Asset Manifest

Tiny set of utilities to compute subresource integrity and cache busting checksums for static assets.

Asset Manifest provides two main utilities:

• An executable that you should run when compiling your static assets. This generates a manifest file that includes the SRI hash and a checksum (for cache-busting purposes) of each of your assets.

• A ruby library to generate the proper links to the assets taking into account the data from your manifest.

Somewhere in your makefile, you'll want to generate the public/manifest.json file like so:

# Assuming you keep an ASSETS list with all the assets you're compiling...
ASSETS += public/css/app.css
ASSETS += public/css/app.min.css
ASSETS += public/js/app.js
ASSETS += public/js/app.min.js

# ...you'd want a rule like this one:
public/manifest.json: $(ASSETS)
	asset-manifest -d public $^ > $@

Then, in your app, you'll want to initialize AssetManifest::Helpers passing the contents of this JSON file:

assets = AssetManifest::Helpers.new(
  JSON.parse(File.read("./public/manifest.json")),
  { minify: ENV["RACK_ENV"] == "production" }
)

Finally, you'll want to pass this object to your views, so that you can do the following:

<%= assets.stylesheet_tag("/css/app.css") %>
<%= assets.script_tag("/js/app.js") %>

This would produce output similar to:

<link rel="stylesheet"
      href="/css/app-07915293e2bb992a67c618e1aa335d978efc3734.css"
      integrity="sha256-yqqr5VJwz1IM5iTlSram51zrBvuE21FbiYgNnD2fwgE=">
<script src="/js/app-1a4aefaba81b61c7ea763d42fcb39584e5784c32.js"
        integrity="sha256-ZmTdnjlqI4ppv9cW8Y5i6PixtV4CQlVUvxB2iySwU94="></script>

(Whitespace added for clarity)

Assuming you have an ASSETS constant with the list of assets, you can use the following example in your Rakefile to re-generate your manifest when your assets change.

rule "public/manifest.json" => ASSETS do |t|
  sh "asset-manifest -d public #{t.prerequisites.join(" ")} > #{t.name}"
end

task assets: ASSETS
task assets: "public/manifest.json"

You can add this file to your assets so that the file is automatically generated when your source assets change.

If you use Cuba, then you can just do this:

require "asset_manifest/cuba"
Cuba.plugin AssetManifest::Cuba

This will set up an assets helper available in all your apps that you can use / pass to the templates.

This project is shared under the MIT license. See the attached LICENSE file for details.