remote_asset.rb

# frozen_string_literal: true
module ThemeCheck
  class RemoteAsset < HtmlCheck
    severity :suggestion
    categories :html, :performance
    doc docs_url(__FILE__)

    TAGS = %w[img script link source]
    PROTOCOL = %r{(https?:)?//}
    ABSOLUTE_PATH = %r{\A/[^/]}im
    RELATIVE_PATH = %r{\A(?!#{PROTOCOL})[^/\{]}oim
    CDN_ROOT = "https://cdn.shopify.com/"

    def on_element(node)
      return unless TAGS.include?(node.name)

      resource_url = node.attributes["src"] || node.attributes["href"]
      return if resource_url.nil? || resource_url.empty?

      # Ignore if URL is Liquid, taken care of by AssetUrlFilters check
      return if resource_url.start_with?(CDN_ROOT)
      return if resource_url =~ ABSOLUTE_PATH
      return if resource_url =~ RELATIVE_PATH
      return if url_hosted_by_shopify?(resource_url)

      # Ignore non-stylesheet link tags
      rel = node.attributes["rel"]
      return if node.name == "link" && rel != "stylesheet"

      add_offense(
        "Asset should be served by the Shopify CDN for better performance.",
        node: node,
      )
    end

    private

    def url_hosted_by_shopify?(url)
      asset_url?(url) || looks_like_hosted_by_shopify?(url) || url_is_setting_variable?(url)
    end

    # There are some cases where it's kind of hard to tell if it's
    # hosted by Shopify or not.
    #
    # e.g. {{ image }} is hosted on primary domain (not CDN)
    #
    # e.g. media.sources are on the CDN
    # {% for source in media.sources %}
    #   {{ source.url }}
    # {% endfor %}
    #
    # So I'll go 80/20 here and assume that people name their variable
    # source in `for source in media.sources`.
    def looks_like_hosted_by_shopify?(url)
      liquid_variable?(url) && url =~ /source\.url/
    end

    def url_is_setting_variable?(url)
      liquid_variable?(url) && url =~ /settings\./
    end

    def asset_url?(url)
      liquid_variable?(url) &&
        AssetUrlFilters::ASSET_URL_FILTERS.any? { |filter| url.include?(filter) }
    end

    def liquid_variable?(url)
      url.start_with?(Liquid::VariableStart)
    end
  end
end