not_active_record_callbacks_because_those_are_evil.rb

service = MyService.new

service.doit("foo").on_done do |result|
  # this is the happy path--doit did whatever it was supposed to
end.on_still_in_progress do
  # this could be an alternate path, e.g. the request is still in progress
end.on_error do |exception|
  # This could be the error path, where you get an exception passed
end

class MyService
  def doit(foo)
    
    # ....

    if completed
      Done.new(result)
    else
      InProgress.new
    end
  rescue => ex
    OnError.new(ex)
  end

  class Callbacks
    def on_done(*); self; end
    def still_in_progress(*); self; end
    def on_error(*); self; end
  end
  class Done < Callbacks
    def initialize(result)
      @result = result
    end
    def on_done(&block)
      block.call(@result)
      super
    end
  end
  
  # ... and so forth
end