From 7ba9e1f3ee3c774aca498400619fa50fe38d11e1 Mon Sep 17 00:00:00 2001
From: DevmateUtgenCppGeneralFBOrg Bot <noreply+1563563004708334@fb.com>
Date: Tue, 21 Jul 2026 11:03:39 -0700
Subject: [PATCH] 
 xplat/js/react-native-github/packages/react-native/ReactCommon/react/renderer/mounting/MountingTransaction.cpp
 (#57585)

Summary: Pull Request resolved: https://github.com/react/react-native/pull/57585

Reviewed By: cortinico

Differential Revision: D112079573
---
 .../tests/MountingTransactionTest.cpp         | 123 ++++++++++++++++++
 1 file changed, 123 insertions(+)
 create mode 100644 packages/react-native/ReactCommon/react/renderer/mounting/tests/MountingTransactionTest.cpp

diff --git a/packages/react-native/ReactCommon/react/renderer/mounting/tests/MountingTransactionTest.cpp b/packages/react-native/ReactCommon/react/renderer/mounting/tests/MountingTransactionTest.cpp
new file mode 100644
index 00000000000..cdc8b1fe56e
--- /dev/null
+++ b/packages/react-native/ReactCommon/react/renderer/mounting/tests/MountingTransactionTest.cpp
@@ -0,0 +1,123 @@
+/*
+ * 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.
+ */
+
+#include <gtest/gtest.h>
+
+#include <react/renderer/mounting/MountingTransaction.h>
+#include <react/renderer/mounting/ShadowView.h>
+#include <react/renderer/mounting/ShadowViewMutation.h>
+#include <react/renderer/telemetry/TransactionTelemetry.h>
+
+namespace facebook::react {
+
+namespace {
+
+ShadowView makeShadowView(Tag tag) {
+  ShadowView view;
+  view.tag = tag;
+  return view;
+}
+
+MountingTransaction makeTransaction(
+    SurfaceId surfaceId,
+    MountingTransaction::Number number,
+    const std::vector<Tag>& createTags,
+    int telemetryRevision = 0) {
+  ShadowViewMutationList mutations;
+  mutations.reserve(createTags.size());
+  for (auto tag : createTags) {
+    mutations.push_back(
+        ShadowViewMutation::CreateMutation(makeShadowView(tag)));
+  }
+  TransactionTelemetry telemetry;
+  telemetry.setRevisionNumber(telemetryRevision);
+  return MountingTransaction{
+      surfaceId, number, std::move(mutations), std::move(telemetry)};
+}
+
+} // namespace
+
+// Verifies that `mergeWith` concatenates mutations from the incoming
+// transaction after the existing ones, preserving relative order across both.
+// Catches regressions where the merged list is reversed, overwrites the
+// original, or drops mutations from either side.
+TEST(MountingTransactionTest, testMergeWithAppendsMutationsInOrder) {
+  auto transaction = makeTransaction(
+      /* surfaceId */ 1, /* number */ 1, /* tags */ {10, 11});
+  auto incoming = makeTransaction(
+      /* surfaceId */ 1, /* number */ 2, /* tags */ {20, 21, 22});
+
+  transaction.mergeWith(std::move(incoming));
+
+  const auto& merged = transaction.getMutations();
+  ASSERT_EQ(merged.size(), 5u);
+  EXPECT_EQ(merged[0].newChildShadowView.tag, 10);
+  EXPECT_EQ(merged[1].newChildShadowView.tag, 11);
+  EXPECT_EQ(merged[2].newChildShadowView.tag, 20);
+  EXPECT_EQ(merged[3].newChildShadowView.tag, 21);
+  EXPECT_EQ(merged[4].newChildShadowView.tag, 22);
+}
+
+// Verifies that `mergeWith` advances `number` to that of the incoming
+// transaction while leaving `surfaceId` untouched. Catches regressions where
+// the number is not updated or the surfaceId is accidentally overwritten.
+TEST(MountingTransactionTest, testMergeWithUpdatesNumberButPreservesSurfaceId) {
+  auto transaction = makeTransaction(
+      /* surfaceId */ 7, /* number */ 3, /* tags */ {100});
+  auto incoming = makeTransaction(
+      /* surfaceId */ 7, /* number */ 42, /* tags */ {200});
+
+  transaction.mergeWith(std::move(incoming));
+
+  EXPECT_EQ(transaction.getSurfaceId(), 7);
+  EXPECT_EQ(transaction.getNumber(), 42);
+}
+
+// Verifies that `mergeWith` replaces the existing telemetry with that of the
+// incoming transaction (per the documented "use the latest instance"
+// behavior). We use a distinguishing revision number so the assertion checks a
+// computed value rather than restating a source constant.
+TEST(MountingTransactionTest, testMergeWithAdoptsIncomingTelemetry) {
+  auto transaction = makeTransaction(
+      /* surfaceId */ 1,
+      /* number */ 1,
+      /* tags */ {},
+      /* telemetryRevision */ 5);
+  auto incoming = makeTransaction(
+      /* surfaceId */ 1,
+      /* number */ 2,
+      /* tags */ {},
+      /* telemetryRevision */ 99);
+
+  transaction.mergeWith(std::move(incoming));
+
+  EXPECT_EQ(transaction.getTelemetry().getRevisionNumber(), 99);
+}
+
+// Verifies that merging an empty incoming transaction is a no-op for the
+// mutation list — the existing mutations remain intact, just with the number
+// bumped. Guards against a naive `= std::move(other.mutations_)` implementation
+// that would clobber the original list.
+TEST(
+    MountingTransactionTest,
+    testMergeWithEmptyIncomingPreservesOriginalMutations) {
+  auto transaction = makeTransaction(
+      /* surfaceId */ 1, /* number */ 1, /* tags */ {10, 11, 12});
+  auto incoming = makeTransaction(
+      /* surfaceId */ 1, /* number */ 2, /* tags */ {});
+
+  transaction.mergeWith(std::move(incoming));
+
+  const auto& merged = transaction.getMutations();
+  ASSERT_EQ(merged.size(), 3u);
+  EXPECT_EQ(merged[0].newChildShadowView.tag, 10);
+  EXPECT_EQ(merged[1].newChildShadowView.tag, 11);
+  EXPECT_EQ(merged[2].newChildShadowView.tag, 12);
+  EXPECT_EQ(transaction.getNumber(), 2);
+}
+
+} // namespace facebook::react