Fabric: Implementation of BackgroundExecutor on iOS (and gating)

Summary:
This is an implementation of `BackgroundExecutor` on iOS.
In case if the experiment is successful, we will make it mandatory for all platforms.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: JoshuaGross

Differential Revision: D22743722

fbshipit-source-id: 7756d2947e962720b9970d48c74929ab407c0440

React/Fabric/RCTSurfacePresenter.mm

@@ -60,6 +60,33 @@ static inline LayoutContext RCTGetLayoutContext()
           .fontSizeMultiplier = RCTFontSizeMultiplier()};
 }
 
+static dispatch_queue_t RCTGetBackgroundQueue()
+{
+  static dispatch_queue_t queue;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    dispatch_queue_attr_t attr =
+        dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_USER_INTERACTIVE, 0);
+    queue = dispatch_queue_create("com.facebook.react.background", attr);
+  });
+  return queue;
+}
+
+static BackgroundExecutor RCTGetBackgroundExecutor()
+{
+  return [](std::function<void()> &&callback) {
+    if (RCTIsMainQueue()) {
+      callback();
+      return;
+    }
+
+    auto copyableCallback = callback;
+    dispatch_async(RCTGetBackgroundQueue(), ^{
+      copyableCallback();
+    });
+  };
+}
+
 @interface RCTSurfacePresenter () <RCTSchedulerDelegate, RCTMountingManagerDelegate>
 @end
 
@@ -312,6 +339,10 @@ - (RCTScheduler *)_createScheduler
     return std::make_unique<MainRunLoopObserver>(activities, owner);
   };
 
+  if (reactNativeConfig && reactNativeConfig->getBool("react_fabric:enable_background_executor_ios")) {
+    toolbox.backgroundExecutor = RCTGetBackgroundExecutor();
+  }
+
   if (reactNativeConfig && reactNativeConfig->getBool("react_fabric:enable_run_loop_based_event_beat_ios")) {
     toolbox.synchronousEventBeatFactory = [runtimeExecutor](EventBeat::SharedOwnerBox const &ownerBox) {
       auto runLoopObserver =