defer.rb

require 'fiber'
require 'coolio'

class Defer < Coolio::AsyncWatcher
  def self.defer loop=Coolio::Loop.default, &block
    new(loop, &block).result
  end

  attr_reader :result

  def initialize loop=Coolio::Loop.default
    super()
    @fiber = Fiber.current
    attach(loop)
    Thread.new{
      @result = yield
      puts "SIGNAL"
      signal
    }
    Fiber.yield
  end

  def on_signal
    puts "ON SIGNAL"
    @fiber.resume
    detach
  end
end

Fiber.new{
  puts Defer.defer{ sleep 1; "DONE" }
}.resume

puts "GO"

Coolio::Loop.default.run