Wire the Android pull model in C++ behind the feature flag

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

@@ -27,6 +27,7 @@
 import androidx.annotation.AnyThread;
 import androidx.annotation.Nullable;
 import androidx.annotation.UiThread;
+import androidx.annotation.VisibleForTesting;
 import androidx.core.view.ViewCompat.FocusDirection;
 import com.facebook.common.logging.FLog;
 import com.facebook.infer.annotation.Assertions;
@@ -65,6 +66,7 @@
 import com.facebook.react.fabric.mounting.mountitems.MountItem;
 import com.facebook.react.fabric.mounting.mountitems.MountItemFactory;
 import com.facebook.react.fabric.mounting.mountitems.PrefetchResourcesMountItem;
+import com.facebook.react.fabric.mounting.mountitems.PullTransactionMountItem;
 import com.facebook.react.fabric.mounting.mountitems.SynchronousMountItem;
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlags;
 import com.facebook.react.internal.featureflags.ReactNativeNewArchitectureFeatureFlags;
@@ -934,7 +936,8 @@ private void scheduleMountItem(
       long layoutEndTime,
       long finishTransactionStartTime,
       long finishTransactionEndTime,
-      int affectedLayoutNodesCount) {
+      int affectedLayoutNodesCount,
+      boolean synchronous) {
     // When Binding.cpp calls scheduleMountItems during a commit phase, it always calls with
     // a BatchMountItem. No other sites call into this with a BatchMountItem, and Binding.cpp only
     // calls scheduleMountItems with a BatchMountItem.
@@ -948,8 +951,9 @@ private void scheduleMountItem(
     } else {
       shouldSchedule = mountItem != null;
     }
-    // In case of sync rendering, this could be called on the UI thread. Otherwise,
-    // it should ~always be called on the JS thread.
+    // In the push model, this is ~always called on the JS thread, or on the UI
+    // thread in case of sync rendering.
+    // In the pull model, this is always called on the UI thread, at pull time.
     for (UIManagerListener listener : mListeners) {
       listener.didScheduleMountItems(this);
     }
@@ -964,16 +968,22 @@ private void scheduleMountItem(
 
     if (shouldSchedule) {
       Assertions.assertNotNull(mountItem, "MountItem is null");
-      mMountItemDispatcher.addMountItem(mountItem);
-      if (UiThreadUtil.isOnUiThread()) {
-        Runnable runnable =
-            new GuardedRunnable(mReactApplicationContext) {
-              @Override
-              public void runGuarded() {
-                mMountItemDispatcher.tryDispatchMountItems();
-              }
-            };
-        runnable.run();
+      if (synchronous) {
+        // Pull model: we are already on the UI thread, inside the dispatcher's loop executing
+        // a PullTransactionMountItem. We don't schedule the item, we execute it directly.
+        mountItem.execute(mMountingManager);
+      } else {
+        mMountItemDispatcher.addMountItem(mountItem);
+        if (UiThreadUtil.isOnUiThread()) {
+          Runnable runnable =
+              new GuardedRunnable(mReactApplicationContext) {
+                @Override
+                public void runGuarded() {
+                  mMountItemDispatcher.tryDispatchMountItems();
+                }
+              };
+          runnable.run();
+        }
       }
     }
 
@@ -1009,6 +1019,26 @@ public void runGuarded() {
     }
   }
 
+  /**
+   * Pull model: called from C++ via JNI (usually on the commit thread) to signal that a transaction
+   * is available for {@code surfaceId}. Enqueues a PullTransactionMountItem so the UI thread pulls
+   * and applies the transaction itself, preserving mount-item ordering.
+   */
+  @SuppressWarnings("unused")
+  @AnyThread
+  @ThreadConfined(ANY)
+  @VisibleForTesting
+  void onTransactionAvailable(int surfaceId) {
+    FabricUIManagerBinding binding = mBinding;
+    if (binding == null) {
+      return;
+    }
+    mMountItemDispatcher.addMountItem(new PullTransactionMountItem(surfaceId, binding));
+    if (UiThreadUtil.isOnUiThread()) {
+      mMountItemDispatcher.tryDispatchMountItems();
+    }
+  }
+
   @SuppressWarnings("unused")
   @AnyThread
   @ThreadConfined(ANY)

