From: "David A. Black" <dblack@...>
Date: 2008-10-08T04:29:17+09:00
Subject: Re: [Q] best way to inject new functionality into a class

Hi --

On Wed, 8 Oct 2008, Chuck Remes wrote:

> I've pretty much made up my mind how to modify my classes, but I thought I 
> would throw this issue out to the community at large to see if I'm missing 
> something glaringly obvious.
>
> When I write code for classes & modules that need to collaborate, I typically 
> inject the functionality of one into the other one of two ways (I rarely 
> subclass). My code will explain.
>
> #1
>
> module Foo
> def baz; # do something;   end
> end
>
> class Bar
> include Foo # Bar takes on Foo's methods
> end
>
>
> #2
>
> class Foo
> def baz; # do something;   end
> end
>
> class Bar
> attr_accessor :strategy  # implement the strategy pattern
> end
>
> bar = Bar.new
> bar.strategy = Foo.new
>
>
> Recent projects at work are forcing me to be a little more dynamic than #1 
> can be, and #2 can be error-prone since there is a lot more setup & wire code 
> required.
>
> As I continue to explore the ruby language, I ran across another mechanism 
> for class modification.
>
> #3
>
> module Foo
> def baz; # do something;   end
> end
>
> class Bar
> end
>
> bar = Bar.new
> bar.extend(Foo)
>
>
> This third way looks like the cleanest way of all. It has the added benefit 
> that "later" during runtime I could change my mind and replace the Foo module 
> with another one that implements the same methods.
>
> bar.extend(Baz) # overrides all methods with the same signature as Bar or Foo
>
>
> Do you, in general, agree that using #extend is at least superior to #2?

I think they all have their uses, but I'll mention that #extend has
always been my favorite choice for modifying core functionality:

   module DifferentArray
     ...
   end

   a = [].extend(DifferentArray)

etc. If you're writing your own classes then including a module in a
class is quite legit and powerful.


David

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