Transducer-based renderer about cljfx HOT 1 OPEN

(defn observe [xf f source]
  (let [key (Object.)
        f (xf f)]
    (add-watch source key (fn [_ _ _ v]
                            (if (reduced? v)
                              (do
                                (remove-watch source key)
                                (f @v))
                              ;; nil? there is some state we are reducing?..
                              (f nil v))))
    (f nil @source)))