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

@@ -85,6 +85,8 @@ internal class FabricUIManagerBinding : HybridClassBase() {
 
   external fun reportMount(surfaceId: Int)
 
+  external fun pullAndExecuteTransaction(surfaceId: Int)
+
   external fun mergeReactRevision(surfaceId: Int)
 
   fun register(

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/mountitems/PullTransactionMountItem.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.fabric.mounting.mountitems
+
+import com.facebook.proguard.annotations.DoNotStripAny
+import com.facebook.react.fabric.FabricUIManagerBinding
+import com.facebook.react.fabric.mounting.MountingManager
+
+/**
+ * Pull model mount item. Enqueued (on the commit thread) when C++ notifies that a transaction is
+ * available for a surface. When it executes on the UI thread it asks C++ to pull the surface's
+ * pending transaction and apply it synchronously, so the diff + batch construction happens on the
+ * UI thread instead of the commit thread (matching iOS).
+ */
+@DoNotStripAny
+internal class PullTransactionMountItem(
+    private val surfaceId: Int,
+    private val binding: FabricUIManagerBinding,
+) : MountItem {
+
+  override fun execute(mountingManager: MountingManager) {
+    binding.pullAndExecuteTransaction(surfaceId)
+  }
+
+  override fun getSurfaceId(): Int = surfaceId
+
+  override fun toString(): String = "PullTransactionMountItem [surfaceId: $surfaceId]"
+}

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlags.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<df3f81d0083a8ebe9d0fbcb86e652405>>
+ * @generated SignedSource<<8508c768e49199737ce5fd1071ca1d68>>
  */
 
 /**
@@ -252,6 +252,12 @@ public object ReactNativeFeatureFlags {
   @JvmStatic
   public fun enableModuleArgumentNSNullConversionIOS(): Boolean = accessor.enableModuleArgumentNSNullConversionIOS()
 
+  /**
+   * When enabled, Android mounts transactions with the pull model (like iOS): the commit thread no longer pulls and builds the mount batch in schedulerShouldRenderTransactions. Instead the UI thread pulls the transaction itself via a PullTransactionMountItem enqueued in the MountItemDispatcher, builds the IntBufferBatchMountItem, and applies it synchronously. Implies rawProps accumulation (enableAccumulatedUpdatesInRawPropsAndroid behavior).
+   */
+  @JvmStatic
+  public fun enableMountingCoordinatorPullModelAndroid(): Boolean = accessor.enableMountingCoordinatorPullModelAndroid()
+
   /**
    * Enables the MutationObserver Web API in React Native.
    */

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxAccessor.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<c31110405f1c9cd5ae0627fb114585a6>>
+ * @generated SignedSource<<eeb2c4625ae5039dc087f364713050fe>>
  */
 
 /**
@@ -57,6 +57,7 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
   private var enableLayoutAnimationsOnAndroidCache: Boolean? = null
   private var enableLayoutAnimationsOnIOSCache: Boolean? = null
   private var enableModuleArgumentNSNullConversionIOSCache: Boolean? = null
+  private var enableMountingCoordinatorPullModelAndroidCache: Boolean? = null
   private var enableMutationObserverByDefaultCache: Boolean? = null
   private var enableNativeCSSParsingCache: Boolean? = null
   private var enableNetworkEventReportingCache: Boolean? = null
@@ -441,6 +442,15 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
     return cached
   }
 
