example.rb

module Async
  def async(method)
    prepend(Module.new {
      define_method(method) do |*args|
        Thread.new(*args) do |*args|
          puts Thread.current.object_id.to_s(16)
          super(*args)
        end
      end
    })
    
    method
  end
end

class Example
  extend Async
  
  async def background_method
    puts "I am in a separate thread!"
  end
end