asset_path_tag.rb

<<~aid -*- coding: utf-8 -*- ruby -*- asset_path_tag.rb ************************

  A Jekyll plugin to output a relative URL for assets based on the post or page


Syntax:
   {% asset_path filename post_id %}
   {% asset_path "filename with whitespace" post_id %}

Examples:
{% asset_path kitten.png %} on post 2013-01-01-post-title
{% asset_path pirate.mov %} on page page-title
{% asset_path document.pdf /2012/05/25/another-post-title %}
{% asset_path "document with spaces in name.pdf" /2012/05/25/another-post-title %}
{% asset_path image.jpg /my_collection/document_in_collection %}

Output:
/assets/posts/post-title/kitten.png
/assets/page-title/pirate.mov
/assets/posts/another-post-title/document.pdf
/assets/posts/another-post-title/document with spaces in name.pdf
/assets/my_collection/document_in_collection/image.jpg

Looping example using a variable for the pathname:

File _data/image.csv contains:
  file
  image_one.png
  image_two.png

{% for image in site.data.images %}{% asset_path {{ image.file }} %}{% endfor %}
on post 2015-03-21-post-title

Output:
/assets/posts/post-title/image_one.png
/assets/posts/post-title/image_two.png

Looping example over posts:

Site contains posts:
  post-title
  another-post-title

{% for post in site.posts %}{% asset_path cover.jpg {{post.id}} %}{% endfor %} ]
on index.html

Output:
/assets/posts/post-title/cover.jpg
/assets/posts/another-post-title/cover.jpg

*****************************************************************************aid

##
#
# @license MIT OR Apache-2.0 OR BlueOak-1.0.0
# @copyright The OpenINF Authors and Friends
# @author Sam Rayner
# @author Otto Urpelainen
# @author The OpenINF Authors and Friends
module Jekyll
  def self.get_post_path(page_id, collections)
    #loop through all collections to find the matching document and get its slug
    collections.each do |collection|
      doc = collection.docs.find { |doc| doc.id == page_id }
      if doc != nil
        slug = Jekyll::VERSION  >= '3.0.0' ? doc.data["slug"] : doc.slug
        return "#{collection.label}/#{slug}"
      end
    end

    ''
  end

  # Returns the path to the specified asset.
  class AssetPathTag < Liquid::Tag
    @markup = nil

    def initialize(tag_name, markup, tokens)
      # strip leading and trailing spaces
      @markup = markup.strip
      super
    end

    def render(context)
      if @markup.empty?
        return 'Error processing input, expected syntax: ' \
               '{% asset_path filename post_id %}'
      end

      # render the markup
      filename, post_id = parse_parameters context
      path = post_path context, post_id

      # strip filename
      path = File.dirname(path) if path =~ /\.\w+$/

      # fix double slashes
      "#{context.registers[:site].config['baseurl']}/assets/#{path}/#{filename}"
        .gsub(%r{/{2,}}, '/')
    end

    private

    def parse_parameters(context)
      parameters = Liquid::Template.parse(@markup).render context
      parameters.strip!

      if ['"', "'"].include? parameters[0]
        # Quoted filename, possibly followed by post id
        last_quote_index = parameters.rindex(parameters[0])
        filename = parameters[1...last_quote_index]
        post_id = parameters[(last_quote_index + 1)..-1].strip
        return filename, post_id
      end
      # Unquoted filename, possibly followed by post id
      parameters.split(/\s+/)
    end

    def post_path(context, post_id)
      page = context.environments.first['page']

      post_id = page['id'] if post_id.nil? || post_id.empty?
      if post_id
        # Posts as well as documents in other collections have a post_id
        collections = context.registers[:site].collections.map { |c| c[1] }
        Jekyll.get_post_path(post_id, collections)
      else
        page['url']
      end
    end
  end
end

Liquid::Template.register_tag('asset_path', Jekyll::AssetPathTag)