why_update.cljs

(ns mk.fe.core.why-update
  (:require [cljsjs.react]))

(defn comp-did-update [prev-props prev-state]
  (this-as this
    (let [name       (or (aget this "displayName")
                       (aget this "constructor" "displayName")
                       (aget this "constructor" "name"))
          prev-state (aget prev-state ":rum/state")
          state      (aget (.-state this) ":rum/state")]
      (when (= prev-state state)
        (println name "update was avoidable!")))))

(def orig-create-class js/React.createClass)
(def orig-did-update js/React.Component.prototype.componentDidUpdate)
(def Mixin #js {:componentDidUpdate comp-did-update})

(defn enable! []
  (set! js/React.createClass
    (fn [obj]
      (when (nil? (aget obj "mixins"))
        (aset obj "mixins" #js []))
      (aset obj "mixins" (.concat #js [Mixin] (aget obj "mixins")))
      (.call orig-create-class js/React obj)))
  :wow)

(defn disable! []
  (set! js/React.createClass orig-create-class))

(defonce much (enable!))