Prevent View Preallocation (#45163)

Summary:
Pull Request resolved: #45163

Avoid view preallocation when rendering on the main thread

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D58833983

fbshipit-source-id: a942d1fac684be5a8073941dbf043ba1d738e3a0

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -764,6 +764,14 @@ private void preallocateView(
             isLayoutable));
   }
 
+  @SuppressLint("NotInvokedPrivateMethod")
+  @SuppressWarnings("unused")
+  @AnyThread
+  @ThreadConfined(ANY)
+  private boolean isOnMainThread() {
+    return UiThreadUtil.isOnUiThread();
+  }
+
   @SuppressWarnings("unused")
   @AnyThread
   @ThreadConfined(ANY)

packages/react-native/ReactAndroid/src/main/jni/react/fabric/Binding.cpp

@@ -513,7 +513,7 @@ void Binding::schedulerDidRequestPreliminaryViewAllocation(
   if (!mountingManager) {
     return;
   }
-  mountingManager->preallocateShadowView(shadowNode);
+  mountingManager->maybePreallocateShadowView(shadowNode);
 }
 
 void Binding::schedulerDidDispatchCommand(

packages/react-native/ReactAndroid/src/main/jni/react/fabric/FabricMountingManager.cpp

@@ -784,11 +784,16 @@ void FabricMountingManager::executeMount(
   env->DeleteLocalRef(intBufferArray);
 }
 
-void FabricMountingManager::preallocateShadowView(
+void FabricMountingManager::maybePreallocateShadowView(
     const ShadowNode& shadowNode) {
   if (!shadowNode.getTraits().check(ShadowNodeTraits::Trait::FormsView)) {
     return;
   }
+  static thread_local bool onMainThread = isOnMainThread();
+  if (onMainThread) {
+    // View preallocation is not beneficial when rendering on the main thread
+    return;
+  }
 
   SystraceSection section("FabricMountingManager::preallocateShadowView");
 
@@ -844,6 +849,13 @@ void FabricMountingManager::preallocateShadowView(
       isLayoutableShadowNode);
 }
 
+bool FabricMountingManager::isOnMainThread() {
+  static auto isOnMainThread =
+      JFabricUIManager::javaClassStatic()->getMethod<jboolean()>(
+          "isOnMainThread");
+  return isOnMainThread(javaUIManager_);
+}
+
 void FabricMountingManager::dispatchCommand(
     const ShadowView& shadowView,
     const std::string& commandName,

packages/react-native/ReactAndroid/src/main/jni/react/fabric/FabricMountingManager.h

@@ -32,7 +32,7 @@ class FabricMountingManager final {
 
   void onSurfaceStop(SurfaceId surfaceId);
 
-  void preallocateShadowView(const ShadowNode& shadowNode);
+  void maybePreallocateShadowView(const ShadowNode& shadowNode);
 
   void executeMount(const MountingTransaction& transaction);
 
@@ -55,6 +55,8 @@ class FabricMountingManager final {
   void onAllAnimationsComplete();
 
  private:
+  bool isOnMainThread();
+
   jni::global_ref<JFabricUIManager::javaobject> javaUIManager_;
 
   std::recursive_mutex commitMutex_;