ocp_decorator.rb

class PdfFormatter
  def initialize ; ... ; end
  def output ; ... ; end
end
class PdfWithPasswordFormatter < PdfFormatter
  def initialize(base) ; @base = base ; end
  def protect_with_password(original_output) ; ... ; end
  def output ; protect_with_password @base.output ; end
end
class PdfWithWatermarkFormatter < PdfFormatter
  def initialize(base) ; @base = base ; end
  def add_watermark(original_output) ; ... ; end
  def output ; add_watermark @base.output ; end
  end
end
# If we just want a plain PDF
formatter = PdfFormatter.new
# If we want a "draft" watermark
formatter = PdfWithWatermarkFormatter.new(PdfFormatter.new)
# Both password protection and watermark
formatter = PdfWithWatermarkFormatter.new(
  PdfWithPasswordFormatter.new(PdfFormatter.new))