gistfile1.rb

# not real code
module Rack
  class Director
    def call(env)
      @request = Rack::Request.new(env)
      status, headers, body = @app.call(env)
      @response = Rack::Response.new(body, status, headers)
      if authenticated?
        # inject toolbar
      end
      @response.finish
    end
    
    def authenticated?
      config.authencation_proc.call(@request) == true
    end
  end
end

# in initializer
Rack::Director.configure do |c|
  c.authentication_proc = proc { |request| request.env['warden'].user.authenticated? }
end