From: Brian Candler <b.candler@...>
Date: 2009-01-26T00:41:18+09:00
Subject: Re: Help me understand this technique from an open source app?

Pito Salas wrote:
> n the code at the bottom (from HTTParty), the module is included in
> some other class ("O"), and among the effects is that (for example)
> the method default_params becomes available in "O".

Well, consider this first:

module ClassMethods
  def foo
    puts "hello from class"
  end
end

module InstanceMethods
  def bar
    puts "hello from instance"
  end
end

class Parent
  include InstanceMethods
  extend ClassMethods
end

Parent.foo
Parent.new.bar

However this pattern is very common, so it's refactored in two ways.

1. put the module containing class methods *inside* the module 
containing the instance methods

module MyTools
  ... instance methods go here

  module ClassMethods
    ... class methods go here
  end
end

2. use the "included" hook so that when you include MyTools, it 
automatically does extend MyTools::ClassMethods at the same time.

The example above becomes:

module MyTools
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def foo
      puts "hello from class"
    end
  end

  def bar
    puts "hello from instance"
  end
end

class Parent
  include MyTools
end

Parent.foo
Parent.new.bar

At least, I *think* that's what you were asking about. There's also the 
ModuleLevelInheritableAttributes there, but you didn't post all the code 
for that. But notice that the include hook is calling

  mattr_inheritable :default_options

in the base class, and also setting a class instance variable.

HTH,

Brian.
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