From f200b70354562c82ca8a9fa2d4f11852fd17ab9f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rub=C3=A9n=20Norte?= <rubennorte@meta.com>
Date: Fri, 21 Feb 2025 03:21:07 -0800
Subject: [PATCH] Add Fantom test for synchronous state updates + UI
 consistency (#49572)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49572

Changelog: [internal]

This adds a test to verify the behavior of 2 feature flags: synchronous state updates and UI consistency.

Reviewed By: javache

Differential Revision: D69932313

fbshipit-source-id: 341cfff3fa533503a293f6ccd0282442ba63d430
---
 .../__tests__/UIConsistency-itest.js          | 158 ++++++++++++++++++
 1 file changed, 158 insertions(+)
 create mode 100644 packages/react-native/src/private/renderer/consistency/__tests__/UIConsistency-itest.js

diff --git a/packages/react-native/src/private/renderer/consistency/__tests__/UIConsistency-itest.js b/packages/react-native/src/private/renderer/consistency/__tests__/UIConsistency-itest.js
new file mode 100644
index 00000000000..fa82907105a
--- /dev/null
+++ b/packages/react-native/src/private/renderer/consistency/__tests__/UIConsistency-itest.js
@@ -0,0 +1,158 @@
+/**
+ * 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.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ * @fantom_flags enableAccessToHostTreeInFabric:true
+ * @fantom_flags enableUIConsistency:true
+ * @fantom_flags enableSynchronousStateUpdates:true
+ */
+
+import ScrollView from '../../../../../Libraries/Components/ScrollView/ScrollView';
+import Text from '../../../../../Libraries/Text/Text';
+import ensureInstance from '../../../utilities/ensureInstance';
+import ReactNativeElement from '../../../webapis/dom/nodes/ReactNativeElement';
+import Fantom from '@react-native/fantom';
+import * as React from 'react';
+import {useLayoutEffect} from 'react';
+
+import '../../../../../Libraries/Core/InitializeCore';
+
+describe('UIConsistency', () => {
+  it('should provide consistent data from the tree within the same synchronous function', () => {
+    const root = Fantom.createRoot();
+
+    let maybeScrollViewNode;
+
+    Fantom.runTask(() => {
+      root.render(
+        <ScrollView
+          ref={node => {
+            maybeScrollViewNode = node;
+          }}
+          style={{height: 100}}
+          contentContainerStyle={{height: 1000}}
+        />,
+      );
+    });
+
+    const scrollViewNode = ensureInstance(
+      maybeScrollViewNode,
+      ReactNativeElement,
+    );
+
+    Fantom.runTask(() => {
+      expect(scrollViewNode.scrollTop).toBe(0);
+
+      Fantom.runOnUIThread(() => {
+        Fantom.scrollTo(scrollViewNode, {x: 0, y: 100});
+      });
+
+      expect(scrollViewNode.scrollTop).toBe(0);
+    });
+
+    expect(scrollViewNode.scrollTop).toBe(100);
+  });
+
+  it('should provide up-to-date data in the first access to the tree', () => {
+    const root = Fantom.createRoot();
+
+    let maybeScrollViewNode;
+
+    Fantom.runTask(() => {
+      root.render(
+        <ScrollView
+          ref={node => {
+            maybeScrollViewNode = node;
+          }}
+          style={{height: 100}}
+          contentContainerStyle={{height: 1000}}
+        />,
+      );
+    });
+
+    const scrollViewNode = ensureInstance(
+      maybeScrollViewNode,
+      ReactNativeElement,
+    );
+
+    Fantom.runTask(() => {
+      // We never accessed the tree before the state update
+
+      Fantom.runOnUIThread(() => {
+        Fantom.scrollTo(scrollViewNode, {x: 0, y: 100});
+      });
+
+      // The value is up-to-date immediately
+      expect(scrollViewNode.scrollTop).toBe(100);
+    });
+  });
+
+  it('should provide up-to-date data after commit', () => {
+    const root = Fantom.createRoot();
+
+    let maybeScrollViewNode;
+
+    function InnerComponent(props: {
+      text: string,
+      renderFn?: () => void,
+      effectFn?: () => void,
+    }): React.Node {
+      const {text, renderFn, effectFn} = props;
+
+      renderFn?.();
+
+      useLayoutEffect(() => {
+        effectFn?.();
+      }, [text, effectFn]);
+
+      return <Text>{text}</Text>;
+    }
+
+    Fantom.runTask(() => {
+      root.render(
+        <ScrollView
+          ref={node => {
+            maybeScrollViewNode = node;
+          }}
+          style={{height: 100}}
+          contentContainerStyle={{height: 1000}}>
+          <InnerComponent text="A" />
+        </ScrollView>,
+      );
+    });
+
+    const scrollViewNode = ensureInstance(
+      maybeScrollViewNode,
+      ReactNativeElement,
+    );
+
+    Fantom.runTask(() => {
+      root.render(
+        <ScrollView
+          style={{height: 100}}
+          contentContainerStyle={{height: 1000}}>
+          <InnerComponent
+            text="B"
+            renderFn={() => {
+              expect(scrollViewNode.scrollTop).toBe(0);
+
+              Fantom.runOnUIThread(() => {
+                Fantom.scrollTo(scrollViewNode, {x: 0, y: 100});
+              });
+
+              expect(scrollViewNode.scrollTop).toBe(0);
+            }}
+            effectFn={() => {
+              expect(scrollViewNode.scrollTop).toBe(100);
+            }}
+          />
+        </ScrollView>,
+      );
+    });
+  });
+});