Adding a before filter to module instance methods

gistfile1.rb

# All Walkman commands require the Player to be running so
# let's raise an error if it's not
class Walkman
  module Commands
    def self.included(base)
      base.extend ClassMethods

      base.instance_methods(false).each do |method_name|
        base.add_player_check(method_name)
      end

      # check every method added to see if we need to create
      # a before filter for it or not
      base.instance_eval do
        def method_added(name)
          # remove our prefix(es)
          name = name.to_s
          name.slice!("old_")

          # check if we've already defined new methods
          if !method_defined?("old_#{name}")
            add_player_check(name.to_sym)
          end
        end
      end
    end

    module ClassMethods
      # create a before filter that checks if the player is running
      # by creating a new method with our filter and aliasing the old
      def add_player_check(name)
        class_eval do
          unless method_defined?("old_#{name}")
            alias_method "old_#{name}".to_sym, name
            original_method = instance_method(name)

            define_method(name) do |*args, &block|
              raise "Player is not running" unless player.running?
              original_method.bind(self).call(*args, &block)
            end
          end
        end
      end
    end
  end
end

class Walkman
  module Commands
    module Controls
      include Walkman::Commands
      
      def play
        puts "play"
      end
    end
  end
end

class Walkman
  include Commands::Controls
end

walkman = Walkman.new

walkman.player.running? # => false
walkman.play # => RuntimeError: Player is not running

# start player
walkman.player.running? # => true
walkman.play # => "play"