From: benjohn@...
Date: 2006-10-26T17:42:56+09:00
Subject: Re: Beginner would like help with oo-modelling


It's usually a good idea to separate out how something presents itself
from how it is stored. You mentioned that you want to render not just to
SVG, but perhaps to ps and PDF too? That makes me think it would be
sensible to have a RulerRenderer subclass with implementations for each
rendering method. The SVG renderer (and probably the others too) may be
well represented with a "template" - take a look at ERB.

In my opinion (would like to hear others)...

The most important thing I've learned about OO is that object
hierarchies are not fundamental. There is no right way to model a given
set of objects. If you try to find one, you will be engaging in
premature hierarchy building, and you'll be wasting time.

Given some objects and a particular use for them, there are hierarchies
that work well. A good hierarchy will continue to work well as the
requirements for the object change.

In particular settings, there are known extremely good hierarchies.
These are known because so many people have gone up the learning curve
of building a hierarchy and refactoring it / starting again. Over time,
consensus about a sensible object hierarchy emerges. Some of the
patterns are examples (Model View Controller springs to mind).

I think that in this domain, you've limited opportunity for making
sensible use of OO modelling (please disagree everyone). There's also a
danger that you will build a hierarchy of objects that doesn't really
model anything particularly useful.

However: actual advice? Play about and see how far you get :) Try
writing procedural code, and when it gets annoying, see how you can
introduce some objects that split the problem in to manageable chunks.
Go to a good book shop and look at a few books on patterns and OO
decomposition - get some that appeal to you.

Cheers,
  Benj