From: Phrogz <gavin@...>
Date: 2005-07-16T08:50:52+09:00
Subject: Customizable Mixin

Summary:
I'm writing a module that needs a specific property/method to be
supplied by the user. How do you handle this?

Details:
For my module to be effective, it needs the user to implement either an
instance property (or method that returns a value)that 'describes' the
instance. This should _not_ be unique for each instance, so #object_id
is out.

It would be effective to simply say "Hey, if you include this module in
your class, you need to supply @foo or else it'll break". But for some
reason that seems wrong to me.

The best way I've come up with is the following. Do you have a better
suggestion?

module Foo
  def self.included( klass )
    klass.class_eval{
      def self.foo_name=( block )
        @name_block = block
      end
      def self.foo_name
        @name_block
      end
    }
  end

  def foo_name
    self.class.foo_name.call( self )
  end

  def same_as?( other_foo )
    self.foo_name == other_foo.foo_name
  end
end

class Bar
  include Foo
  self.foo_name = lambda{ |bar| bar.name }

  attr_reader :name
  def initialize( name )
    @name = name
  end
end

b1a = Bar.new( "Fred" )
b1b = Bar.new( "Fred" )
b2 = Bar.new( "Jim" )

p b1a == b1b           #=> false
p b1a.same_as?( b1b )  #=> true
p b1a.same_as?( b2 )   #=> false