recur.cljs

(let [opacity-ch (async/chan (async/dropping-buffer 1))]

  (async/go-loop [last-set (:opacity @app-state)
                  last-received nil]
    (let [t (async/timeout 100)
          [ch val] (async/alts! [t opacity-ch])
          [new-set new-received] (cond
                                   (= ch opacity-ch)
                                   [last-set val]

                                   (not= last-set last-received)
                                   (do
                                     (swap! app-state assoc :opacity last-received)
                                     [last-received last-received])

                                   :else
                                   [last-set last-received])]
      (recur new-set new-received)))

  (defn opacity-change
    [e]
    (let [opacity (-> e .-target .-value (/ 100.0))]
      (async/go (>! opacity-ch opacity)))))