From: daz <dooby@...10.karoo.co.uk>
Date: 2005-10-02T21:36:46+09:00
Subject: Re: beginners YAML question


On 2005/09/06, Adam Shelly wrote:
> On 2005/09/02, daz wrote:
> > In short, @size is an attribute of Box ... but you're
> > using the inherited Array#to_yaml to serialise your object.
> >
> [...]
>
> That's exactly what I want.  A "Box" object which acts like an array,
> but has other attributes too:
> For example:
> [...]
>
> What I can't figure out is how to get the extra attributes in the
> YAML.  Do I have to override the to_yaml method?
>

Hey, Adam!

  So sorry - I missed this post.
  (Just found it whilst browsing through the archives.)

Yes, you'll need to override the to_yaml method that
is already overridden by Array.  Ideally, you'd just
want to undo the override but I don't think that's
advisable, even if it's possible.

Either:

1) Add this to Box#initialize

  self.class.class_eval do
    ( Object.instance_methods - instance_methods(false) ).grep(/yaml/) do |meth|
        define_method( meth, Object.instance_method(meth) )
    end
  end

That'll copy all yaml methods from Object into Box that were
concealed as a consequence of inheriting the same-named methods
from Array. (Array is interposed between Box and Object).

 - OR -

2) The canonical way, AFAICS, is to redefine them
   in the Box class, which would look similat to this:
      (see:  ruby/lib/ruby/1.8/yaml/rubytypes.rb)

  def to_yaml_type
    "!ruby/object:#{self.class}"
  end

  def to_yaml_properties
    instance_variables      # all ... OR ...
    ### ['@label', '@loc']  # your selection
  end

  def to_yaml( opts = {} )
    YAML.quick_emit(nil, opts) do |out|
      out.map(to_yaml_type) do |map|
        to_yaml_properties.sort.each do |iv|
          map.add( iv[1..-1], instance_eval(iv) )
        end
      end
    end
  end

-------------

I think the definition of 'to_yaml' above looks like it's
doing something tailored to your class, so here's another
alternative:

  def to_yaml( opts = {} )
    Object.instance_method(:to_yaml).bind(self).call(opts)
  end

-------------

You probably worked it out yourself but this is
something to compare against.

BTW, an alternative to Box < Array is to delegate.
The top of your class would look like this:

   require 'delegate'

   class Box < DelegateClass(Array)
     def initialize label, location, initial_size ...
       super Array.new(initial_size, initial_value)

Have a look here:
http://www.rubycentral.com/book/lib_patterns.html#S1


Cheers,

daz