router.cljs

(defn tap
  ([in f] (tap in f (chan)))
  ([in f out]
    (go (loop []
          (let [v (<! in)]
            (if-not (nil? v)
              (do (f v)
                (>! out v)
                (recur))
              :done))))
    out))

(defprotocol IObservable
  (-subscribe! [observable key chan])
  (-unsubscribe! [observable key chan]))

(defn dispatch
  ([lookup] (dispatch lookup {}))
  ([lookup coll]
    (let [table (atom coll)]
      (reify
        IObservable
        (-subscribe! [_ key chan]
          (swap! table update-in [key]
            (fnil conj []) chan))
        (-unsubscribe! [_ key chan]
          (swap! table update-in [key]
            #(into [] (filter #{chan} (get % key)))))
        IFn
        (-invoke [_ v]
          (doseq [c (get @table (lookup v))]
            (put! c v)))))))