Optimise debouncing by checking if a debounce is already scheduled

This lets us avoid making 100s of calls to set and cancel the timeout
on every debounce call. The tradeoff is accuracy of debounce time,
but that is not important to us here.

src/re_frame/trace.cljc

@@ -24,6 +24,7 @@
 
 (def trace-cbs (atom {}))
 (defonce traces (atom []))
+(defonce next-delivery (atom 0))
 
 (defn register-trace-cb
   "Registers a tracing callback function which will receive a collection of one or more traces.
@@ -47,11 +48,25 @@
    :child-of  (or child-of (:id *current-trace*))
    :start     (interop/now)})
 
+;; On debouncing
+;;
+;; We debounce delivering traces to registered cbs so that
+;; we can deliver them in batches. This aids us in efficiency
+;; but also importantly lets us avoid slowing down the host
+;; application by running any trace code in the critical path.
+;;
+;; We add a lightweight check on top of goog.functions/debounce
+;; to avoid constant setting and cancelling of timeouts. This
+;; means that we will deliver traces between 10-50 ms from the
+;; last trace being created, which still achieves our goals.
+
+(def debounce-time 50)
+
 (defn debounce [f interval]
   #?(:cljs (goog.functions/debounce f interval)
      :clj  (f)))
 
-(def run-tracing-callbacks!
+(def schedule-debounce
   (debounce
     (fn tracing-cb-debounced []
       (doseq [[k cb] @trace-cbs]
@@ -61,8 +76,21 @@
              #?(:cljs (catch :default e
                         (console :error "Error thrown from trace cb" k "while storing" @traces e)))))
       (reset! traces []))
-    50))
+    debounce-time))
 
+(defn run-tracing-callbacks! [now]
+  ;; Optimised debounce, we only re-debounce
+  ;; if we are close to delivery time
+  ;; to avoid constant setting and cancelling
+  ;; timeouts.
+
+  ;; If we are within 10 ms of next delivery
+  (when (< (- @next-delivery 10) now)
+    (schedule-debounce)
+    ;; The next-delivery time is not perfectly accurate
+    ;; as scheduling the debounce takes some time, but
+    ;; it's good enough for our purposes here.
+    (reset! next-delivery (+ now debounce-time))))
 
 (macros/deftime
   (defmacro finish-trace [trace]
@@ -72,7 +100,7 @@
           (swap! traces conj (assoc ~trace
                                :duration duration#
                                :end (interop/now)))
-          (run-tracing-callbacks!))))
+          (run-tracing-callbacks! end#))))
 
  (defmacro with-trace
      "Create a trace inside the scope of the with-trace macro