app.rb

class Application
  def call(request, response)
    response.write(request, "hello", 200, {})

    10.times do |body|
      response.write(request, body)
    end

    response.close
  end
end

class Filter
  def initialize(filter)
    @filter = filter
  end

  def write(request, body, status = nil, headers = nil)
    p "FILTERING: #{body}"
    @filter.write(request, body, status, headers)
  end

  def close
    @filter.close
  end
end

class Context
  def initialize(app, filters, callbacks)
    @app       = app
    @callbacks = callbacks
    @chain     = filters.reverse.inject(self) do |filter, klass|
      klass.new(filter)
    end
  end

  def serve(request)
    @callbacks.each { |cb| cb.before(request) }
    @app.call(request, @chain)
    @callbacks.reverse_each { |cb| cb.after(request) }
  end

  def write(request, body, status, headers)
    p "OUTPUTTING #{body}"
  end

  def close; end
end

class Handler
  def before(request)
    p "before: hi"
  end

  def after(request)
    p "after: bye"
  end
end

ctx = Context.new(Application.new, [Filter, Filter], [Handler.new])
ctx.serve({})