From: "jan aerts (RI)" <jan.aerts@...>
Date: 2006-11-13T23:50:55+09:00
Subject: classes within classes

Hi all,

I'm trying to create a little library for drawing my data using SVG. One
of the inherent properties of the drawings that I have to make, is that
each picture can contain one or more tracks with each track containing
one or more features (see ASCII art below).

 +----------------------------picture-+
 | +----------------------track1----+ |
 | |                                | |
 | |  x          x                  | |
 | |         x               x      | |
 | |                                | |
 | |                                | |
 | +--------------------------------+ |
 |                                    |
 | +----------------------track2----+ |
 | |                                | |
 | |                     xx         | |
 | +--------------------------------+ |
 +------------------------------------+

As a track can _only_ be defined within a picture, and a feature can
_only_ be defined within a track, I have set up the classes as follows:
 class Picture
   class Track
     class Feature
     end
   end
 end

The problem is that some of the properties of a Picture object have to
be readable for its Track objects. This is _not_ simple inheritance,
because Picture and Picture::Track are two completely different things.
As I see it, there are several options:
(A) either pass those properties as arguments every time you create a
new Picture::Track object. Not optimal, because the same piece of data
is copied over and over again.
(B) set those features as global variables. Not optimal as well.
(C) something more elegant?

More elaborately (showing some of the key features)
<LIB CODE SNIPPET>
 class Picture
   def initialize(width)
     @width = width
     @tracks = Array.new
   end
   attr_accessor :width, :tracks

   def add_track(name)
     @tracks.push(Picture::Track.new(name)
     return @tracks[-1]
   end

   class Track
     # DO NOT CALL INITIALIZE METHOD DIRECTLY: use Picture#add_track
     def initialize(name)
       @name = name
     end
     attr_accessor :name

     def to_svg
       # I NEED THE WIDTH OF THE PICTURE HERE
       return
some_xml_that_includes_the_width_which_was_defined_in_Picture
     end

     class Feature
       ...
     end
   end
 end
</LIB CODE SNIPPET>

<USAGE SNIPPET>
p = Picture.new(800)  # Creates a new picture of 800pt width.
p.add_track('first_track')
p.add_track('second_track')
</USAGE SNIPPET>

In the end, the question is: should I make Picture#width a global
variable, should I add width as an argument to
Picture::Track#initialize, or is there a more elegant solution? (I hope
it's the last one...)

Any help would be very much appreciated,
jan.

Dr Jan Aerts
Bioinformatics Group
Roslin Institute
Roslin, Scotland, UK
+44 131 527 4200

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