Ruby: Adding things to initialize method through modules

Question

In Ruby I want to have a class include a series of modules and have these individual modules execute a block or method (or just find some way to edit an instance variable) when initializing that class.

I know I can do this by creating a method in the module and then calling it in the class' initialize method, but I want some way to do this by simply including the module and calling one method to execute any code the modules add to initialize, that way I can have a large amount of things included in a class without worrying about adding a line of code in the initialize method for every single module included.

I've checked out aliasing, super, and related things but haven't gotten anything...

If it helps to understand what I'm hoping to accomplish here's some pseudocode:

module Mod1  
    call_this_block_on_initialize { @a.push 4 }  
end  
  
module Mod2  
    call_this_block_on_initialize { @a.push 5 }  
end  
  
class Test  
    attr_accessor :a  
    include Mod1  
    include Mod2  
  
    def initialize  
      @a = [1, 2, 3]  
      call_those_blocks_set_by_mods  
    end  
end  

t = Test.new
t.a # returns [1, 2, 3, 4, 5]

This may be a bit wordy but I think the title sums up what I'm trying to do. Thanks for any help!

Zach Kemp · Accepted Answer · 2012-11-06 19:45:50Z

11

There are a few ways you can do this. This example will redefine the initialize method and add whatever extra code you want:

module MyModule
  def self.included(base) # built-in Ruby hook for modules
    base.class_eval do    
      original_method = instance_method(:initialize)
      define_method(:initialize) do |*args, &block|
        original_method.bind(self).call(*args, &block)
        @a.push 4 # (your module code here)
      end
    end
  end
end

class Test  
  attr_accessor :a  

  def initialize  
    @a = [1, 2, 3]    
  end  

  # It must be included after the `initialize` 
  # definition or the module won't find the method:
  include MyModule
end

However: I think what you really want is subclassing. If you have a lot of classes with similar behavior, as it seems you do, ask yourself if there is a natural abstract parent class. Can you explain what you did with super and why it didn't work?

edited Nov 6, 2012 at 19:45

answered Nov 6, 2012 at 19:39

Zach Kemp

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2 Comments

thIIIrd Over a year ago

You're right, I looked into how super works and I'm able to execute code for any number of modules added no matter what combination or which modules. Basically made a chain of supers for the modules:

Joshua Pinter Over a year ago

This is so money, thank you. I used this to separate out ability.rb files for a Rails CanCan authorization pattern: stackoverflow.com/a/25723811/293280

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