From 7ea0ef7a90e18f33cb4e15f673168cee1ab0b64b Mon Sep 17 00:00:00 2001
From: Luna Wei <luwe@meta.com>
Date: Fri, 18 Jul 2025 14:16:22 -0700
Subject: [PATCH] Implement windowFocus on ReactVirtualViewExperiment (#52690)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52690

Changelog: [Internal] - Add the window focus experiment to ReactVirtualViewExperimental

Reviewed By: yungsters

Differential Revision: D78502991

fbshipit-source-id: 3e72561835925040e5b240e71734a088908957ed
---
 .../react/views/scroll/ReactScrollView.java   |  3 ++
 .../views/scroll/VirtualViewContainer.kt      | 36 +++++++++++++++++--
 2 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java
index a0a906f6c10..fd5ec0e73db 100644
--- a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java
@@ -407,6 +407,9 @@ public class ReactScrollView extends ScrollView
     if (mMaintainVisibleContentPositionHelper != null) {
       mMaintainVisibleContentPositionHelper.stop();
     }
+    if (mVirtualViewContainerState != null) {
+      mVirtualViewContainerState.cleanup();
+    }
   }
 
   @Override
diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/VirtualViewContainer.kt b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/VirtualViewContainer.kt
index 66bd3d167df..0dae4459618 100644
--- a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/VirtualViewContainer.kt
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/VirtualViewContainer.kt
@@ -9,6 +9,7 @@ package com.facebook.react.views.scroll
 
 import android.graphics.Rect
 import android.view.ViewGroup
+import android.view.ViewTreeObserver
 import com.facebook.common.logging.FLog
 import com.facebook.react.common.build.ReactBuildConfig
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlags
@@ -46,13 +47,34 @@ private fun rectsOverlap(rect1: Rect, rect2: Rect): Boolean {
   return true
 }
 
-internal class VirtualViewContainerState(private val scrollView: ViewGroup) {
+internal class VirtualViewContainerState {
 
   private val prerenderRatio: Double = ReactNativeFeatureFlags.virtualViewPrerenderRatio()
+  private val detectWindowFocus = ReactNativeFeatureFlags.enableVirtualViewWindowFocusDetection()
+
   private val virtualViews: MutableSet<VirtualView> = mutableSetOf()
   private val emptyRect: Rect = Rect()
   private val visibleRect: Rect = Rect()
   private val prerenderRect: Rect = Rect()
+  private val onWindowFocusChangeListener =
+      ViewTreeObserver.OnWindowFocusChangeListener {
+        debugLog("onWindowFocusChanged")
+        updateModes()
+      }
+  private val scrollView: ViewGroup
+
+  constructor(scrollView: ViewGroup) {
+    this.scrollView = scrollView
+    if (detectWindowFocus) {
+      scrollView.viewTreeObserver.addOnWindowFocusChangeListener(onWindowFocusChangeListener)
+    }
+  }
+
+  public fun cleanup() {
+    if (detectWindowFocus) {
+      scrollView.viewTreeObserver.removeOnWindowFocusChangeListener(onWindowFocusChangeListener)
+    }
+  }
 
   public fun onChange(virtualView: VirtualView) {
     if (virtualViews.add(virtualView)) {
@@ -72,7 +94,7 @@ internal class VirtualViewContainerState(private val scrollView: ViewGroup) {
 
   // Called on ScrollView onLayout or onScroll
   public fun updateState() {
-    debugLog("VirtualViewContainer.updateState")
+    debugLog("updateState")
     updateModes()
   }
 
@@ -92,8 +114,16 @@ internal class VirtualViewContainerState(private val scrollView: ViewGroup) {
       when {
         rect.isEmpty -> {}
         rectsOverlap(rect, visibleRect) -> {
-          mode = VirtualViewMode.Visible
           thresholdRect = visibleRect
+          if (detectWindowFocus) {
+            if (scrollView.hasWindowFocus()) {
+              mode = VirtualViewMode.Visible
+            } else {
+              mode = VirtualViewMode.Prerender
+            }
+          } else {
+            mode = VirtualViewMode.Visible
+          }
         }
         rectsOverlap(rect, prerenderRect) -> {
           mode = VirtualViewMode.Prerender