avoid scheduling frame callback if there are no events

Summary:
changelog: [internal]

FabricEventDispatcher does not need to run on every frame. Whenever a new event is added to Fabric's event queue, it will call `FabricUIManager.onRequestEventBeat` in Java. This in turn calls `FabricEventDispatcher.maybePostFrameCallbackFromNonUI` and adds a frame callback on the Choreographer.

This makes code simpler, as we do not need to manage the frame callback subscription.

Reviewed By: sammy-SC

Differential Revision: D50604303

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/events/FabricEventDispatcher.java

@@ -11,6 +11,7 @@
 import com.facebook.react.bridge.LifecycleEventListener;
 import com.facebook.react.bridge.ReactApplicationContext;
 import com.facebook.react.bridge.UiThreadUtil;
+import com.facebook.react.config.ReactFeatureFlags;
 import com.facebook.react.modules.core.ReactChoreographer;
 import com.facebook.react.uimanager.common.UIManagerType;
 import com.facebook.systrace.Systrace;
@@ -84,7 +85,9 @@ public void removeBatchEventDispatchedListener(BatchEventDispatchedListener list
 
   @Override
   public void onHostResume() {
-    maybePostFrameCallbackFromNonUI();
+    if (!ReactFeatureFlags.enableOnDemandReactChoreographer) {
+      maybePostFrameCallbackFromNonUI();
+    }
   }
 
   @Override
@@ -94,17 +97,21 @@ public void onHostPause() {
 
   @Override
   public void onHostDestroy() {
-    stopFrameCallback();
+    if (!ReactFeatureFlags.enableOnDemandReactChoreographer) {
+      stopFrameCallback();
+    }
   }
 
   public void onCatalystInstanceDestroyed() {
-    UiThreadUtil.runOnUiThread(
-        new Runnable() {
-          @Override
-          public void run() {
-            stopFrameCallback();
-          }
-        });
+    if (!ReactFeatureFlags.enableOnDemandReactChoreographer) {
+      UiThreadUtil.runOnUiThread(
+          new Runnable() {
+            @Override
+            public void run() {
+              stopFrameCallback();
+            }
+          });
+    }
   }
 
   private void stopFrameCallback() {
@@ -133,7 +140,7 @@ private class ScheduleDispatchFrameCallback implements Choreographer.FrameCallba
     public void doFrame(long frameTimeNanos) {
       UiThreadUtil.assertOnUiThread();
 
-      if (mShouldStop) {
+      if (ReactFeatureFlags.enableOnDemandReactChoreographer || mShouldStop) {
         mIsPosted = false;
       } else {
         post();