Question

I got the following code from @avdi’s Ruby Tapas episode for today:

module Eventful
  def self.included(other)
    other.extend(Macros)
  end
  def add_listener(listener)
    (@listeners ||= []) << listener
  end
  def notify_listeners(event, *args)
    (@listeners || []).each do |listener|
      listener.public_send("on_#{event}", *args)
    end
  end

  module Macros
    def event(name)
      module_eval(%Q{
        def #{name}(*args)
          notify_listeners(:#{name}, *args)
        end
      })
    end
  end
end

class Dradis
  include Eventful
  event :new_contact
end

class ConsoleListener
  def on_new_contact(direction, range)
    puts "DRADIS contact! #{range} kilometers, bearing #{direction}"
  end
end

dradis = Dradis.new
dradis.add_listener(ConsoleListener.new)
dradis.new_contact(120, 23000)

I understand the concept of events and listeners and the observer pattern, but don’t get how/why this syntax is working, and haven’t seen it in any manuals. The class Dradis has this:

 event :new_contact

At first, I thought that event was a method and :new_contact was an argument so that I would call event on an instance of Dradis, something like:

dradis = Dradis.new
dradis.event

but instead, new_contact is called on an instance of Dradis like:

dradis = Dradis.new
dradis.add_listener(ConsoleListener.new)
dradis.new_contact(120, 23000)

and that triggers the event method in the Macro module.

Can anyone explain why it works like this? calling :new_contact on an instance dradis to trigger the event method?

Answer 1

I didn’t watch the episode, but look, it’s right there.

  module Macros
    def event(name)
      module_eval(%Q{
        def #{name}(*args)
          notify_listeners(:#{name}, *args)
        end
      })
    end
  end

event is a method which defines another method (new_contact) which calls notify_listeners from Eventful.

and that triggers the event method in the Macro module

Incorrect. That method has finished its work a long time ago and it doesn’t get invoked again. It produced a new method using module_eval / def and that new method (new_contact) is what’s getting called.

It’s important to understand that event method runs only once, when the Dradis class is parsed and loaded. It does not get run on every instantiation of Dradis.