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.