Shower-thoughts on response objects

ResponseObject.rb

class ResponseObject
  def initialize
    @handlers = Hash.new { [] }
    yield self
  end
  
  def do(state, *args)
    @state = state
    @args = args
  end
  
  def on(state, &response)
    @handlers[state] << response
  end
  
  def respond(handler)
    handler.call(self)
    
    if @state.nil?
      raise IndeterminateStateException, "Response state was not declared"
    elsif @handlers.has_key?(@state)
      @handlers[@state].call(@args)
    else
      raise UnhandledStateException, "State #{@state} has no defined handler"
    end
  end
end

class Foos
  def with_new_foo(attributes={}, &block)
  
    ResponseObject.new do |r|
      r.do(:success, create_foo(attributes))
      
    # I'm not sure that do..end defines a rescue scope, but it's prettier, so I'll assume it does
    rescue FooExisits
      r.do(:duplicate, find_foo(attributes))
      
    rescue ValidationError => e
      $statsd.increment("foo_validation_failures")
      r.do(:validation_failure, e)
      
    end.respond(block)
  end
end

class FooController
  def create
    Foos.with_new_foo(params[:foo]) do |r|
      r.on(:success) do |foo|
        render_foo(foo)
      end
      
      r.on(:duplicate) do |foo|
        render_foo(foo)
      end
      
      r.on(:validation_failure) do |e|
        render_validation_failure(e)
      end
    end
  end
end