Preload assets to make rails dev faster

asset_preloader.rb

require 'parallel'
require 'thread'

# The asset preloader is designed to precompute and cache all precompilable
# assets in parallel to avoid doing it in serial on the first request. As of
# Sprockets 3, all assets on the precompile list (i.e. config.assets.precompile)
# are compiled on the first request whether the current page has asked for them
# or not. Obviously such behavior can mean a very slow initial request (we were
# seeing load times on the order of 10-11 minutes). By preloading, or warming the
# sprockets cache, initial page load times can be reduced to ~15 seconds (with
# an additional ~2 minutes spent during boot). Preloading only happens once, so
# subsequent requests should be fast. Preloading is different from precompiling,
# as the latter does not appear to cache the assets once they've been compiled.
# Generally speaking, preloading should be done in development and precompiling
# should be done in production.
class AssetPreloader
  class << self
    def preload!
      list = precompile_list

      count = 0
      count_mutex = Mutex.new

      options = {
        in_processes: 4,
        finish: -> (*) {
          count_mutex.synchronize { count += 1 }
          STDOUT.write("\r#{count}/#{list.size} preloading assets... ")
        }
      }

      Parallel.each(list, options) do |path|
        # compat: false causes #resolve to return fully resolved asset URIs,
        # complete with correct MIME type, etc
        uri, _ = assets.resolve(path, compat: false)
        next unless uri

        # only load the asset if it hasn't been cached yet
        unloaded = Sprockets::UnloadedAsset.new(uri, assets)
        key = unloaded.dependency_history_key
        assets.load(uri) unless assets.cache.get(key)

        nil
      end

      puts 'done'
    end

    private

    # find all assets that have been added to the precompile list
    # code adapted from Sprockets::Manifest and Sprockets::Legacy
    def precompile_list
      paths, filters = config.assets.precompile.partition do |arg|
        Sprockets::Manifest.simple_logical_path?(arg)
      end

      filters = filters.map do |arg|
        Sprockets::Manifest.compile_match_filter(arg)
      end

      paths + assets.logical_paths.each_with_object([]) do |(logical_path, filename), ret|
        if filters.any? { |f| f.call(logical_path, filename) }
          ret << logical_path
        end
      end
    end

    def config
      Rails.application.config
    end

    def assets
      Rails.application.assets
    end
  end
end