In grunt, if I'm watching multiple files and two or more change, how can I only run tasks on the changed files?

In grunt, if I'm watching multiple files and two or more change, how can I
only run tasks on the changed files?

I've got an initConfig with this code in it:
grunt.initConfig({
pkg: grunt.file.readJSON('package.json'),
watch: {
options: {
spawn: false
},
coffee: {
files: [
'src/**/*.coffee'
],
tasks: ['coffee', 'coffeelint', 'concat', 'qunit']
},
...
coffee: {
glob_to_multiple: {
expand: true,
flatten: false,
cwd: '.',
src: ['src/**/*.coffee'],
ext: '.js'
}
},
...
grunt.event.on('watch', function (action, filepath) {
if (grunt.file.isMatch("**/*.coffee", filepath)) {
grunt.config(['coffee', 'glob_to_multiple', 'src'], filepath);
}
});
This is supposed to compile only the .coffee files that have changed. This
works pretty well. But I just noticed that if I modify multiple at once,
it will output this:
Waiting...src\test\resources\app\js\FILE1.coffee
src\main\resources\app\js\FILE2.coffee
OK
>> File "src\test\resources\app\js\FILE1.coffee" changed.
>> File "src\main\resources\app\js\FILE2.coffee" changed.
Running "coffee:glob_to_multiple" (coffee) task
File src/main/resources/app/js/FILE2.js created.
...
As you can see, I've changed two files, but it's only running the tasks on
"FILE2.js". How can I avoid this? I want it to run coffee:glob_to_multiple
on FILE1 and FILE2, not just one of them.



NOTE: I'm pretty sure the documentation explains how to do this:
If you save multiple files simultaneously you may opt for a more robust
method:
var changedFiles = Object.create(null);
var onChange = grunt.util._.debounce(function() {
grunt.config(['jshint', 'all'], Object.keys(changedFiles));
changedFiles = Object.create(null);
}, 200);
grunt.event.on('watch', function(action, filepath) {
changedFiles[filepath] = action;
onChange();
});
But I'm not sure how this works. Could someone explain it to me?