http-kit connection timeout handling, client tracking. ping-clients should probably be named differently. assumes traffic is json.

handler_clients.clj

(def clients (atom {}))

(defn remove-client [ch] (close ch)(swap! clients dissoc ch))

(defn purge-clients! [] 
  (doseq [c @clients] 
      (if (> (- (.getTime (java.util.Date.)) (.getTime (:last-seen (val c)))) 60000)
        (remove-client (key c)))))

(defn broadcast-clients [data] "pass map to be reconstructed as json"
   (doall(map #(send! (key %) (json/write-str data)) @clients)))

(def ping-clients (future (loop [] 
                            (Thread/sleep (* 1000 35)) 
                            (broadcast-clients {:ping ""})
                            (purge-clients!)
                            (recur) )))

;(future-cancel ping-clients)

(defn handler [req]
  (with-channel req ch
    (if (websocket? ch)
      (do
        (println "client connect " ch)
        (swap! clients conj (hash-map ch {:token nil,:last-seen (java.util.Date.)}))

        (on-close ch (fn [status] (println "client disconnect " status)
                       (if (= :normal status)  ;clean disconnect?
                         (do(println "removing client" ch)(swap! clients dissoc ch))
                         ;;note: delayed client removal may be unnecessary, depending on how mobile interacts while android GC rolls around
                         (do(println "delaying client removal")(swap! clients assoc-in [ch :last-seen] (java.util.Date.))) )))

        (on-receive ch (fn [data]
             (println "receiving data " ch)
             (let [rj (json/read-str data :key-fn keyword)]
                (if (:echo rj) (send! ch (json/write-str {:echo (:echo rj)})))
                (if (:broadcast rj) (broadcast-clients rj))
                (if (:pong rj) (swap! clients assoc-in [ch :last-seen] (java.util.Date.)))
                )))
        ))))

(defroutes all-routes
  (GET "/" [] #'handler))

(run-server (site #'all-routes) {:port 8080})