How do you watch multiple files, but only run task on changed file, in Grunt.js?

In learning how to use grunt, I am trying to make a simple coffee-script watcher/compiler. The problem is, if I tell the watch task to watch several files, and one changes, it is going to pass all of the files to the coffee command. This means when you change 1 file, it's going to recompile all of the files matching the src pattern. Instead, I want to only recompile the single file that changed that matches the src pattern.

Here is the grunt.js:

module.exports = function(grunt) {
  grunt.initConfig({
    coffee: {
      app: {
        src: ['test/cases/controller/*.coffee'],
        dest: 'tmp',
        options: {
          bare: true,
          preserve_dirs: true
        }
      }
    },
    watch: {
      files: ['<config:coffee.app.src>'],
      tasks: ['coffee:app']
    }
  });

  grunt.loadNpmTasks('grunt-coffee');
  grunt.registerTask('default', 'coffee');
};

This is using grunt-coffee, which is basically this: https://gist.github.com/2373159.

When I run grunt watch, and I save a file in test/cases/controller/*.coffee, it compiles all of the matching files (putting them in tmp/*).

How do you instead only compile the changed file using grunt?

The upcoming (and currently in-development) v0.4.0a grunt has the grunt.file.watchFiles object, which was designed expressly for this purpose. The grunt-coffee plugin may have added support for this feature already, I'm not sure.

Either way, if you're interested in trying an in-development version of grunt in your project, check out the When will I be able to use in-development feature 'X'? FAQ entry.

I got this working when compiling my less files. You should be able to mess with this configuration a little bit to git it working with the coffeescript plugin. The portion of interest is the grunt.event.on('watch', ...). In this event handler I'm updating the files property in the less command to only contain the changed file.

path = require('path');

module.exports = function(grunt) {

  // Project configuration.
  grunt.initConfig({

    pkg: grunt.file.readJSON('package.json'),

    less: {
      development: {
        options: {
          paths: ["./library/less"],
        },
        files: [
          { src: "./library/less/bootstrap.less", dest: "./library/css/bootstrap.css"},
          { src: "./library/less/app.less", dest: "./library/css/app.css"}
        ]
      }
    },

    watch: {
      styles: {
        files: "./library/less/*",
        tasks: ["less"],
        options: {
          nospawn: true,
        },
      },
    },
  });

  // Event handling
  grunt.event.on('watch', function(action, filepath){
    // Update the config to only build the changed less file.
    grunt.config(['less', 'development', 'files'], [
      {src: filepath, dest: './library/css/' + path.basename(filepath, '.less') + '.css'}
    ]);
  });

  // Load the plugins
  grunt.loadNpmTasks('grunt-contrib-less');
  grunt.loadNpmTasks('grunt-contrib-watch');

  // Tasks
  grunt.registerTask('default', ['watch']);
};

None of these answers worked very well for me. Here is my solution if anyone is interested (I know I'm a little late in replying to this question).

https://gist.github.com/ryansmith94/8569178

In this issue, Kyle Robinson suggests using the watchevent. It's very important to set watch task nospawn property to true to make it work. I modified his solution to selectively run the tasks:

grunt.event.on('watch', function(action, filepath) {
    if (minimatch(filepath, grunt.config('watch.stylesheets.files'))) {
        grunt.config('compass.dist.options.specify', [filepath]);
    }

    if (minimatch(filepath, grunt.config('watch.scripts.files'))) {
        var uglifySrc = filepath.replace(grunt.config('uglify.dist.cwd'), '');
        grunt.config('jshint.dist.src', [filepath]);
        grunt.config('uglify.dist.src', [uglifySrc]);
    }
});

Here is the complete solution: https://gist.github.com/luissquall/5408257