From: 7stud -- <bbxx789_05ss@...>
Date: 2011-04-15T08:15:08+09:00
Subject: Re: What do you do when you need to attach data to an object instance?

Aaron D. Gifford wrote in post #992841:
> What do you do when you see a need to be able to attach some data to
> an object instance for later use somewhere else in a body of code?
> Lately I've resorted to this:
>
> ## Generic utility to allow one to attach data with a getter/setter to
> ## any instance of any object so long as there isn't a method name
> ## collision:
> def attach_data(obj, name, data)
>   getter = name.to_sym
>   setter = (name.to_s + '=').to_sym
>   raise "method name collision for #{obj.class} instance" if
> obj.respond_to?(getter) || obj.respond_to?(setter)
>   ## The 'value' local variable will remain in existence in the lambda
> closures below:
>   value = data
>   meta = class << obj ; self ; end
>   meta.send(:define_method, getter, lambda { value })            ##
> Getter closure
>   meta.send(:define_method, setter, lambda {|val| value = val }) ##
> Setter closure
>   value
> end
>
> For example, in an application using SSH keys, I didn't want to create
> a new subclass, nor use an array or hash container instance just to
> carry an OpenSSL::PKey::RSA object instance around the code.  But I
> needed to associate a user ID (user@host) to a key so it could be
> accessed somewhere else.  I figured it was easiest to just attach it
> to the OpenSSL::PKey::RSA instance (see code above) directly.  That
> made the code cleaner, portions that only required the RSA key
> directly, yet still gave the benefit of the key instance containing
> the additional meta data I required.
>
> What do you do when you need stuff like that?  Monkey patch?  Use a
> container and pass it around instead?  Or?
>

How about a decorator pattern?


> Is there a module version of OpenStruct that one can just include in
> whatever class one wants to attach additional arbitrary data to?  So I
> could have done this instead:
>
> require 'ostructmod'
> class OpenSSL::PKey
>   include OpenStructModule
> end
>
> ???
>
> Aaron out.

class SSL
  def talk
    puts 'hi'
  end
end

class MyWrapper
  def initialize(ssl_obj, key)
    @ssl_obj = ssl_obj
    @key = key
  end

  attr_accessor :ssl_obj, :key

  def method_missing(name, *args)
    @ssl_obj.send(name, *args)
  end
end

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