Is there a way to be notified when a clojure future finishes?

Question

Is there a way to set a watch on a future so that it triggers a callback when it is done?

something like this?

> (def a (future (Thread/sleep 1000) \"         


        

Answer 1

An extension to the accepted answer

please note the following caveat:

with the above when-done implementation the callback will not be invoked if the future is cancelled. That is

(do
  (def f0 (future (Thread/sleep 1000)))
  (when-done f0 (fn [e] (println "THEN=>" e)))
  (Thread/sleep 500)
  (future-cancel f0))

will not print because the deref call on the cancelled future will raise an exception.

If you need callbacks to happen i suggest:

(defn then
  "Run a future waiting on another, and invoke
  the callback with the exit value if the future completes,
  or invoke the callback with no args if the future is cancelled"
  [fut cb]
  (future
    (try
      (cb @fut)
      (catch java.util.concurrent.CancellationException e
        (cb)))))

So now this will print:

  (do
    (def f0 (future (Thread/sleep 1000)))
    (then f0 (fn
               ([] (println "CANCELLED"))
               ([e] (println "THEN=>" e))))
    (Thread/sleep 500)
    (future-cancel f0))