intercession.rb

class Intercession
  def initialize app, behavior
    @app      = app
    @behavior = behavior

    @before, @after = %w(before after).map { |m| @behavior.instance_methods.map(&:to_s).include? m }
  end

  def call env
    session = env["rack.session"] and session.extend @behavior
    session.before if @before.to_s == "true"
    @app.call(env).tap { session.after if @after.to_s == "true" }
  end
end