Multiple callbacks as blocks

callbacks.rb

class CallbackList
  NOOP_BLOCK = ->(*) { :ok }
  def self.[](block)
    block ||= NOOP_BLOCK
    new.tap { |proxy| block.call(proxy) }
  end

  def respond_with(callback, *args)
    callbacks.fetch(callback, NOOP_BLOCK).call(*args)
  end

  def method_missing(method, *args, &block)
    block ? register_callback(method, block) : super
    self
  end

  private

  def callbacks
    @callbacks ||= {}
  end

  def register_callback(method, block)
    callbacks[method] = block
  end
end