From: Joel Pearson <lists@...>
Date: 2013-07-02T17:12:52+09:00
Subject: Re: How do I make lots of classes aware of each other?

Perhaps an example would help. I struggled for a while with the links 
between classes in an Excel-style structure that I made.

Here's the documentation:
http://rubydoc.info/gems/rubyexcel/0.2.7/index

This is the structure I used:

All classes are namespaced under "module RubyExcel"

"class Workbook" holds multiple instances of "class Sheet" in an 
instance variable called @sheets. Method "Workbook#sheets" allows you to 
loop through each sheet or select specific ones with an argument.

Each instance of "class Sheet" holds a reference to its "parent" 
Workbook, and its data, held in "class Data". The Data and its link

Each instance of classes Row, Column, Range, and Cell hold references to 
their "parent" Sheet, which in turn provides a path to the Data and the 
Workbook.
Because these classes exist merely to access specific sections of the 
Sheet/Data, they can be garbage collected once they're finished with, 
and don't need persistent tracking since all they do is reference 
sections of the Data. They can be recreated when required.

When I say "parent", I'm not talking about class inheritance, merely an 
instance variable which holds a reference to an instance of another 
class which is associated with the instance of the current class.


Here's a simple example of the structure (without all the long object 
ids in the output):

class Parent
  attr_accessor :children
  def initialize
    @children = []
  end
end

class Child
  attr_accessor :parent
  def initialize( parent )
    @parent = parent
    parent.children << self
  end
end

parent = Parent.new
child1 = Child.new parent
child2 = Child.new parent

parent.children
=> child1, child2

child1.parent
=> parent

child2.parent
=> parent

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