Technique for extending a method from a module

February 14, 2010 Pivotal Labs

Update: Read the follow-up post Second thoughts on initializing modules

I was recently presented the problem of appending to the initialize method from a module that was being included. To do this it would need to override the class’s initialize method with my own but keep the functionality of the original initialize method.

Whenever I need to do something in Ruby that I know will require some experimentation I like to move outside of my application and reproduce the problem in a simple way. For this problem I created a Person class that mixes in a Teacher module.

module Teacher
  def initialize
    puts "initializing teacher"
  end
end

class Person
  include Teacher

  def initialize
    puts "initializing person"
  end
end

The goal is to get the following output when a Person object is created:

> Person.new
initializing teacher
initializing person

The basic program fails as expected; Teacher.new prints “initializing person” because Person’s initialize is trumping Teacher’s. Our immediate goal is to replace Person’s initialize with Teacher’s but in a way that preserves the original initialize method. By using alias_method we can create a copy of the original initialize method that we can call later.

module Teacher
  def self.included(base)
    base.class_eval do
      alias_method :original_initialize, :initialize
      def initialize
        puts "initializing teacher"
        original_initialize
      end
    end
  end
end

This solution is the simplest thing that could possibly work, unfortunately it also has one major limitation. For it to work the call to include Teacher in Person has to come after Person’s definition of initialize. This is may be fine in situations where you have total control over the Person class, but what if Teacher is going to be part of a library you are distributing? Asking your users to place the include line to your module in a specific spot is unacceptable.

To make this work we need to be able to capture definitions of the method we want to redefine even after our module has been included. This sounds like a good time to use Ruby’s method_added hook.

module Teacher

  def self.included(base)
    base.extend ClassMethods
    base.overwrite_initialize
    base.instance_eval do
      def method_added(name)
        return if name != :initialize
        overwrite_initialize
      end
    end
  end

  module ClassMethods
    def overwrite_initialize
      class_eval do
        unless method_defined?(:custom_initialize)
          define_method(:custom_initialize) do
            puts "teacher initialized"
            original_initialize
          end
        end

        if instance_method(:initialize) != instance_method(:custom_initialize)
          alias_method :original_initialize, :initialize
          alias_method :initialize, :custom_initialize
        end
      end
    end
  end

end

Whoa! As you can see a lot of complexity has been added to Teacher. However, what it’s doing is actually really cool. Here is the breakdown:

What self.included is doing:

  1. The ClassMethods module containing overwrite_initialize is added to base (Person).
  2. overwrite_initialize is invoked.
  3. method_added is defined on Person at the class level.

What overwrite_initialize does:

  1. If a method called custom_initialize does not exist it defines one. custom_initialize runs Teacher’s initialize logic and then defers to Person’s initialize.
  2. If the current initialize method is not our custom_initialize method then initialize is preserved as original_initialize and a copy of custom_initialize is made to replace initialize.

What method_added is doing:

  1. Watches for new methods with the name “initialize”.
  2. When an initialize method is defined method_added calls overwrite_initialize to put the chain from custom_initialize to this new initialize method in place.

What is particularly nice is that this implementation is flexible enough to handle multiple redefinitions of initialize. This is important because a subclass of Person may also define initialize. It is not perfect though—if the initialize in the subclass of Person calls super the program will go into an infinite loop where custom_initialize and the subclass’s initialize call each other indefinitely. If anyone has a suggestion on how to get around this please post a comment or fork the gist on Github.

Read the follow-up to this post Second thoughts on initializing modules.

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