Render without allocating ratom unless necessary

src/reagent/impl/component.cljs

@@ -128,16 +128,22 @@
             (do-render C))
           res)))))
 
+(defn run-reactively [C run on-dirty]
+  (let [rat (.-cljsRatom C)]
+    (if (nil? rat)
+      (let [res (ratom/capture-derefed run C)
+            derefed (.-captured C)]
+        (when (and (not (nil? derefed))
+                   tmpl/isClient)
+          (set! (.-cljsRatom C)
+                (ratom/make-reaction run
+                                     :auto-run on-dirty
+                                     :derefed derefed)))
+        res)
+      (ratom/run rat))))
+
 (defn reactive-render [C]
-  (assert C)
-  (when (nil? (.-cljsRatom C))
-    (set! (.-cljsRatom C)
-          (ratom/make-reaction
-           #(do-render C)
-           :auto-run (if tmpl/isClient
-                       #(queue-render C)
-                       identity))))
-  (ratom/run (.-cljsRatom C)))
+  (run-reactively C #(do-render C) #(queue-render C)))
 
 
 ;;; Function wrapping
@@ -179,7 +185,9 @@
 
     :componentWillUnmount
     (fn [C]
-      (ratom/dispose! (.-cljsRatom C))
+      (let [ratom (.-cljsRatom C)]
+        (if-not (nil? ratom)
+          (ratom/dispose! ratom)))
       (set! (.-cljsIsDirty C) false)
       (when f (f C)))
 

src/reagent/ratom.cljs

@@ -4,11 +4,13 @@
 
 (declare ^:dynamic *ratom-context*)
 
+(def debug false)
+
 (def -running (clojure.core/atom 0))
 
 (defn running [] @-running)
 
-(defn- capture-derefed [f obj]
+(defn capture-derefed [f obj]
   (set! (.-captured obj) nil)
   (binding [*ratom-context* obj]
     (f)))
@@ -115,7 +117,7 @@
       (when (not= derefed watching)
         (-update-watching this derefed))
       (when-not active?
-        (swap! -running inc)
+        (when debug (swap! -running inc))
         (set! active? true))
       (set! dirty? false)
       (set! state res)
@@ -142,7 +144,7 @@
     (set! state nil)
     (set! dirty? true)
     (when active?
-      (swap! -running dec)
+      (when debug (swap! -running dec))
       (set! active? false))
     (when on-dispose
       (on-dispose)))
@@ -159,8 +161,14 @@
   IHash
   (-hash [this] (goog/getUid this)))
 
-(defn make-reaction [f & {:keys [auto-run on-set on-dispose]}]
-  (let [runner (if (= auto-run true) run auto-run)]
-    (Reaction. f nil true false
-               nil {}
-               runner on-set on-dispose)))
+(defn make-reaction [f & {:keys [auto-run on-set on-dispose derefed]}]
+  (let [runner (if (= auto-run true) run auto-run)
+        active (not (nil? derefed))
+        dirty (not active)
+        reaction (Reaction. f nil dirty active
+                            nil {}
+                            runner on-set on-dispose)]
+    (when-not (nil? derefed)
+      (when debug (swap! -running inc))
+      (-update-watching reaction derefed))
+    reaction))

test/testratom.cljs

@@ -6,6 +6,8 @@
   (:require [cemerick.cljs.test :as t]
             [reagent.ratom :as rv]))
 
+(set! rv/debug true)
+
 (defn running [] (rv/running))
 (defn dispose [v] (rv/dispose! v))