Question

It is often a pattern that I wish to poll a file for changes (when it was last written). When the file does change from its previous value, I wish to execute some function. Something of the form.

(poll-for-changes file-str on-change-fx current-value)

where

1. file-str is just a string that specifies the files location
2. on-change-fx is the function that should be called when the file at file-str changes. Let us say that the on-change-fx should take the File object pointing to file-str as a argument.
3. current-value the current value of the file in milliseconds. You might set to 0 to guarantee that this function will run at least once, or to the actual value to only run this function when you actually detect a change.

I would just like this function implemented in the clearest, most concise, Clojurist way possible. Thank you.

Answer 1

If you’re looking to poll a directory or files and act on it, I think watchtower is pretty good to look at.

Java 7 has a WatchService, which uses file system events to react to changes. In this case, you don’t poll at all, but block on a future file event. I don’t think there are any projects in Clojure that are out there leveraging that, although I spent some time toying with it to write a small library. The source for it is here

I don’t claim my library is even complete, but it does use the Java 7 service, so you could use that for inspiration on your own project.