Add athens.tracing dev tool for profiling

Added athens.tracing which depends on tufte for profiling CLJS
functions. It's not a great integration, as currently Athens is on an
old version of re-frame-10x that does not have the appropriate
monkeypatchin hooks.

The assumption is that a developer will enable it by uncommenting the
`athens.tracing` in shadow-cljs preloads and checking the results in
console.log.

project.clj

@@ -68,6 +68,7 @@
    {:dependencies [[binaryage/devtools "1.0.0"]
                    [day8.re-frame/re-frame-10x "0.6.0"]
                    [day8.re-frame/tracing "0.5.3"]
+                   [com.taoensso/tufte "2.2.0"]
                    [cider/cider-nrepl "0.25.1"]]
 
     :source-paths ["dev"]}

shadow-cljs.edn

@@ -29,7 +29,8 @@
                                         :closure-defines {re-frame.trace.trace-enabled? true}
                                         :output-feature-set :es-next}
                      :devtools {:preloads [devtools.preload
-                                           day8.re-frame-10x.preload]}}
+                                           day8.re-frame-10x.preload
+                                           #_athens.tracing]}}
 
           ;; backend for electron (node.js)
           :main {:target :node-script

src/cljs/athens/tracing.cljs

@@ -0,0 +1,62 @@
+(ns athens.tracing
+  (:require
+   [reagent.impl.batching :as batching]
+   ;; This exists only in newer versions of re-frame-10x
+   ;; [day8.reagent.impl.batching :as day8.batching]
+   [taoensso.tufte :as tufte]))
+
+
+(defonce reagent-next-tick batching/next-tick)
+
+;; This exists only in newer version of re-frame-10x:
+;; (defonce reframe10x-next-tick day8.batching/next-tick)
+
+
+(defonce stats-accumulator
+  (tufte/add-accumulating-handler! {:ns-pattern "*"}))
+
+
+(defonce stats-total
+  (atom nil))
+
+
+(defn format-stats [stats]
+  (-> stats
+      (tufte/format-pstats
+       {:columns [:n-calls :p50 :p95 :p99 :max :clock :total]
+        :format-id-fn str})
+      (println)))
+
+
+(defn next-tick
+  [f]
+  ;; Schedule a trace to be emitted after a render if
+  ;; there is nothing else scheduled after that render.
+  ;; This signals the end of the epoch.
+
+  (reagent-next-tick
+   ;; TODO: ideally, should also be wrapping re-frame-10x:
+   ;; reframe10x-next-tick day8.batching/next-tick
+   (fn []
+     (tufte/profile {} (f))
+     (when (false? (.-scheduled? batching/render-queue))
+       (when-some [m (not-empty @stats-accumulator)]
+         (when (some-> m vals first deref :stats)
+           (let [stats (-> m vals first)]
+             (println "Epoch Stats:")
+             (format-stats stats)
+             (if @stats-total
+               (swap! stats-total tufte/merge-pstats stats)
+               (reset! stats-total stats))
+             (println "Total Stats:")
+             (format-stats @stats-total))))))))
+
+
+(defn patch-next-tick
+  []
+  (println "Patching with tufte profiling")
+  (set! batching/next-tick next-tick))
+
+
+;; TODO: assuming this namespace will only be loaded via shadow preloads
+(patch-next-tick)