Rake task to generate appcache manifest for Rails assets

appcache.rake

namespace :appcache do
  desc "Create manifest"
  task :create do
    sprockets_manifest = Rails.root.join("public/assets/manifest.yml")
    abort "#{sprockets_manifest} not found" unless sprockets_manifest.exist?
    prefix = Rails.configuration.assets.prefix
    files = YAML.load(sprockets_manifest.open).map { |_, file| "#{prefix}/#{file}" }
    Rails.root.join("public/manifest.appcache").open("w") do |out|
      out << "CACHE MANIFEST\n\n"
      out << "# " << `git rev-parse HEAD` << "\n"
      out << files.join("\n")
      out << "\n"
    end
  end
end