Perf: Defer per-row reactive allocations [perf-eval] (#218)

Author: jlukic

packages/reactivity/src/reaction.js

@@ -7,34 +7,33 @@ export class Reaction {
   static create(callback, options = {}) {
     const reaction = new Reaction(callback, options);
     if (options.firstRun !== false) {
-      reaction.boundRun();
+      reaction.run();
     }
     return reaction;
   }
 
   constructor(callback, { context } = {}) {
     this.callback = callback;
     this.dependencies = new Set();
-    this.cleanups = [];
+    this.cleanups = null; // lazy — most reactions register none
     this.firstRun = true;
     this.active = true;
     if (context && isTracing()) {
       this.setContext(context);
     }
-    this.boundRun = this.run.bind(this);
   }
 
   // callbacks fire before next run() and on stop. use to scope inner reactions to parent
   onCleanup(callback) {
-    this.cleanups.push(callback);
+    (this.cleanups ??= []).push(callback);
   }
 
   fireCleanups() {
-    if (this.cleanups.length === 0) {
+    if (this.cleanups === null) {
       return;
     }
     const callbacks = this.cleanups;
-    this.cleanups = [];
+    this.cleanups = null;
     for (let i = 0; i < callbacks.length; i++) {
       callbacks[i]();
     }

packages/renderer/src/engines/native/reactive-context.js

@@ -145,7 +145,7 @@ function trapGet(target, prop) {
     }
     return values[prop];
   }
-  if (!target.keysSealed) { target.keySetVersion.depend(); }
+  if (!target.keysSealed) { (target.keySetVersion ??= new Dependency()).depend(); }
   return target.parent[prop];
 }
 
@@ -269,7 +269,11 @@ export class ReactiveDataContext {
     // userland breaks Signal.equalityFunction after this RDC is live,
     // both Signal and RDC fail the same way; no divergence.
     this.equalityFunction = Signal.equalityFunction;
-    this.keySetVersion = new Dependency();
+    // Lazy: only a reader falling through on the unsealed path allocates it
+    // (spread-mode late keys). as-mode reads run post-seal, so it stays null —
+    // the per-row saving. A writer fires it only if a reader already created it
+    // (a changed() on a never-subscribed dep was always a no-op).
+    this.keySetVersion = null;
     this.proxy = new Proxy(this, HANDLER);
 
     if (registerItemContext) {
@@ -288,7 +292,7 @@ export class ReactiveDataContext {
       if (key !== this.asKey || value === null || typeof value !== 'object') {
         this.deps[key] = new Dependency();
       }
-      if (!this.keysSealed) { this.keySetVersion.changed(); }
+      if (!this.keysSealed && this.keySetVersion !== null) { this.keySetVersion.changed(); }
       return;
     }
     const old = this.values[key];