Implement BackgroundExecutor for layouts

Summary:
As a followup to D22743723 (https://github.com/facebook/react-native/commit/d53fc8a3cd44c7ec7e6eade985daf3d4feb2d736) on the iOS side, I implement a BackgroundExecutor that can be used from C++ to schedule layouts on their own thread, off the JS and UI thread.

This is a potential perf improvement that we will experiment with.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D22826795

fbshipit-source-id: 899bd67fe1b86f52910951e9536b59a1414a304c

ReactAndroid/src/main/java/com/facebook/react/bridge/BackgroundExecutor.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.bridge;
+
+import com.facebook.proguard.annotations.DoNotStrip;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+@DoNotStrip
+public class BackgroundExecutor {
+  private static final String TAG = "FabricBackgroundExecutor";
+  private final ExecutorService mExecutorService;
+
+  @DoNotStrip
+  private BackgroundExecutor() {
+    mExecutorService = Executors.newFixedThreadPool(1);
+  }
+
+  @DoNotStrip
+  private void queueRunnable(Runnable runnable) {
+    // Very rarely, an NPE is hit here - probably has to do with deallocation
+    // race conditions and the JNI.
+    // It's not clear yet which of these is most prevalent, or if either is a concern.
+    // If we don't find these logs in production then we can probably safely remove the logging,
+    // but it's also cheap to leave it here.
+
+    if (runnable == null) {
+      ReactSoftException.logSoftException(TAG, new ReactNoCrashSoftException("runnable is null"));
+      return;
+    }
+
+    final ExecutorService executorService = mExecutorService;
+    if (executorService == null) {
+      ReactSoftException.logSoftException(
+          TAG, new ReactNoCrashSoftException("executorService is null"));
+      return;
+    }
+
+    executorService.execute(runnable);
+  }
+}

ReactAndroid/src/main/java/com/facebook/react/fabric/jni/Binding.cpp

@@ -278,6 +278,12 @@ void Binding::installFabricUIManager(
   toolbox.synchronousEventBeatFactory = synchronousBeatFactory;
   toolbox.asynchronousEventBeatFactory = asynchronousBeatFactory;
 
+  if (reactNativeConfig_->getBool(
+          "react_fabric:enable_background_executor_android")) {
+    backgroundExecutor_ = std::make_unique<JBackgroundExecutor>();
+    toolbox.backgroundExecutor = backgroundExecutor_->get();
+  }
+
   if (enableLayoutAnimations) {
     animationDriver_ = std::make_shared<LayoutAnimationDriver>(this);
   }

ReactAndroid/src/main/java/com/facebook/react/fabric/jni/Binding.h

@@ -19,6 +19,7 @@
 #include <mutex>
 #include "ComponentFactoryDelegate.h"
 #include "EventBeatManager.h"
+#include "JBackgroundExecutor.h"
 
 namespace facebook {
 namespace react {
@@ -115,6 +116,7 @@ class Binding : public jni::HybridClass<Binding>,
   virtual void onAllAnimationsComplete() override;
   LayoutAnimationDriver *getAnimationDriver();
   std::shared_ptr<LayoutAnimationDriver> animationDriver_;
+  std::unique_ptr<JBackgroundExecutor> backgroundExecutor_;
 
   std::shared_ptr<Scheduler> scheduler_;
   std::mutex schedulerMutex_;

ReactAndroid/src/main/java/com/facebook/react/fabric/jni/JBackgroundExecutor.cpp

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <fbjni/fbjni.h>
+#include <react/jni/JNativeRunnable.h>
+
+#include "JBackgroundExecutor.h"
+
+namespace facebook {
+namespace react {
+
+using namespace facebook::jni;
+
+using facebook::react::JNativeRunnable;
+using facebook::react::Runnable;
+
+BackgroundExecutor JBackgroundExecutor::get() {
+  auto self = JBackgroundExecutor::create();
+
+  return [self](std::function<void()> &&runnable) {
+    static auto method =
+        findClassStatic(JBackgroundExecutor::JBackgroundExecutorJavaDescriptor)
+            ->getMethod<void(Runnable::javaobject)>("queueRunnable");
+
+    auto jrunnable = JNativeRunnable::newObjectCxxArgs(std::move(runnable));
+    method(self, static_ref_cast<Runnable>(jrunnable).get());
+  };
+}
+
+} // namespace react
+} // namespace facebook

ReactAndroid/src/main/java/com/facebook/react/fabric/jni/JBackgroundExecutor.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <fbjni/fbjni.h>
+#include <react/uimanager/primitives.h>
+
+namespace facebook {
+namespace react {
+
+using namespace facebook::jni;
+
+class JBackgroundExecutor : public JavaClass<JBackgroundExecutor> {
+ public:
+  static auto constexpr kJavaDescriptor =
+      "Lcom/facebook/react/bridge/BackgroundExecutor;";
+
+  constexpr static auto JBackgroundExecutorJavaDescriptor =
+      "com/facebook/react/bridge/BackgroundExecutor";
+
+  static global_ref<javaobject> create() {
+    return make_global(newInstance());
+  }
+
+  BackgroundExecutor get();
+};
+
+} // namespace react
+} // namespace facebook