序能够使用注册回调的返回值

#使分配程序能够使用注册回调的返回值
A simple change to the  EventDispatcher class makes it possible for the dispatcher to use the return values of the registered  callbacks. The original implementation of  EventDispatcher#notify called the registered code blocks, but ignored their return value. This version of EvenTDispatcher#notify yields the return values to a block passed in to notify:

	module EventDispatcher
	  def notify(event, *args)
	      if @event_dispatcher_listeners[event]
	      @event_dispatcher_listeners[event].each do |m|
	        yield(m.call(*args)) if m.respond_to? :call
	      end
	    end
	    return nil
	  end
	end

Here's an insultingly simple portal rendering engine. It lets boxes register to be rendered inside an HTML table, on one of two rows on the portal page:

	class Portal
	  include EventDispatcher

	  def initialize
	    setup_listeners
	  end

	  def render
	    puts '<table>'
	    render_block = Proc.new { |box| puts " <td>#{box}</td>" }
	    [:row1, :row2].each do |row|
	      puts ' <tr>'
	      notify(row, &render_block)
	      puts ' </tr>'
	    end
	  puts '</table>'
	  end
	end

	portal = Portal.new
	portal.subscribe(:row1) { 'Stock Ticker' }
	portal.subscribe(:row1) { 'Weather' }
	portal.subscribe(:row2) { 'Pointless, Trivial News' }
	portal.render # <table>
	# <table>
	#  <tr>
	#   <td>Stock Ticker</td>
	#   <td>Weather</td>
	#  </tr>
	#  <tr>
	#   <td>Pointless, Trivial News</td>
	#  </tr>
	# </table>