+  override fun enableMountingCoordinatorPullModelAndroid(): Boolean {
+    var cached = enableMountingCoordinatorPullModelAndroidCache
+    if (cached == null) {
+      cached = ReactNativeFeatureFlagsCxxInterop.enableMountingCoordinatorPullModelAndroid()
+      enableMountingCoordinatorPullModelAndroidCache = cached
+    }
+    return cached
+  }
+
   override fun enableMutationObserverByDefault(): Boolean {
     var cached = enableMutationObserverByDefaultCache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxInterop.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<87b76f5c2af82462dc66c7721cd66ac4>>
+ * @generated SignedSource<<9a171a03d15885e64b84f03442b304b4>>
  */
 
 /**
@@ -102,6 +102,8 @@ public object ReactNativeFeatureFlagsCxxInterop {
 
   @DoNotStrip @JvmStatic public external fun enableModuleArgumentNSNullConversionIOS(): Boolean
 
+  @DoNotStrip @JvmStatic public external fun enableMountingCoordinatorPullModelAndroid(): Boolean
+
   @DoNotStrip @JvmStatic public external fun enableMutationObserverByDefault(): Boolean
 
   @DoNotStrip @JvmStatic public external fun enableNativeCSSParsing(): Boolean

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsDefaults.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<8a117541421f413e517e1be1acf8be7d>>
+ * @generated SignedSource<<4fd762a8e3ab6de1276a15368075f40c>>
  */
 
 /**
@@ -97,6 +97,8 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun enableModuleArgumentNSNullConversionIOS(): Boolean = false
 
+  override fun enableMountingCoordinatorPullModelAndroid(): Boolean = false
+
   override fun enableMutationObserverByDefault(): Boolean = false
 
   override fun enableNativeCSSParsing(): Boolean = false

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsLocalAccessor.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<9a2806b3934d130010954a92a7b48e0f>>
+ * @generated SignedSource<<4a4a234004196595b5fb2949bd91f9c4>>
  */
 
 /**
@@ -61,6 +61,7 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
   private var enableLayoutAnimationsOnAndroidCache: Boolean? = null
   private var enableLayoutAnimationsOnIOSCache: Boolean? = null
   private var enableModuleArgumentNSNullConversionIOSCache: Boolean? = null
+  private var enableMountingCoordinatorPullModelAndroidCache: Boolean? = null
   private var enableMutationObserverByDefaultCache: Boolean? = null
   private var enableNativeCSSParsingCache: Boolean? = null
   private var enableNetworkEventReportingCache: Boolean? = null
@@ -482,6 +483,16 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
     return cached
   }
 
+  override fun enableMountingCoordinatorPullModelAndroid(): Boolean {
+    var cached = enableMountingCoordinatorPullModelAndroidCache
+    if (cached == null) {
+      cached = currentProvider.enableMountingCoordinatorPullModelAndroid()
+      accessedFeatureFlags.add("enableMountingCoordinatorPullModelAndroid")
+      enableMountingCoordinatorPullModelAndroidCache = cached
+    }
+    return cached
+  }
+
   override fun enableMutationObserverByDefault(): Boolean {
     var cached = enableMutationObserverByDefaultCache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsProvider.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<49c76b8072c1bd29135086c9e0a5da2f>>
+ * @generated SignedSource<<0b4bf7de06ad677b0868640c98ff0aa9>>
  */
 
 /**
@@ -97,6 +97,8 @@ public interface ReactNativeFeatureFlagsProvider {
 
   @DoNotStrip public fun enableModuleArgumentNSNullConversionIOS(): Boolean
 
+  @DoNotStrip public fun enableMountingCoordinatorPullModelAndroid(): Boolean
+
   @DoNotStrip public fun enableMutationObserverByDefault(): Boolean
 
   @DoNotStrip public fun enableNativeCSSParsing(): Boolean

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

@@ -52,6 +52,13 @@ void FabricMountingManager::onSurfaceStop(SurfaceId surfaceId) {
   allocatedViewRegistry_.erase(surfaceId);
 }
 
