gistfile1.txt

class Rack::ResponseTimeInjector
  def initialize(app, options = {})
    @app = app
    @format = options[:format] || "%f"
  end
  
  def call(env)
    t0 = Time.now
    returning @app.call(env) do |response|
      response.last.body.gsub! /\$responsetime(?:\((.+)\))?/ do
        diff = Time.now - t0
        if @format.respond_to? :call
          @format.call(diff)
        else
          ($1 || @format) % diff
        end
      end
    end
  end
end