Asset Manifest

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

How it works

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.

Usage

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)

With Rake instead of Make

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.

Cuba plugin

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.

License

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