equivalent_xml_matcher.rb

require 'rexml/document'
require 'rexml/formatters/transitive'
require 'rexml/formatters/pretty'

Spec::Matchers.define :be_equivalent_xml_to do |expected_xml|
  match do |target_xml|
    @expected_xml = expected_xml
    formatter = REXML::Formatters::Pretty.new

    formatted_target_xml, formatted_expected_xml = "", ""

    formatter.write(REXML::Document.new(target_xml),    formatted_target_xml)
    formatter.write(REXML::Document.new(@expected_xml), formatted_expected_xml)

    formatted_target_xml.strip!
    formatted_expected_xml.strip!
    
    formatted_target_xml == formatted_expected_xml
  end
  
  failure_message_for_should do |target_xml|
    "Expected the XML\n" +
    "=====\n" +
    target_xml + "\n" +
    "=====\n" +
    "to be equivalent to\n" +
    "=====\n" +
    @expected_xml + "\n" +
    "=====\n" +
    "but it wasn't."
  end
end