From: Matthew Moss <matthew.moss.coder@...>
Date: 2006-01-31T01:36:07+09:00
Subject: Re: [QUIZ] Port a Library (#64)

There aren't any particular libraries I've used anytime recently... 
all my work is in-house code.  But I took a quick glance over CPAN for
something relatively small and simple...  the latter because I stopped
coding in Perl years ago and don't remember all the syntax very well,
especially the stuff that has been added for objects.

In any case, I found a simple library called Trampoline by Steven
Lembark which allows you to create an object but delay actual
construction...  which is useful to have something with expensive
construction cost ready to go but not actually constructed until used.

Below I provide a really basic implementation that is probably not
rock-solid and could probably be done better ...  I'm still such a
n00b, especially when it comes to metaclasses (or eigenclasses, or
whatever they want to be called this week). It also doesn't do
everything the Perl lib did, just what I found useful and could
understand.

Anyway, here's the code (trampoline.rb), with a couple of use examples
at the bottom.

module Trampoline
   # Instance methods
   class Bounce
      def initialize(cons, klass, *args)
         @klass, @cons, @args = klass, cons, args
      end

      def method_missing(method, *args)
         @obj = @klass.send(@cons, *@args) unless @obj
         @obj.send(method, *args)
      end
   end

   # Class methods
   class << Bounce
      alias_method :old_new, :new

      def new(*args)
         old_new(:new, *args)
      end

      def method_missing(method, *args)
         old_new(method, *args)
      end
   end
end



And now, example use. Obviously, this class is not in need of delayed
construction; just using it as an example.

require 'trampoline'
class Logger
   def initialize(prefix)
      puts 'Constructing Logger...'
      @prefix = prefix
   end

   def Logger.make(prefix)
      Logger.new(prefix)
   end

   def log(msg)
      puts "#{@prefix}: #{msg}"
   end
end

puts "start"
errors = Trampoline::Bounce.new(Logger, 'ERROR')
puts "made bouncer, about to log message"
errors.log('Hello, world!')
puts "about to log second message"
errors.log('Goodbye, world!')
puts "message logged"

# This is really the same, but eventually calls Logger.make to construct.
puts "start"
warns = Trampoline::Bounce.make(Logger, 'WARNING')
puts "made bouncer, about to log message"
warns.log('Hello, world!')
puts "about to log second message"
warns.log('Goodbye, world!')
puts "message logged"


Output from the example code:

start
made bouncer, about to log message
Constructing Logger...
ERROR: Hello, world!
about to log second message
ERROR: Goodbye, world!
message logged
start
made bouncer, about to log message
Constructing Logger...
WARNING: Hello, world!
about to log second message
WARNING: Goodbye, world!
message logged