From: James French <James.French@...>
Date: 2012-04-06T01:16:09+09:00
Subject: Re: Design pattern question



-----Original Message-----
From: Robert Klemme [mailto:shortcutter@googlemail.com] 
Sent: 05 April 2012 16:08
To: ruby-talk ML
Subject: Re: Design pattern question

On Thu, Apr 5, 2012 at 4:25 PM, James French <James.French@naturalmotion.com> wrote:

> Using  the Jay Fields link and slightly modifying I've got something 
> I'm happy with. Thanks for the help Robert, as always :)
>
> module SourceControl
>  def self.init(subject)
>    subject.public_methods(false).each do |meth|
>      (class << self; self; end).class_eval do
>        define_method meth do |*args|
>          subject.send meth, *args
>        end
>      end
>    end
>  end
> end

That's just a mechanism to generate delegation.  And you redefine methods all the time.  I don't think meta programming is necessary at all here.

> class SourceControlAPI
>
>  def checkout(repository, dest, options={})
>  end
>
>  def update(path, options={})
>  end

Those are not really needed.  And if you want to have them then I'd make them either contain a default implementation if there exists one or throw an exception which explains that this method must be implemented for the class.


True, but I want to document the API somewhere central. I raise exceptions saying the method must be implement.

> end
>
> class SVN < SourceControlAPI
>
>  def checkout(repository, dest, options={})
>    puts "Checking out from SVN"
>  end
>
>  def update(path, options={})
>  end
>
> end
>
> SourceControl.init(SVN.new)
> SourceControl.checkout('foo', 'bar')

You are hiding specific state in a constant.  I don't like that because it will prevent accessing multiple different source control systems in the same program or from multiple threads.  I find that requirement to be able to invoke all the methods through the constant leads to a situation where you severely cripple modularity and reusability without really gaining something.


Also true, if I wanted to do that, but I don't. I just need to initialise it at startup and use the source control system that was automatically determined from then on, without knowing which one it is. I think this API is working really well for me... I also just like the API of SourceControl.checkout etc.


Kind regards

robert

--
remember.guy do |as, often| as.you_can - without end http://blog.rubybestpractices.com/