roster.rb

require "rubygems"
require "builder"

class Roster

  def initialize
    @members = []
    @fresh   = true
    @content = nil
  end

  attr_reader :members

  def <<(member)
    @members << member
    
    @fresh = false
  end

  def to_xml
    if @fresh
      @content ||= render_xml_roster
    else
      @content = render_xml_roster
      @fresh   = true
    end

    return @content
  end
  
  def dirty?
    !@fresh
  end

  private

  def render_xml_roster
    raise "Cache Broked" if @fresh
    
    builder = Builder::XmlMarkup.new

    builder.roster do |r|
      members.each do |m|
        r.member(m)
      end
    end
  end
end

roster_test.rb

require 'roster'
require 'test/unit'

class RosterTest < Test::Unit::TestCase
  def setup
    @roster = Roster.new
    @roster << "Greg Brown"
    @roster.to_xml # Populate cache and clear dirty flag
  end
  
  def test_dirty_is_true_after_a_member_is_added
    assert [email protected]?, "Array is dirty"
    
    @roster << "Jordan Byron"
    
    assert @roster.dirty?, "Array is not dirty"
  end
  
  def test_cache_does_not_change_if_not_dirty
    xml_id = @roster.to_xml.object_id
    
    assert_equal xml_id, @roster.to_xml.object_id
  end
  
  def test_cache_does_change_if_dirty
    xml_id = @roster.to_xml.object_id
    
    @roster << "Jordan Byron"
    
    assert_not_equal xml_id, @roster.to_xml.object_id
  end
end