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I remember spending some time implementing a plugin framework in C++ some ~10 years ago. Things have changed greatly since, and when I had to do the same in Ruby some days ago for my Ruby script for the wmii window manager it took me but a few minutes.

Goals

This is yet another task that becomes so easy in Ruby it seems to fade out, so to say, the same way many design patterns are trivial and hardly deserve to be named anymore.

There are at least two things a plugin framework should do:

• allow the user to drop plugins into some directory and have them loaded
• register the functionality exposed by each plugin so as to use it either automatically or on demand

The first part is but a simple iteration

Dir["#{PLUGIN_DIR}/*.rb"].each{|x| load x }

Registering

As for the second, there are many ways to do it, but it definitely looks better when plugin registration is automagical and the plugin definition seems purely declarative. Several ideas come to mind, as there are quite a few hooks we can use for that.

Class#inherited

Class#inherited is the most obvious choice (I feel); it'd look like this

class PluginBase
  def self.inherited(child)
    # register child somewhere
    # just to give a complete example:
    PluginBase.registered_plugins << child
  end
  @registered_plugins = []
  class << self; attr_reader :registered_plugins end
end