+void FabricMountingManager::onTransactionAvailable(SurfaceId surfaceId) {
+  static auto onTransactionAvailable =
+      JFabricUIManager::javaClassStatic()->getMethod<void(jint)>(
+          "onTransactionAvailable");
+  onTransactionAvailable(javaUIManager_, surfaceId);
+}
+
 bool FabricMountingManager::isViewAllocated(SurfaceId surfaceId, Tag tag) {
   std::lock_guard lock(allocatedViewsMutex_);
   auto it = allocatedViewRegistry_.find(surfaceId);
@@ -339,7 +346,10 @@ jni::local_ref<jobject> getProps(
 
     return ReadableNativeMap::newObjectCxxArgs(std::move(diff));
   }
-  if (ReactNativeFeatureFlags::enableAccumulatedUpdatesInRawPropsAndroid()) {
+  // With the pull model we need to have the accumulated props, as we might skip
+  // intermediate commits
+  if (ReactNativeFeatureFlags::enableAccumulatedUpdatesInRawPropsAndroid() ||
+      ReactNativeFeatureFlags::enableMountingCoordinatorPullModelAndroid()) {
     if (oldProps == nullptr) {
       return ReadableNativeMap::newObjectCxxArgs(newProps->rawProps);
     } else {
@@ -568,7 +578,8 @@ inline void writeUpdateOverflowInsetMountItem(
 } // namespace
 
 void FabricMountingManager::executeMount(
-    const MountingTransaction& transaction) {
+    const MountingTransaction& transaction,
+    bool synchronous) {
   TraceSection section("FabricMountingManager::executeMount");
 
   std::scoped_lock lock(commitMutex_);
@@ -830,7 +841,8 @@ void FabricMountingManager::executeMount(
                                           jlong,
                                           jlong,
                                           jlong,
-                                          jint)>("scheduleMountItem");
+                                          jint,
+                                          jboolean)>("scheduleMountItem");
 
   if (batchMountItemIntsSize == 0) {
     auto finishTransactionEndTime = telemetryTimePointNow();
@@ -845,7 +857,8 @@ void FabricMountingManager::executeMount(
         telemetryTimePointToMilliseconds(telemetry.getLayoutEndTime()),
         telemetryTimePointToMilliseconds(finishTransactionStartTime),
         telemetryTimePointToMilliseconds(finishTransactionEndTime),
-        telemetry.getAffectedLayoutNodesCount());
+        telemetry.getAffectedLayoutNodesCount(),
+        static_cast<jboolean>(synchronous));
     return;
   }
 
@@ -1012,7 +1025,8 @@ void FabricMountingManager::executeMount(
       telemetryTimePointToMilliseconds(telemetry.getLayoutEndTime()),
       telemetryTimePointToMilliseconds(finishTransactionStartTime),
       telemetryTimePointToMilliseconds(finishTransactionEndTime),
-      telemetry.getAffectedLayoutNodesCount());
+      telemetry.getAffectedLayoutNodesCount(),
+      static_cast<jboolean>(synchronous));
 
   env->DeleteLocalRef(buffer.ints);
 }

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

@@ -54,7 +54,23 @@ class FabricMountingManager final {
    */
   bool isViewAllocated(SurfaceId surfaceId, Tag tag);
 
-  void executeMount(const MountingTransaction &transaction);
+  /*
+   * Converts the transaction's mutations into an IntBufferBatchMountItem and
+   * hands it to Java.
+   *
+   * In the push model (`synchronous` = false), the batch is
+   * scheduled onto the UI thread asynchronously.
+   *
+   * In the pull model (`synchronous` = true) the batch is
+   * applied immediately on the calling (UI) thread.
+   */
+  void executeMount(const MountingTransaction &transaction, bool synchronous = false);
+
+  /*
+   * Pull model: notify Java that a transaction is available for `surfaceId` so
+   * the UI thread can pull it via a PullTransactionMountItem.
+   */
+  void onTransactionAvailable(SurfaceId surfaceId);
 
   void dispatchCommand(const ShadowView &shadowView, const std::string &commandName, const folly::dynamic &args);