How to create a rails 4 offline webapp?

I'm trying to create an app with Rails 4 that should be usable without internet connection. I've heard about html5 application cache and the rack-offline gem which is the approach I took. Now, it seems that it's not working properly on Rails 4 since the /application.manifest only shows:

CACHE MANIFEST
# dd1ba6bba9339ef83f9c1225c70289dd6326d3caae01b0d52b502381030dc78f

404.html
422.html
500.html

NETWORK:
*

Also, I'm using assets precompile so the application.js, application.css and the image files has a fingerprint in their names, something like application-e8cc2fba8275c884c.js.

Solution

I made my own solution in a generate_appcahe_manifest.rake file and put it in /lib/tasks folder.

task :generate_appcache_file => ['deploy:precompile_assets', 'html5_manifest']

desc "Create html5 manifest.appcache"
task :html5_manifest => :environment do
  puts 'Creating appcache manifest file...'

  File.open("public/manifest.appcache", "w") do |f|
    f.write("CACHE MANIFEST\n")
    f.write("# Version #{Time.now.to_i}\n\n")
    f.write("CACHE:\n")
    assets = Dir.glob(File.join(Rails.root, 'public/assets/**/*'))
    assets.each do |asset|
      if File.extname(asset) != '.gz' && File.extname(asset) != '' && File.extname(asset) != '.json'
        filename_path = /#{Rails.root.to_s}\/public\/(assets\/.*)/.match(File.absolute_path(asset))[1].to_s
        # f.write("assets/#{File.basename(asset)}\n")
        f.write(filename_path.concat("\n"))
      end
    end
    f.write("\nNETWORK:\n")
    f.write("*\n")
    f.write("http://*\n")
    f.write("https://*\n")
  end
  puts 'Done.'
end

namespace :deploy do
  task :precompile_assets do
    require 'fileutils'
    if File.directory?("#{Rails.root.to_s}/public/assets")
      FileUtils.rm_r "#{Rails.root.to_s}/public/assets"
    end

    puts 'Precompiling assets...'
    puts `RAILS_ENV=production bundle exec rake assets:precompile`
    puts 'Done.'
  end
end

So when I run rake generate_appcache_file on the terminal, I got a /public/manifest.appcache file with the compiled assets like this:

CACHE MANIFEST
# Version 1409045103

CACHE:

assets/app/backgrounds/strips-05561384267a3286ab382c852f1e8b0d.jpg
assets/app/backgrounds/team-12e0fc5f670794c4eba3db117fba6746.jpg
assets/app-a7de6b02d1d39b783fe4e7ab7906ec95.css
assets/app-ae958853aa13f9011e35121cbe5b3cfe.js

NETWORK:
*
http://*
https://*

Finally, I call that file on my /app/views/layouts/app.html.erb file:

<!DOCTYPE html>
<html lang="en" manifest="/manifest.appcache">

More info about offline application cache that helps me a lot can be found here.