Merge custom assets with webpack assets

lib-tasks-custom_assets.rake

namespace :assets do
  task add_custom_assets: :environment do
    source_manifest_file = "#{ActionView::Base.assets_manifest.dir}/custom-manifest.json"
    unless File.exists?(source_manifest_file)
      puts "source manifest file not found at #{source_manifest_file}"
    else
      # make sure the file exists and has content. better to use the below approach
      # rails_manifest_file = Sprockets::ManifestUtils.find_directory_manifest(ActionView::Base.assets_manifest.dir)
      # assets = JSON.parse(open(rails_manifest_file).read)
      # new_assets = open(source_manifest_file).read
      # assets['assets'].merge!(JSON.parse(new_assets))
      # manifest = open(rails_manifest_file, 'w+')
      # manifest.write(assets)
      # manifest.save

      # TODO - use only after https://github.com/rails/sprockets/pull/481/files is merged and released.
      manifest = Sprockets::Manifest.new(ActionView::Base.assets_manifest.environment, ActionView::Base.assets_manifest.dir)
      new_assets = open(source_manifest_file).read
      manifest.add_assets(JSON.parse(new_assets))
      manifest.save
      puts "merged assets with rails assets"
    end
  end
end

if defined?(AssetSync) && AssetSync.config.run_on_precompile == true
  Rake::Task['assets:sync'].enhance ['assets:add_custom_assets']
else
  Rake::Task['assets:precompile'].enhance do
    Rake::Task['assets:add_webpack'].invoke
  end
end