How can I use grunt-contrib-watch and grunt-contrib-coffee to compile CoffeeScript only as needed?

Question

I'd like to run coffee lint and coffee compile on only the single file that I'm saving. There are hundreds of CoffeeScript files in my project, and compiling all of them takes too much time.

Here's my Gruntfile:

module.exports = (grunt) ->

  grunt.initConfig

    pkg: grunt.file.readJSON 'package.json'

    coffee:
      all:
        expand: true
        bare: true
        cwd: 'src/coffeescript/'
        src: '**/*.coffee'
        dest: 'public/js/compiled'
        ext: '.js'

    coffeelint:
      all: ['src/coffeescript/**/*.coffee']

    watch:
      coffeescript:
        files: ['src/**/*.coffee']
        tasks: ['coffeelint', 'coffee']
        options:
          spawn: false

  grunt.event.on 'watch', (action, filepath) ->
    grunt.config(['coffeelint', 'all'], filepath)
    grunt.config(['coffee', 'all'], filepath)

  grunt.loadNpmTasks 'grunt-coffeelint'
  grunt.loadNpmTasks 'grunt-contrib-coffee'
  grunt.loadNpmTasks 'grunt-contrib-watch'

  grunt.registerTask 'default', ['coffeelint', 'coffee', 'watch']

The coffeelint task runs successfully only on the changed file.

The coffee compilation doesn't produce any JS files, even though grunt says it runs.

Here's the output after saving a single coffee file:

OK
>> File "src/coffeescript/app.coffee" changed.


Running "coffeelint:all" (coffeelint) task
>> 1 file lint free.

Running "coffee:all" (coffee) task

Running "watch" task
Completed in 0.009s at Sat Feb 01 2014 13:10:07 GMT-0600 (CST) - Waiting...

What's wrong here? Any help would be greatly appreciated!

Update:

Here's a working example:

module.exports = (grunt) ->

  fs = require 'fs'
  isModified = (filepath) ->
    now = new Date()
    modified =  fs.statSync(filepath).mtime
    return (now - modified) < 10000

  grunt.initConfig

    coffee:
      options:
        sourceMap: true
        bare: true
        force: true # needs to be added to the plugin
      all:
        expand: true
        cwd: 'src/coffeescript/'
        src: '**/*.coffee'
        dest: 'public/js/compiled'
        ext: '.js'
      modified:
        expand: true
        cwd: 'src/coffeescript/'
        src: '**/*.coffee'
        dest: 'public/js/compiled'
        ext: '.js'
        filter: isModified

    coffeelint:
      options:
        force: true
      all:
        expand: true
        cwd: 'src/coffeescript/'
        src: '**/*.coffee'
      modified:
        expand: true
        cwd: 'src/coffeescript/'
        src: '**/*.coffee'
        filter: isModified

    watch:
      coffeescript:
        files: ['src/**/*.coffee']
        tasks: ['coffeelint:modified', 'coffee:modified']

  grunt.loadNpmTasks 'grunt-coffeelint'
  grunt.loadNpmTasks 'grunt-contrib-coffee'
  grunt.loadNpmTasks 'grunt-contrib-watch'

  grunt.registerTask 'default', ['coffeelint:all', 'coffee:all', 'watch']

Answer 1

Try to add something like this to your c task

 coffee:
  all:
    filter: (filepath) ->
        fs = require('fs')
        now = new Date()
        modified =  fs.statSync(filepath).mtime
        return (now - modified) < 10000 # or another difference in millyseconds

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