www.moparisthebest.com/plugins/post_filters.rb

module Jekyll

  # Extended plugin type that allows the plugin
  # to be called on varous callback methods.
  #
  # Examples:
  # https://github.com/tedkulp/octopress/blob/master/plugins/post_metaweblog.rb
  # https://github.com/tedkulp/octopress/blob/master/plugins/post_twitter.rb
  class PostFilter < Plugin

    #Called before post is sent to the converter. Allows
    #you to modify the post object before the converter
    #does it's thing
    def pre_render(post)
    end

    #Called after the post is rendered with the converter.
    #Use the post object to modify it's contents before the
    #post is inserted into the template.
    def post_render(post)
    end

    #Called after the post is written to the disk.
    #Use the post object to read it's contents to do something
    #after the post is safely written.
    def post_write(post)
    end
  end

  # Monkey patch for the Jekyll Site class. For the original class,
  # see: https://github.com/mojombo/jekyll/blob/master/lib/jekyll/site.rb
  class Site

    # Instance variable to store the various post_filter
    # plugins that are loaded.
    attr_accessor :post_filters

    # Instantiates all of the post_filter plugins. This is basically
    # a duplication of the other loaders in Site#setup.
    def load_post_filters
      self.post_filters = Jekyll::PostFilter.subclasses.select do |c|
        !self.safe || c.safe
      end.map do |c|
        c.new(self.config)
      end
    end
  end

  # Monkey patch for the Jekyll Post class. For the original class,
  # see: https://github.com/mojombo/jekyll/blob/master/lib/jekyll/post.rb
  class Post

    # Copy the #write method to #old_write, so we can redefine #write
    # method.
    alias_method :old_write, :write

    # Write the generated post file to the destination directory. It
    # then calls any post_write methods that may exist.
    #   +dest+ is the String path to the destination dir
    #
    # Returns nothing
    def write(dest)
      old_write(dest)
      post_write if respond_to?(:post_write)
    end
  end

  # Monkey patch for the Jekyll Page class. For the original class,
  # see: https://github.com/mojombo/jekyll/blob/master/lib/jekyll/page.rb
  class Page

    # Copy the #write method to #old_write, so we can redefine #write
    # method.
    alias_method :old_write, :write

    # Write the generated post file to the destination directory. It
    # then calls any post_write methods that may exist.
    #   +dest+ is the String path to the destination dir
    #
    # Returns nothing
    def write(dest)
      old_write(dest)
      post_write if respond_to?(:post_write)
    end
  end

  # Monkey patch for the Jekyll Convertible module. For the original class,
  # see: https://github.com/mojombo/jekyll/blob/master/lib/jekyll/convertible.rb
  module Convertible

    def is_post?
      self.class.to_s == 'Jekyll::Post'
    end

    def is_page?
      self.class.to_s == 'Jekyll::Page'
    end

    def is_filterable?
      is_post? or is_page?
    end

    # Call the #pre_render methods on all of the loaded
    # post_filter plugins.
    #
    # Returns nothing
    def pre_render
      self.site.load_post_filters unless self.site.post_filters

      if self.site.post_filters and is_filterable?
        self.site.post_filters.each do |filter|
          filter.pre_render(self)
        end
      end
    end

    # Call the #post_render methods on all of the loaded
    # post_filter plugins.
    #
    # Returns nothing
    def post_render
      if self.site.post_filters and is_filterable?
        self.site.post_filters.each do |filter|
          filter.post_render(self)
        end
      end
    end

    # Call the #post_write methods on all of the loaded
    # post_filter plugins.
    #
    # Returns nothing
    def post_write
      if self.site.post_filters and is_filterable?
        self.site.post_filters.each do |filter|
          filter.post_write(self)
        end
      end
    end

    # Copy the #transform method to #old_transform, so we can
    # redefine #transform method.
    alias_method :old_transform, :transform

    # Transform the contents based on the content type. Then calls the
    # #post_render method if it exists
    #
    # Returns nothing.
    def transform
      old_transform
      post_render if respond_to?(:post_render)
    end

    # Copy the #do_layout method to #old_do_layout, so we can
    # redefine #do_layout method.
    alias_method :old_do_layout, :do_layout

    # Calls the pre_render method if it exists and then adds any necessary
    # layouts to this convertible document.
    #
    # payload - The site payload Hash.
    # layouts - A Hash of {"name" => "layout"}.
    #
    # Returns nothing.
    def do_layout(payload, layouts)
      pre_render if respond_to?(:pre_render)
      old_do_layout(payload, layouts)
    end

    # Returns the full url of the post, including the
    # configured url
    def full_url
      self.site.config['url'] + self.url
    end
  end
end