callback example - before_trashing

# callbacks based on http://snippets.dzone.com/posts/show/5226 and
# http://github.com/benhoskings/hammock/tree/master/lib/hammock/callbacks.rb (ben_h)
module Trashable
  include ActiveSupport::Callbacks
  define_callbacks :before_trashing

  def trash!
    run_callbacks(:before_trashing) {|result, object| result == false}
    self.trashed = true
    self.save!
  end

  def recycle!
    self.trashed = false
    self.save!
  end

  def trashed?
    self.trashed
  end

  def self.included(base)

    base.class_eval {
      include ActiveSupport::Callbacks
      class_eval <<-"end_eval"
        def self.before_trashing(*methods, &block)
          callbacks = CallbackChain.build(:before_trashing, *methods, &block)
          @before_trashing_callbacks ||= CallbackChain.new
          @before_trashing_callbacks.concat callbacks
        end

        def self.before_trashing_callback_chain
          @before_trashing_callbacks ||= CallbackChain.new

          if superclass.respond_to?(:before_trashing_callback_chain)
            CallbackChain.new(superclass.before_trashing_callback_chain + @before_trashing_callbacks)
          else
            @before_trashing_callbacks
          end
        end
      end_eval
    }
  end
end