(ns re-frame.subs
 (:require
   [reagent.ratom  :refer [make-reaction] :refer-macros [reaction run!]]
   [re-frame.db    :refer [app-db]]
   [re-frame.utils :refer [first-in-vector warn error]]))


;; maps from handler-id to handler-fn
(def ^:private key->fn (atom {}))


(defn clear-handlers!
  "Unregisters all subscription handlers"
  []
  (reset! key->fn {}))


(defn register
  "Registers a handler function for an id"
  [key-v handler-fn]
  (if (contains? @key->fn key-v)
    (warn "re-frame: overwriting subscription-handler for: " key-v))   ;; allow it, but warn.
  (swap! key->fn assoc key-v handler-fn))


(defn subscribe
  "Returns a reagent/reaction which observes a part of app-db"
  ([v]
   (let [key-v (first-in-vector v)
         handler-fn (get @key->fn key-v)]
     (if (nil? handler-fn)
       (error "re-frame: no subscription handler registered for: \"" key-v "\". Returning a nil subscription.")
       (handler-fn app-db v))))
  ([v dynv]
   (let [key-v (first-in-vector v)
         handler-fn (get @key->fn key-v)]
     (if (nil? handler-fn)
       (error "re-frame: no subscription handler registered for: \"" key-v "\". Returning a nil subscription.")
       (let [dyn-vals (reaction (mapv deref dynv))
             sub (reaction (handler-fn app-db v @dyn-vals))]
         ;; handler-fn returns a reaction which is then wrapped in the sub reaction
         ;; need to double deref it to get to the actual value.
         (reaction @@sub))))))