gistfile1.rb

# Store somewhere
class SitemapGenerator
  
  def initialize(app)
    @app = app
  end

  def manipulate_resource_list(resources)
    sitemap_builder = ::Middleman::Sitemap::Resource.new(
      @app.sitemap,
      "sitemap.xml", # The URL
      File.join(File.dirname(__FILE__), 'source', 'sitemap.xml.builder') # Path to your template
    )
    resources << sitemap_builder
    resources
  end

end

# In your extension register/setup method
app.after_configuration do
  sitemap.register_resource_list_manipulator(:sitemap_generator,
                                             SitemapGenerator.new(self),
                                             false)
end

Hey Thomas -
I've tried adding this to an example extension I made. However, when I run middleman build --verbose, It appears an additional path to the template is being prefixed onto my specified path:

== Request: /extension_page.html
       error  build/extension_page.html
No such file or directory - /home/dave/middleman-test/source/home/dave/middleman-test/vendor/gems/middleman-example-extension/lib/middleman-example-extension/source/extension_page.md

Here's the code causing this:

# middleman-example-extension/lib/extension_page.rb
extension_page = ::Middleman::Sitemap::Resource.new(
  @app.sitemap,
  "extension_page.html", # The URL
  File.join(File.dirname(__FILE__), 'source', 'extension_page.md') # => should return "middleman-example-extension/source/extension_page.md"
)

So the correct path is getting prefixed with /home/dave/middleman-test/source/. Can you see what I'm doing wrong? I've updated my repo so you can reproduce this on your machine, if you want. Thanks again for the help :) .

Update: Thomas fixed this issue in c9290c6.