Use Ferrum for PDF

Gemfile

@@ -35,6 +35,8 @@ gem "decidim-user_extension", path: "decidim-user_extension"
 
 gem "slack-ruby-client"
 
+gem "ferrum"
+
 group :development, :test do
   gem "byebug", "~> 11.0", platform: :mri
   gem "figaro"

Gemfile.lock

@@ -455,6 +455,11 @@ GEM
       multipart-post (~> 2)
     faraday-net_http (3.1.0)
       net-http
+    ferrum (0.13)
+      addressable (~> 2.5)
+      concurrent-ruby (~> 1.1)
+      webrick (~> 1.7)
+      websocket-driver (>= 0.6, < 0.8)
     ffi (1.16.3)
     figaro (1.2.0)
       thor (>= 0.14.0, < 2)
@@ -900,6 +905,7 @@ GEM
     webpush (1.1.0)
       hkdf (~> 0.2)
       jwt (~> 2.0)
+    webrick (1.8.1)
     websocket-driver (0.7.6)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
@@ -932,6 +938,7 @@ DEPENDENCIES
   dotenv-rails
   factory_bot_rails
   faker (~> 3.2)
+  ferrum
   figaro
   fog-aws
   image_processing

config/initializers/decidim.rb

@@ -490,6 +490,7 @@
 Decidim.register_assets_path File.expand_path("app/packs", Rails.application.root)
 
 require "decidim/map/provider/static_map/cfj_osm"
+require "decidim/exporters/pdf"
 
 ## Set default OGP description length limit. It's used in Decidim::Blogs components
 Rails.application.config.default_blog_ogp_description_limit = ENV.fetch("DECIDIM_BLOG_OGP_DESCRIPTION_LIMIT", 150).to_i

lib/decidim/exporters/pdf.rb

@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+# To be correctly loaded by Zeitwerk
+Decidim::Forms::Admin::QuestionnaireAnswerPresenter # rubocop:disable Lint/Void
+
+module Decidim
+  module Exporters
+    # Exports a PDF using the provided hash, given a collection and a
+    # Serializer. This is an abstract class that should be inherited
+    # to create PDF exporters, with each PDF exporter class setting
+    # the desired template, layout and orientation.
+    #
+    class PDF < Exporter
+      # Public: Exports a PDF version of the collection by rendering
+      # the template into html and then converting it to PDF.
+      #
+      # Returns an ExportData instance.
+      def export
+        html = controller.render_to_string(
+          template: template,
+          layout: layout,
+          locals: locals
+        )
+
+        document = pdf_from_string(html, orientation: orientation)
+
+        ExportData.new(document, "pdf")
+      end
+
+      # may be overwritten if needed
+      def orientation
+        "Portrait"
+      end
+
+      # implementing classes should return a valid ERB path here
+      def template
+        raise NotImplementedError
+      end
+
+      # implementing classes should return a valid ERB path here
+      def layout
+        raise NotImplementedError
+      end
+
+      # This method may be overwritten if the template needs more local variables
+      def locals
+        { collection: collection }
+      end
+
+      protected
+
+      def controller
+        raise NotImplementedError
+      end
+
+      private
+
+      def pdf_from_string(html, orientation:)
+        document = nil
+
+        Dir.mktmpdir do |dir|
+          html_path = File.join(dir, "tmp.html")
+          File.write(html_path, html)
+          url = URI::File.build([nil, html_path])
+
+          browser = Ferrum::Browser.new
+          browser.go_to(url)
+          document = browser.pdf(path: nil,
+                                 encoding: :binary,
+                                 landscape: orientation != "Portrait",
+                                 printBackground: true,
+                                 scale: 0.8,
+                                 format: :A4)
+        end
+
+        document
+      end
+    end
+  end
+end