From cf75d032d0e92812128227b7203aaa9e3a96ced7 Mon Sep 17 00:00:00 2001
From: Samuel Susla <samuelsusla@meta.com>
Date: Fri, 19 Apr 2024 11:02:54 -0700
Subject: [PATCH] disable event loop (#44169)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44169

## Changelog:
[General][Changed] Disable new event loop behavior when bridgeless (new architecture) is enabled.

# What is the problem
With event loop, specifically with `batchRenderingUpdatesInEventLoop`, prop change is not delivered to Android mounting layer if the prop change was initiated from state update inside of `useLayoutEffect`, `componentDidMount` or `componentDidUpdate`.  Note this has to be a prop change affecting mounting layer directly, not something consumed by Yoga, e.g. background colour or border colour.

This affects android only.

Minimal repro :
```
import React, {useLayoutEffect, useState} from 'react';
import {Button, SafeAreaView, View} from 'react-native';
function Foo() {
  const [bgColor, setBgColor] = React.useState('red');
  useLayoutEffect(() => {
    console.log('useLayoutEffect');
    setBgColor('blue');
  }, []);
  return (
    <View
      style={{
        backgroundColor: bgColor,
        width: '100%',
        height: '100%',
      }}
    />
  );
}
function RNTesterApp() {
  const [show, setShow] = useState(false);
  return (
    <SafeAreaView>
      <Button title="Toggle" onPress={() => setShow(!show)} />
      {show && <Foo />}
    </SafeAreaView>
  );
}
export default RNTesterApp;
```

# The underlaying problem
The problem is in batched rendering updates and how props are delivered to Android mounting layer.

Here is a step by step what happens in the repro above:
1. React issues asks Fabric to create new shadow node A with background colour **red**.
2. Fabric asks Android to allocate a view for shadow node A with background colour **red**.
3. React commits tree **T1** and calls layout effects. Meanwhile Fabric waits, without trying to mount the tree **T1**, to prevent painting state that is about to be updated and prevent flickering.
4. React clones node A, changing the background colour to **blue** and commits the new tree **T2**.
5. Fabric, will now go ahead and mount the latest tree **T2**. While creating mount instructions, it will drop prop updates because it believes prop updates where delivered already as part of step 2.

At first this might appear as a problem with view preallocation. But the underlaying trouble is that on Android, we currently have no way of knowing how to combine changesets from React into single folly::dynamic.

Reviewed By: javache, cortinico

Differential Revision: D56355863

fbshipit-source-id: f8616ee48e10fc10e129bb632c5d398842220d24
---
 .../Libraries/AppDelegate/RCTAppDelegate.mm      | 16 +---------------
 .../defaults/DefaultNewArchitectureEntryPoint.kt |  6 ------
 2 files changed, 1 insertion(+), 21 deletions(-)

diff --git a/packages/react-native/Libraries/AppDelegate/RCTAppDelegate.mm b/packages/react-native/Libraries/AppDelegate/RCTAppDelegate.mm
index 60d0362722f..81a4b5b20f2 100644
--- a/packages/react-native/Libraries/AppDelegate/RCTAppDelegate.mm
+++ b/packages/react-native/Libraries/AppDelegate/RCTAppDelegate.mm
@@ -299,21 +299,7 @@
 
 #pragma mark - Feature Flags
 
-class RCTAppDelegateBridgelessFeatureFlags : public facebook::react::ReactNativeFeatureFlagsDefaults {
- public:
-  bool useModernRuntimeScheduler() override
-  {
-    return true;
-  }
-  bool enableMicrotasks() override
-  {
-    return true;
-  }
-  bool batchRenderingUpdatesInEventLoop() override
-  {
-    return true;
-  }
-};
+class RCTAppDelegateBridgelessFeatureFlags : public facebook::react::ReactNativeFeatureFlagsDefaults {};
 
 - (void)_setUpFeatureFlags
 {
diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultNewArchitectureEntryPoint.kt b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultNewArchitectureEntryPoint.kt
index 103f8c74207..2b67e1ca17b 100644
--- a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultNewArchitectureEntryPoint.kt
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultNewArchitectureEntryPoint.kt
@@ -49,12 +49,6 @@ public object DefaultNewArchitectureEntryPoint {
     if (bridgelessEnabled) {
       ReactNativeFeatureFlags.override(
           object : ReactNativeFeatureFlagsDefaults() {
-            override fun useModernRuntimeScheduler(): Boolean = true
-
-            override fun enableMicrotasks(): Boolean = true
-
-            override fun batchRenderingUpdatesInEventLoop(): Boolean = true
-
             override fun useNativeViewConfigsInBridgelessMode(): Boolean = fabricEnabled
           })
     }