Non-Fabric UIModuleManager should not flush View updates if there are…

… no non-Fabric views

Summary:
In Fabric, we currently incur the cost of (frequently!) flushing non-Fabric UI updates, even if there are no non-Fabric views.

For now it is still possible to run both renderers at the same time, so we still largely continue to use the non-Fabric path, but only use UIImplementation if there is actually a RootView being managed outside of Fabric.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D29724538

fbshipit-source-id: 0f1148870c04ca9aaed0edfd6b5c55a3756a2bd7

ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModule.java

@@ -114,6 +114,7 @@ public interface CustomEventNamesResolver {
   private volatile int mViewManagerConstantsCacheSize;
 
   private int mBatchId = 0;
+  private int mNumRootViews = 0;
 
   @SuppressWarnings("deprecated")
   public UIManagerModule(
@@ -442,6 +443,7 @@ public <T extends View> int addRootView(
             -1);
 
     mUIImplementation.registerRootView(rootView, tag, themedRootContext);
+    mNumRootViews++;
     Systrace.endSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE);
     return tag;
   }
@@ -465,6 +467,7 @@ public void stopSurface(final int surfaceId) {
   @ReactMethod
   public void removeRootView(int rootViewTag) {
     mUIImplementation.removeRootView(rootViewTag);
+    mNumRootViews--;
   }
 
   public void updateNodeSize(int nodeViewTag, int newWidth, int newHeight) {
@@ -805,7 +808,12 @@ public void onBatchComplete() {
       listener.willDispatchViewUpdates(this);
     }
     try {
-      mUIImplementation.dispatchViewUpdates(batchId);
+      // If there are no RootViews registered, there will be no View updates to dispatch.
+      // This is a hack to prevent this from being called when Fabric is used everywhere.
+      // This should no longer be necessary in Bridgeless Mode.
+      if (mNumRootViews > 0) {
+        mUIImplementation.dispatchViewUpdates(batchId);
+      }
     } finally {
       Systrace.endSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE);
     }