timer.clj

(ns timer
  (:import [java.util Timer TimerTask]))

(def the-timer (Timer.))

(defn timer
  "Schedules a callback for delayed execution.
   Returns a function, that accepts a parameter:
     :bump to restart the timer, returning true only if timer was active, hence was extended by that call
     :cancel to abort the timer, returning true only if timer was active, hence was canceled b.t.c."
  [delay callback]
  (let [current (atom 0) ; bump counter, nil is special for a disabled timer
	create-task (fn [rev]
		      (proxy [TimerTask] []
					; only run, when this handler is the latest
			(run [] (when (compare-and-set! current rev nil)
				  (callback)))))
	
	ctl (fn [action]
	      (condp = action
		:bump (when-let [in-current @current]
			(when (compare-and-set! current in-current (inc in-current))
			  (.schedule the-timer 
				     (create-task (inc in-current))
				     (long delay))
			  true))
		
		:cancel (loop [] (when-let [in-cur @current] ; :cancel
				   (if (compare-and-set! current in-cur nil)
				     true (recur))))))]
    (ctl :bump)
    ctl))