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

Question

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?

Answer 1

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.