From: Asfand Yar Qazi <im_not_giving_it_here@..._hate_spam.com>
Date: 2003-12-01T23:02:08+09:00
Subject: 'Decorator' pattern implemented by extending objects with mixins - need guidance.

Hi,

I'm a newb at this language, and now that I've seen it, I think I will 
use it for every project I do from now on (apart from my software 3D 
engine :-).

Anyway, here's the thing: I'm writing a game, and need to implement the 
following behaviour.

A tree is made of Node objects.

Each Node can have 0 or more child-nodes (the Node becomes their parent 
node.)

Each Node can have several Attrs (attributes) attached to it.

Some Attrs are inheritable, i.e. if a parent node has an inheritable 
Attr, its child nodes do as well.  However, not all Attrs are inheritable.

Attrs should be able to be added and removed dynamically.

Some Attrs require other Attrs, either from the current node, an 
ancestor node or from a child node.  This relationship could be 
compulsory, or optional (in which case the relationship should be 
removed if the relavent Attr is removed from its node.)

Some Attrs are Singleton Attrs, i.e. they can only exist on one Node in 
a Tree.  It should be able to find these Singleton Attrs without having 
to traverse the tree, i.e. their location should be cached in a Hash or 
something.


I don't know which of Ruby's features are best for implementing this 
behaviour.  The Delegate model is nice for the inherited Attrs, however 
not all Attrs are meant to be inheritable.

Creating attributes as mixins and extending the Nodes with them is a 
good way of adding attributes dynamicallly, but then they can't be 
removed (I think.)

Just using a Hash and using 'Node.add/remove(Attr)' methods is a 
technique that springs to mind, but I was thinking of a "Ruby" way of 
doing things.

Any thoughts?




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