Sometimes, you don't want to calculate the value of an attribute every time--like the sum of an array. However, you don't want it cached on the outside of the object since that makes for messy code. But you seem to cache things over and over again inside objects. Here's a module AttributeCache that does a little meta programming to make attribute caching a little bit easier. more details at http://webjazz.blogspot.com/2007/05/ruby-snippet-caching-object-attributes.html

   1  
   2  module AttributeCache
   3  
   4    def metaclass; class << self; self; end; end
   5    
   6    def cache(attr_name, options)
   7      instance_variable_set "@#{attr_name}", options[:initial]
   8      instance_variable_set "@#{:sum}_outdated", true
   9  
  10      metaclass.instance_eval do 
  11        define_method("outdate_#{attr_name}") do
  12          puts "in outdated"
  13          instance_variable_set "@#{attr_name}_outdated", true
  14        end
  15      end
  16      
  17      metaclass.class_eval %Q{
  18        def cached_#{attr_name}(&block)
  19          if @#{attr_name}_outdated
  20            puts "in cached to update!"
  21            @#{attr_name}_outdated = false
  22            @#{attr_name} = block.call
  23          else
  24            puts "in cached to give cache!"
  25          end
  26          return @#{attr_name}
  27        end
  28      }
  29  
  30    end
  31    
  32  end
  33  
  34  class Collection
  35    include AttributeCache
  36  
  37    def initialize
  38      @array = []
  39      cache :sum, :initial => 0
  40    end
  41  
  42    def add(x)
  43      outdate_sum
  44      @array << x
  45    end
  46  
  47    def sum
  48      cached_sum { @array.inject {|t, e| t += e} }
  49    end
  50    
  51  end
  52  
  53  c = Collection.new
  54  puts c.public_methods(false).inspect
  55  c.add(2)
  56  c.add(3)
  57  puts c.sum
  58  puts c.sum
  59  c.add(4)
  60  puts c.sum
  61  puts c.sum