carrierwaveuploader/carrierwave

module CarrierWave
  module Uploader
    module Callbacks
      extend ActiveSupport::Concern

      included do
        class_attribute :_before_callbacks, :_after_callbacks,
                        :instance_writer => false
        self._before_callbacks = Hash.new []
        self._after_callbacks = Hash.new []
      end

      def with_callbacks(kind, *args)
        self.class._before_callbacks[kind].each { |c| send c, *args }
        yield
        self.class._after_callbacks[kind].each { |c| send c, *args }
      end

      module ClassMethods
        def before(kind, callback)
          self._before_callbacks = self._before_callbacks.
            merge kind => _before_callbacks[kind] + [callback]
        end

        def after(kind, callback)
          self._after_callbacks = self._after_callbacks.
            merge kind => _after_callbacks[kind] + [callback]
        end
      end # ClassMethods

    end # Callbacks
  end # Uploader
end # CarrierWave