roster.rb

require "rubygems"
require "builder"

class Roster

  def initialize
    @members       = []
    @members_cache = []
    @content       = nil
  end

  attr_reader :members

  def to_xml
    if @members_cache == @members
      @content ||= render_xml_roster
    else
      @content       = render_xml_roster
      @members_cache = @members
    end

    return @content
  end

  private

  def render_xml_roster
    builder = Builder::XmlMarkup.new

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

r = Roster.new
r.members << "Gregory Brown"
r.members << "David Black"
r.members << "Jeremy McAnally"

puts r.to_xml

roster_spec.rb

require 'roster'

describe "roster" do
  before(:each) do
    @r = Roster.new
    @r << "Gregory Brown"
  end
    
  it "should cache if not modified between 1st and 2nd to_xml" do
    xml_id = @r.to_xml.object_id
    @r.to_xml.object_id.should eql(xml_id)
  end
  
  it "should update cache if modified between 1st and 2nd to_xml" do
    xml_id = @r.to_xml.object_id
    @r << "David Black"
    @r.to_xml.object_id.should_not eql(xml_id)
  end
end