todomvc.subs.cljs

>  This Gist is out of date. It has been superseded by  
>  https://gist.github.com/mike-thompson-day8/d61692451166e206a9c43771c8b389fc

;; WARNING:  this code is untested. In fact not even compiled.  
;; It is just meant to be indicative of how `regsub` might work/look. 

(ns todomvc.subs
  (:require-macros [reagent.ratom :refer [reaction]])
  (:require [re-frame.core :refer [register-sub]]))

(regsub :showing
  (fn [db _]        ;; by default, the first parameter, db, is the value in app-db  (not app-db itself which is a ratom)
    (:showing db))) ;; I repeat:  db is a value. Not a ratom.  And this fn doesnot return a reaction

(regsub :sorted-todos
  (fn [[db] _]
    (:todos db)))

;; A subscription is effectively a handler functions which is re-run
;; whenever its inputs change. 
;; In the two examples above, the handler will rerun whenever `app-db` changes, and
;; each time it re-runs the new value in `app-db` ius supplied as the first parameter. 
;; But many subscriptions are not directly dependent on app-db, and instead, dedpend on 
;; some value derived from app-db.  These handler represent "intermediate nodes" in the 
;; signal graph.  Values flow this signal graph. 
;; When writting the handler for such an intermediate node, you must nominate the 
;; input signal. 
;; It recomoputes a new value.  Which is then reactively passed on to 
;; further downstream nodes in the signal graph.
;; The root of the XXXXX
(regsub :todos
  :<- [:sorted-todo]       ;; this subscription itself depends on an input value  (subscribe [:sorted-todo])
  (fn [sorted-todos _]     ;; this signal is created by  
    (vals sorted-todos)))

(regsub :visible-todos
  :<- [:todos]
  :<- [:showing]
  (fn [[todos showing] _]
    (let [filter-fn (case showing
                      :active (complement :done)
                      :done   :done
                      :all    identity)]
      (filter filter-fn todos))))

(regsub :completed-count
  :<- [:todos]
  (fn [todos _]
    (count (filter :done todos))))

(regsub :footer-stats                     ;; different from original. Now does not return showing
  :<- [:todos]
  :<- [:completed-count]
  (fn [[todos completed] _]
    [(- (count todos) completed) completed]))