0_widgets_example.rb

require_relative 'deferrable'
require_relative 'slime'

class WidgetsController
  include ControllerSlime

  def create
    creator = WidgetCreator.new(attributes: params[:widget])

    creator.done do |widget|
      render nothing: true, status: 201, location: widget_path(widget)
    end
    creator.fail { |errors| render json: errors, status: 400 }

    creator.call
  end
end

class WidgetCreator
  include Deferrable

  attr_reader :attributes

  def initialize(args)
    @attributes = args.fetch(:attributes)
  end

  def call
    WidgetRepository.save(Widget.new(attributes), self)
  end
end


class WidgetRepository
  def self.save(widget, listener)
    if [true,false].sample
      listener.resolve! widget
    else
      listener.reject! [{donkey: 'does not hover.'}]
    end
  end
end

5.times { WidgetsController.new.create }

deferrable.rb

module Deferrable
  def done(&block)
    done_blocks << block
  end

  def fail(&block)
    fail_blocks << block
  end

  def resolve!(*args)
    done_blocks.each do |block|
      block.call(*args)
    end
  end

  def reject!(*args)
    fail_blocks.each do |block|
      block.call(*args)
    end
  end

  def done_blocks
    @_done_blocks ||= []
  end

  def fail_blocks
    @_fail_blocks ||= []
  end
end

slime.rb

require 'ostruct'

module ControllerSlime
  def widget_path(widget)
    "/widgets/#{widget.id}"
  end

  def render(opts)
    puts "#render #{opts.inspect}"
  end

  def params
    {widget: {id: 1, foo: 'bar'}}
  end
end

class Widget < OpenStruct; end