'How to use tailwind css gem in a rails 7 engine?

How to use tailwind in a rails engine? According to the documentation supplying a css argument to the Rails generator should work

Rails 7.0.2.2 engine generated using

rails plugin new tailtest --mountable --full -d postgresql --css tailwind

This generates the engine with Postgresql but does nothing with tailwind at all, and following manual installation instructions fail too.

Running, as per documentation, bundle add tailwindcss-rails adds tailwind to the gemfile rather than the engines tailtest.gemspec So after adding the dependency to the gemspec

spec.add_dependency "tailwindcss-rails", "~> 2.0"

and running bundle install does install the engine however the rest of the manual installation fails

then adding the require to lib/engine.rb

require "tailwindcss-rails"
module Tailtest
  class Engine < ::Rails::Engine
    isolate_namespace Tailtest
  end
end

then running the install process fails

rails tailwindcss:install
Resolving dependencies...
rails aborted!
Don't know how to build task 'tailwindcss:install' (See the list of available tasks with `rails --tasks`)
Did you mean?  app:tailwindcss:install

Obviously the app:tailwindcss:install command fails too.

So I am probably missing an initializer of some sort in the engine.rb file but no idea on what it should be.

Solution 1:^[1]

It is the same idea as How to set up importmap-rails in Rails 7 engine?. We don't need to use the install task. Even if you're able to run it, it's not helpful in the engine (see the end of the answer for explanation).

Also rails plugin new doesn't have a --css option. To see available options: rails plugin new -h.

Update engine's gemspec file:

# my_engine/my_engine.gemspec

spec.add_dependency "tailswindcss-rails"

Update engine.rb:

# my_engine/lib/my_engine/engine.rb

module MyEngine
  class Engine < ::Rails::Engine
    isolate_namespace MyEngine

    # NOTE: add engine manifest to precompile assets in production, if you don't have this yet.
    initializer "my-engine.assets" do |app|
      app.config.assets.precompile += %w[my_engine_manifest]
    end
  end
end

Update assets manifest:

# my_engine/app/assets/config/my_engine_manifest.js

//= link_tree ../builds/ .css

Update engine's layout:

# my_engine/app/views/layouts/my_engine/application.html.erb

<!DOCTYPE html>
<html>
  <head>
   <%# 
       NOTE: make sure this name doesn't clash with anything in the main app.
             think of it as `require` and `$LOAD_PATH`,
             but instead it is `stylesheet_link_tag` and `manifest.js`.
    %>
    <%= stylesheet_link_tag "my_engine", "data-turbo-track": "reload" %>
  </head>
  <body> <%= yield %> </body>
</html>

bundle show command will give us the path where the gem is installed, so we can copy a few files:

$ bundle show tailwindcss-rails
/home/alex/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/tailwindcss-rails-2.0.8-x86_64-linux

Copy tailwind.config.js file from tailswindcss-rails:

$ cp $(bundle show tailwindcss-rails)/lib/install/tailwind.config.js config/tailwind.config.js

Copy application.tailwind.css file into any directory to fit your setup:

$ cp $(bundle show tailwindcss-rails)/lib/install/application.tailwind.css app/assets/stylesheets/application.tailwind.css

Because tailwindcss-rails uses standalone executable, we don't need node or rails to compile the stylesheets. We just need to get to the executable itself.

Executable is located here https://github.com/rails/tailwindcss-rails/tree/v2.0.8/exe/. Instead of running the build task https://github.com/rails/tailwindcss-rails/blob/v2.0.8/lib/tasks/build.rake we can just call the executable directly.

$ $(bundle show tailwindcss-rails)/exe/tailwindcss -i app/assets/stylesheets/application.tailwind.css -o app/assets/builds/my_engine.css -c config/tailwind.config.js --minify

Use -w option to start watch mode.

$ $(bundle show tailwindcss-rails)/exe/tailwindcss -i app/assets/stylesheets/application.tailwind.css -o app/assets/builds/my_engine.css -c config/tailwind.config.js --minify -w

The output file should match the name in stylesheet_link_tag "my_engine".

Now that you have a plain my_engine.css file, do with it what you want. Use it in the layout, require it from the main app application.css. The usual rails asset pipeline rules apply.

If you want to put all that into a task, use Engine.root to get the paths.

# my_engine/lib/tasks/my_engine.rake

task :tailwind_engine_watch do
  require 'tailwindcss-rails'
  # NOTE: tailwindcss-rails is an engine
  system "#{Tailwindcss::Engine.root.join("exe/tailwindcss")} \
         -i #{MyEngine::Engine.root.join("app/assets/stylesheets/application.tailwind.css")} \
         -o #{MyEngine::Engine.root.join("app/assets/builds/my_engine.css")} \
         -c #{MyEngine::Engine.root.join("config/tailwind.config.js")} \
         --minify -w"
end

From the engine directory:

$ bin/rails app:tailwind_engine_watch
+ /home/alex/.rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/tailwindcss-rails-2.0.8-x86_64-linux/exe/x86_64-linux/tailwindcss -i /home/alex/code/stackoverflow/my_engine/app/assets/stylesheets/application.tailwind.css -o /home/alex/code/stackoverflow/my_engine/app/assets/builds/my_engine.css -c /home/alex/code/stackoverflow/my_engine/config/tailwind.config.js --minify -w

Rebuilding...
Done in 549ms.

Make your own install task if you have a lot of engines to set up:

desc "Install tailwindcss into our engine"
task :tailwind_engine_install do
  require 'tailwindcss-rails'

  # NOTE: use default app template, which will fail to modify layout, manifest,
  #       and the last command that compiles the initial `tailwind.css`.
  #       It will also add `bin/dev` and `Procfile.dev` which we don't need.
  #       Basically, it's useless in the engine as it is.
  template = Tailwindcss::Engine.root.join("lib/install/tailwindcss.rb")

  # TODO: better to copy the template from 
  #       https://github.com/rails/tailwindcss-rails/blob/v2.0.8/lib/install/tailwindcss.rb
  #       and customize it
  # template = MyEngine::Engine.root("lib/install/tailwindcss.rb")

  require "rails/generators"
  require "rails/generators/rails/app/app_generator"
  
  # NOTE: because the app template uses `Rails.root` it will run the install
  #       on our engine's dummy app. Just override `Rails.root` with our engine
  #       root to run install in the engine directory.
  Rails.configuration.root = MyEngine::Engine.root

  generator = Rails::Generators::AppGenerator.new [Rails.root], {}, { destination_root: Rails.root }
  generator.apply template
end

Install task reference:

https://github.com/rails/rails/blob/v7.0.2.4/railties/lib/rails/tasks/framework.rake#L8

https://github.com/rails/tailwindcss-rails/blob/v2.0.8/lib/tasks/install.rake

Watch task reference:

https://github.com/rails/tailwindcss-rails/blob/v2.0.8/lib/tasks/build.rake#L10