From: Brian Candler <B.Candler@...>
Date: 2008-08-28T17:12:28+09:00
Subject: Re: Having a difficult time with Case statements.

> http://www.ruby-forum.com/attachment/2623/startup.rb

If you're going to keep state in a local variable, then you need to keep
looping within that method, because otherwise the value will be lost as soon
as you return from it.

A much more common Ruby idiom for keeping state is to use an instance
variable:

  class Game
    def initialize
      @state = 0
      @playing = true
    end

    def playing?
      @playing
    end

    def next_action
      case @state
      when 0
        puts "Nephlen".blue
        ...
        @state = 1
      ...
      end
    end
  end

  g = Game.new
  while g.playing?
    g.next_action
  end

This lets you create multiple game instances each with independent state
(useful for a multi-player server!) and also makes unit testing much easier.

A neat trick to avoid the big case statement is to use dynamic method
dispatch:

  class Game
    def initialize
      @state = :startup
    end

    def next_action
      send(@state)    # calls the method name in the @state instance variable
    end

    def startup
      puts "Hello!"
      puts "Moving to dungeon entrance"
      @state = :dungeon
    end

    def dungeon
      puts "You are in a deep dungeon. Go N/S/E/W?"
      case gets
      when /^n/i
        @state = :field
      when /^s/i
        @state = :tunnel
      when /^e/i
        @state = :monster
      when /^w/i
        @state = :monster
      else
        puts "Invalid selection"
      end
    end

    def monster
      puts "You have been eaten by a monster!"
    end

    ... etc
  end

For security reasons, make sure that an untrusted party can't set the @state
variable directly, e.g. by copying @state directly from a user input field
or web form.

Regards,

Brian.