Avoid allocating empty lists

Summary: Changelog: [Internal] Optimize Fabric memory allocations on Android

Reviewed By: genkikondo

Differential Revision: D36098912

fbshipit-source-id: 8d34a652db019865a8306bc070a08f5ddfef96bf

ReactAndroid/src/main/java/com/facebook/react/animated/NativeAnimatedModule.java

@@ -133,9 +133,7 @@ public class NativeAnimatedModule extends NativeAnimatedModuleSpec
 
     @UiThread
     void executeBatch(long maxBatchNumber, NativeAnimatedNodesManager nodesManager) {
-
       List<UIThreadOperation> operations;
-
       if (mSynchronizedAccess) {
         synchronized (this) {
           operations = drainQueueIntoList(maxBatchNumber);
@@ -143,13 +141,19 @@ public class NativeAnimatedModule extends NativeAnimatedModuleSpec
       } else {
         operations = drainQueueIntoList(maxBatchNumber);
       }
-      for (UIThreadOperation operation : operations) {
-        operation.execute(nodesManager);
+      if (operations != null) {
+        for (UIThreadOperation operation : operations) {
+          operation.execute(nodesManager);
+        }
       }
     }
 
     @UiThread
-    private List<UIThreadOperation> drainQueueIntoList(long maxBatchNumber) {
+    private @Nullable List<UIThreadOperation> drainQueueIntoList(long maxBatchNumber) {
+      if (mQueue.isEmpty() && mPeekedOperation == null) {
+        return null;
+      }
+
       List<UIThreadOperation> operations = new ArrayList<>();
       while (true) {
         // Due to a race condition, we manually "carry-over" a polled item from previous batch

ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/MountItemDispatcher.java

@@ -356,13 +356,16 @@ public class MountItemDispatcher {
   @Nullable
   private static <E extends MountItem> List<E> drainConcurrentItemQueue(
       ConcurrentLinkedQueue<E> queue) {
+    if (queue.isEmpty()) {
+      return null;
+    }
     List<E> result = new ArrayList<>();
-    while (!queue.isEmpty()) {
+    do {
       E item = queue.poll();
       if (item != null) {
         result.add(item);
       }
-    }
+    } while (!queue.isEmpty());
     if (result.size() == 0) {
       return null;
     }