29 Jan /Module Decorators

Posted by Alex Miller

Why decorate objects instead of using inheritance?

It’s a more dynamic, flexible and transparent alternative to subclassing. There’s several different ways of using the decorator pattern in Ruby, but I’m just going to show the module/extend approach. You can read about other decorator techniques in Dan Croak’s blog post.

Use a module and extend to add instance methods on an object. This will still maintain the ability to use super as demonstrated in this trivial example:

class Report
  def number_of_pages
    10
  end
end

module CustomerAdmendment
  def number_of_pages
    super + 5
  end
end

module AdminAdmendment
  def number_of_pages
    super + 1
  end
end

report = Report.new
report.number_of_pages # => 10
report.extend(CustomerAdmendment).number_of_pages # => 15
report.extend(AdminAdmendment).number_of_pages  # => 16

With the module/extend technique we can maintain the original interface and delegate through all the decorators.

TAGS:ruby