From 76f41a057842fb101d3cc29d0c82c5cc144b5788 Mon Sep 17 00:00:00 2001
From: Joshua Gross <joshuagross@fb.com>
Date: Fri, 15 Nov 2019 16:57:17 -0800
Subject: [PATCH] Gate fix for T54997838 with a feature flag

Summary:
Gating fix for T54997838 behind a feature flag for experimentation.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D18534767

fbshipit-source-id: 5c0191c454fba6dd4a36d6e179959651ec197cac
---
 .../com/facebook/react/config/ReactFeatureFlags.java   |  8 ++++++++
 .../com/facebook/react/fabric/FabricUIManager.java     | 10 ++++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java b/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java
index 031030f2faa..03665bd10cb 100644
--- a/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java
+++ b/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java
@@ -70,4 +70,12 @@ public class ReactFeatureFlags {
    * and the CatalystInstance will always be destroyed in some future release.
    */
   public static boolean nullifyCatalystInstanceOnDestroy = false;
+
+  /**
+   * Temporary flag that should be removed soon. See FabricUIManager: if this flag is disabled,
+   * mountItems scheduled on the UI thread will *always* be executed synchronously. If this flag is
+   * enabled, users of FabricUIManager may disable immediate execution of scheduled mount items.
+   * TODO T54997838: remove as followup
+   */
+  public static boolean allowDisablingImmediateExecutionOfScheduleMountItems = false;
 }
diff --git a/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java b/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
index 09f1bc08511..1acbd252d97 100644
--- a/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
+++ b/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
@@ -533,8 +533,14 @@ public class FabricUIManager implements UIManager, LifecycleEventListener {
       mMountItems.add(mountItem);
     }
 
-    if (mImmediatelyExecutedMountItemsOnUI && UiThreadUtil.isOnUiThread()) {
-      dispatchMountItems();
+    if (UiThreadUtil.isOnUiThread()) {
+      // We only read these flags on the UI thread.
+      boolean immediateExecutionEnabled =
+          !ReactFeatureFlags.allowDisablingImmediateExecutionOfScheduleMountItems
+              || mImmediatelyExecutedMountItemsOnUI;
+      if (immediateExecutionEnabled) {
+        dispatchMountItems();
+      }
     }
 
     // Post markers outside of lock and after sync mounting finishes its execution