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

Answer 1

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