Front-end Development 101

Part 2 (of 3): Using Grunt to automate CSS compilation

In the second part of this series, I'm taking a look at using Grunt to automate repetitive tasks and to automatically build artefacts when a source file changes. And we'll use Grunt to compile our stylesheets, from our own Less CSS source, and Twitter Bootstrap.

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Since I’m playing around with Go for back end development on my current project, I thought I’d investigate current practices for managing assets on the front end. This is part 2 of a rambling log of what I learned while I was playing around. I’d recommend reading part 1 for some background if you haven’t already.

Installing Grunt

Grunt is a task runner, which manages dependencies amongst tasks, just like make. It seems to be a rite of passage that every programming language on every platform must reinvent make. (I think it’s something to do with an allergic reaction to tab characters.) Let’s install grunt and make it a “development” dependency (i.e. a dependency that’s only required if I’m developing, or packaging, this application, not if I’m just running it in production):

npm install --save-dev grunt

This downloads and unpacks the grunt into the node_modules folder, and adds it to package.json. In addition, in order to run the grunt command line tool, I need to globally install the grunt-cli package:

npm install -g grunt-cli

All this command line tool does is to find the version of grunt that’s associated with the Gruntfile it’s attempting to use, then invoke it.

Building stylesheets with Less

Since Bootstrap is developed with Less (even if there is now an automagic Sass port for the Rails punters who like fewer dependencies), let’s use that for our own stylesheets, too. I’ll use Less’s import mechanism to generate a single CSS file. Since I’m a Rails weenie, and I’m quite happy with its folder structure, I’m looking to turn app/assets/stylesheets/application.less into public/assets/stylesheets/application.css.

Fortunately, grunt has an officially maintained module for that, called grunt-contrib-less. Let’s install that as a development dependency:

npm install --save-dev grunt-contrib-less

Now let’s configure grunt to take some of the grunt work out of producing CSS files. The bulk of the configuration happens in the Gruntfile. Now this file can either be Gruntfile.js or Gruntfile.coffee, depending on what you’d rather write. I’m quite fond of CoffeeScript, but most of the examples are in straight JS, so there’s a fine line between paths of insanity. Let’s give CoffeeScript a go. Create Gruntfile.coffee with the following content:

module.exports = (grunt) ->
  grunt.initConfig
    # Import package metadata from package.json, just in case it's useful.
    pkg: grunt.file.readJSON('package.json')

  grunt.loadNpmTasks('grunt-contrib-less')

All that remains to do is configure the less task. This code block is inserted as part of the JS object that’s passed as an argument to grunt.initConfig():

# less tasks for converting less source to CSS.
less:
  options:
    paths: [ 'bower_components' ]
  development:
    files:
      "public/assets/stylesheets/application.css": "app/assets/stylesheets/application.less"

It’s pretty straightforward, with two things happening:

  • The generated file in public/assets/stylesheets/application.css is generated from the source file, app/assets/stylesheets/application.less. (It in turn imports other files, but that’s our entry point into less-land.)

  • The bower_components folder is added to the search path.

The latter allows more sensible imports inside a less file, to be able to:

@import 'bootstrap/less/bootstrap.less';

instead of:

@import '../../../bower_components/bootstrap/less/bootstrap.less';

which is a bit unwieldy. I can regenerate the CSS by running grunt less:

grunt less
Running "less:development" (less) task
File public/assets/stylesheets/application.css created: 0 B → 21.4 kB

Done, without errors.

Reloading http://localhost:8080/ shows up the page with the stylesheet applied. I’ve got a simple set of stylesheets so far, split into a couple of files just to prove that less is successfully importing files. First of all, app/assets/stylesheets/application.less:

@import 'base.less';

Then app/assets/stylesheets/base.less:

// Pull in the minimal bits of Bootstrap I need right now:

// Core variables and mixins
@import 'bootstrap/less/mixins.less';
@import 'bootstrap/less/variables.less';

// Bootstrap reset
@import 'bootstrap/less/normalize.less';
@import 'bootstrap/less/print.less';

// Core CSS
@import 'bootstrap/less/scaffolding.less';
@import 'bootstrap/less/type.less';

// Customise some of the bootstrap variables.
@import 'variables.less';

And, finally, app/assets/stylesheets/variables.less:

@body-bg: #ccc;

I’ve also added /public/assets to my .gitignore so I don’t accidentally check in generated files if I can possibly avoid it.

Watching for file changes

The trouble is that the workflow here isn’t terribly elegant: every time I make a change to a less source file, I have to run grunt less, wait for it to complete, and reload the page to see the effect of the change. Let’s fix that with the grunt watch plugin (another one that’s officially maintained by the Grunt team). First of all, install it:

npm install --save-dev grunt-contrib-watch

Now to configure it, I’ve added the following configuration inside the grunt.initConfig() arguments:

# Watch source files for changes and rebuild the associated assets
watch:
  options:
    livereload: true
    spawn: false
  gruntfile:
    files: [ "Gruntfile.coffee"]
    options:
      reload: true
  stylesheets:
    files: [
      "app/assets/stylesheets/**/*.less",
      "bower_components/**/*.less"
    ]
    tasks: [ "less" ]

and load the module in the main module.exports method:

grunt.loadNpmTasks('grunt-contrib-watch')

This tells it to watch all the less files in app/assets/stylesheets and bower_components and, if any of them change, trigger the less task, which will rebuild them all. If, in future, I find this is taking too long to complete, and that I have different entry point stylesheets for different parts of the app, I can break this down to be more fine grained, but it’ll do nicely for now.

There’s also a task in there to watch for changes to the Gruntfile.coffee and reload, so I don’t have to restart it manually when I’m mucking around with it. Now start it up with:

grunt watch

Leave that running in a terminal. It’ll let you know when one or more tasks are triggered.

The watch plugin also has built in support for Live Reload so if you’ve got the browser plugin installed and enabled (I’m using the Google Chrome Live Reload extension it will automatically refresh your page when the stylesheets have been rebuilt.

Conclusion

That’s it for today. We’ve figured out how to use Grunt to automate repetitive tasks, and we’ve figured out how to build our stylesheets, starting with Twitter Bootstrap as a solid foundation. Not bad. Next time, we’ll finish off with a whirlwind tour of JavaScript, image optimisation, and a little bit on tidying up after ourselves.

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