Ruby structure for extendable handler/plugin architechture - ruby

Question

I'm writing something that is a bit like Facebook's shared link preview.
I would like to make it easily extendable for new sites by just dropping in a new file for each new site I want to write a custom parser for. I have the basic idea of the design pattern figured out but don't have enough experience with modules to nail the details. I'm sure there are plenty of examples of something like this in other projects.
The result should be something like this:
> require 'link'
=> true
> Link.new('http://youtube.com/foo').preview
=> {:title => 'Xxx', :description => 'Yyy', :embed => '<zzz/>' }
> Link.new('http://stackoverflow.com/bar').preview
=> {:title => 'Xyz', :description => 'Zyx' }
And the code would be something like this:
#parsers/youtube.rb
module YoutubeParser
url_match /(youtube\.com)|(youtu.be)\//
def preview
get_stuff_using youtube_api
end
end
#parsers/stackoverflow.rb
module SOFParser
url_match /stachoverflow.com\//
def preview
get_stuff
end
end
#link.rb
class Link
def initialize(url)
extend self with the module that has matching regexp
end
end

Answer 1

# url_processor.rb
class UrlProcessor
# registers url handler for given pattern
def self.register_url pattern, &block
#patterns ||= {}
#patterns[pattern] = block
end
def self.process_url url
_, handler = #patterns.find{|p, _| url =~ p}
if handler
handler.call(url)
else
{}
end
end
end
# plugins/so_plugin.rb
class SOPlugin
UrlProcessor.register_url /stackoverflow\.com/ do |url|
{:title => 'foo', :description => 'bar'}
end
end
# plugins/youtube_plugin.rb
class YoutubePlugin
UrlProcessor.register_url /youtube\.com/ do |url|
{:title => 'baz', :description => 'boo'}
end
end
p UrlProcessor.process_url 'http://www.stackoverflow.com/1234'
#=>{:title=>"foo", :description=>"bar"}
p UrlProcessor.process_url 'http://www.youtube.com/1234'
#=>{:title=>"baz", :description=>"boo"}
p UrlProcessor.process_url 'http://www.foobar.com/1234'
#=>{}
You just need to require every .rb from plugins directory.

Answer 2

If you're willing to take this approach you should probably scan the filed for the mathing string and then include the right one.
In the same situation I attempted a different approach. I'm extending the module with new methods, ##registering them so that I won't register two identically named methods. So far it works good, though the project I started is nowhere near leaving the specific domain of one tangled mess of a particular web-site.
This is the main file.
module Onigiri
extend self
##registry ||= {}
class OnigiriHandlerTaken < StandardError
def description
"There was an attempt to override registered handler. This usually indicates a bug in Onigiri."
end
end
def clean(data, *params)
dupe = Onigiri::Document.parse data
params.flatten.each do |method|
dupe = dupe.send(method) if ##registry[method]
end
dupe.to_html
end
class Document < Nokogiri::HTML::DocumentFragment
end
private
def register_handler(name)
unless ##registry[name]
##registry[name] = true
else
raise OnigiriHandlerTaken
end
end
end
And here's the extending file.
# encoding: utf-8
module Onigiri
register_handler :fix_backslash
class Document
def fix_backslash
dupe = dup
attrset = ['src', 'longdesc', 'href', 'action']
dupe.css("[#{attrset.join('], [')}]").each do |target|
attrset.each do |attr|
target[attr] = target[attr].gsub("\\", "/") if target[attr]
end
end
dupe
end
end
end
Another way I see is to use a set of different (but behaviorally indistinguishable) classes with a simple decision making mechanism to call a right one. A simple hash that holds class names and corresponding url_matcher would probably suffice.
Hope this helps.