Grunt: Watch multiple files, Compile only Changed

Question

I'm new to Grunt, and so far I'm enjoying it very much. I want Grunt to compile only the changed files when running grunt watch

In my Grunfile.coffee I currently have (relevant parts).
Note: assets/javascript/app.coffee and assets/javascript/app.js are directories

    coffee:
        default:
            expand: true
            cwd: "assets/javascript/app.coffee"
            src: ["*.coffee"]
            dest: "assets/javascript/app.js"
            ext: ".js"

    uglify:
        dev:
            options:
                beautify: true
                compress: false
                mangle: false
                preserveComments: 'all'

            files: 
                "js/app.js": "assets/javascript/app.js/*.js"
                "js/libs.js": "assets/javascript/libs/*.js"

    watch:
        coffeescript:
            files: 'assets/javascript/**/*.coffee'
            tasks: ["coffee"]

        javascript:
            files: "assets/**/*.js"
            tasks: ["uglify:dev"]
        livereload:
            files: ["Gruntfile.coffee", "js/*.js", "*.php", "css/*.css", "images/**/*.{png,jpg,jpeg,gif,webp,svg}", "js/*.js", ]
            options:
                livereload: true

There is probably a shorter way around, but I'm compiling app.coffee to app.js first, so that after I distribute my work, people who aren't comfortable with Coffeescript can browse the code in somewhat reasonable manner.

The problem with all this is that now that I save a Coffeescript file, I get too many steps ( I think ):

>> File "assets/javascript/app.coffee/browse.coffee" changed.

Running "coffee:default" (coffee) task
File assets/javascript/app.js/browse.js created.
File assets/javascript/app.js/filters.js created.

Done, without errors.
Completed in 0.837s at Tue May 28 2013 12:30:18 GMT+0300 (EEST) - Waiting...
OK
>> File "assets/javascript/app.js/browse.js" changed.
>> File "assets/javascript/app.js/filters.js" changed.

Running "uglify:dev" (uglify) task
File "js/app.js" created.
File "js/libs.js" created.

Done, without errors.
Completed in 0.831s at Tue May 28 2013 12:30:19 GMT+0300 (EEST) - Waiting...
OK
>> File "js/app.js" changed.
>> File "js/libs.js" changed.

Completed in 0.000s at Tue May 28 2013 12:30:19 GMT+0300 (EEST) - Waiting...

Currently I'm just setting up my project, but I will have a lot more Coffeescript files, and I don't want Coffeescript to recompile all of the files, on each file change.

Furthermore, libs.js has no part in all of this at all, but I guess it is still compiled, because it also matches the "assets/*/.js" pattern.

Is there a way to make Grunt compile only the files that have changed ?

Answer 1

I've finally found a real solution! And it's super simple too!

npm install grunt-newer --save-dev

Then in your Gruntfile (after loading the task in grunt):

watch:
    coffeescript:
        files: 'assets/javascript/**/*.coffee'
        tasks: ["newer:coffee"]

And that's it! The Awesome grunt-newer is awesome!