Rails Link Preload Headers

README.md

This lets you set the preload headers for your assets, so that the browser can start fetching them before it begins parsing HTML.

application_controller.rb

class ApplicationController < ActionController::Base
  include ::PreloadHeaders
  
  protected
  
  def preload_header_assets
    [
        {path: 'application.css', as: 'style'},
        ({path: 'application.js', as: 'script'} unless Rails.application.config.assets.debug),
        {path: 'logo.png', as: 'image'},
    ].compact
  end  
end

preload_headers.rb

# app/controllers/concerns/preload_headers.rb
module PreloadHeaders
  extend ActiveSupport::Concern
  included do
    after_action :set_preload_headers
  end

  protected

  def set_preload_headers
    return if !request.format.html? || request.xhr? ||
        # Turbolinks 2
        request.headers['X-XHR-Referer'].present? ||
        # Turbolinks 5
        request.env['HTTP_TURBOLINKS_REFERRER']
    response.headers['Link'] = preload_header_assets.map do |asset|
      "<#{view_context.asset_path(asset[:path])}>; rel=preload; as=#{asset[:as]}"
    end
  end
end