From: Alex Young <alex@...>
Date: 2007-07-27T20:45:08+09:00
Subject: Re: eval versus load and plugins

Rover Rhubarb wrote:
> I'm writing a program with a sort of plugin mechanism. Plugins are
> basically single ruby files defining classes. It's not exactly a plugin
> mechanism because there an only be one plugin used.
> 
> Image the program command being
>   ruby myApp -plugin plugins/myPlugin.rb
> 
> 
> I want to load the plugin which defines its own class (which follows a
> certain pattern - since there are no interfaces) then instantiates it.
> 
> At the moment I'm using load to load the plugin file. I don't know the
> name of the class of the specific plugin, but I assume it has a _global_
> method called "new_plugin" which instantiates it.
> 
> Since I'm using load I can be sure that the latest plugin loaded will
> redefine that global method to instantiate the plugin (I can't call it's
> constructor directly because, again, I don't know what name the designer
> is going to give the plugin class - though I can expect them to follow
> the pattern of defining new_plugin)
> 
> Questions:
> 
> 1. I notice that rails uses eval to load the init.rb files of plugins.
> What are the advantages disadvantages of this versus using load?
> (Note also this thread about using eval or load - both are suggested but
> I dont see why we would use eval over load
> http://www.ruby-forum.com/topic/55540#new)
Not sure off the top of my head.  Someone else can probably weigh in here.

> 2. Is there some nicer, cleaner way of doing what I'm doing without
> having the plugin file define a global method?
Make them subclass a base class that you provide, or make them mix in a 
module that you specify.  That gives you the Class#inherited and 
Module#included callbacks to play with, which strikes me as cleaner than 
relying on a method override.

> 3. Is there a neat way of forcing the plugin to match a certain pattern.
> I've seen the "interface" support library, but it seems to be
> shoehorning Ruby into a Java paradigm. What would be the rubyful way?
> Just calling respond_to? on all of the methods I expect and raising
> errors?
If the plugin subclasses a base class you specify, you can either check 
it manually in the callback (which seems a little clumsy to me), or 
provide all the interface members with:

def interface_member
   raise NotImplementedError
end

in the base class.  The former is probably necessary if you need to 
sanity check the class before doing anything with it - you can get away 
with the latter otherwise.

HTH
-- 
Alex