Update the template to load the correct JS engine at runtime. (#35095)

Summary:
Pull Request resolved: #35095

This change will make sure that we load the correct JS engine at runtime,
by using the BuildConfig flag that RNGP sets for us.

This will solve a lot of noise in adb logcat for users seeing
stacktraces mentioning failing to load `jscexecutor` library.

This is also a breaking change, but as the API was not widely used nor
advertised in the template, we should be fine by just mentioning this in the release notes.

Changelog:
[Android] [Changed] - Update the template to load the correct JS engine at runtime

Differential Revision: D40710597

fbshipit-source-id: d47e6c538696428eec591afba53bb65a177ce927

ReactAndroid/src/main/java/com/facebook/react/JSEngineResolutionAlgorithm.java

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and 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;
+
+/**
+ * An enum that specifies the algorithm to use when loading theJS Engine. {@link #JSC} will load
+ * JavaScriptCore first and fail if it is not available. {@link #HERMES} will load Hermes first and
+ * fail if it is not available.
+ */
+public enum JSEngineResolutionAlgorithm {
+  JSC,
+  HERMES
+}

ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManagerBuilder.java

@@ -14,6 +14,7 @@
 import android.app.Application;
 import android.content.Context;
 import androidx.annotation.Nullable;
+import com.facebook.common.logging.FLog;
 import com.facebook.hermes.reactexecutor.HermesExecutor;
 import com.facebook.hermes.reactexecutor.HermesExecutorFactory;
 import com.facebook.infer.annotation.Assertions;
@@ -40,6 +41,8 @@
 /** Builder class for {@link ReactInstanceManager} */
 public class ReactInstanceManagerBuilder {
 
+  private static final String TAG = ReactInstanceManagerBuilder.class.getSimpleName();
+
   private final List<ReactPackage> mPackages = new ArrayList<>();
 
   private @Nullable String mJSBundleAssetUrl;
@@ -64,7 +67,7 @@ public class ReactInstanceManagerBuilder {
   private @Nullable Map<String, RequestHandler> mCustomPackagerCommandHandlers;
   private @Nullable ReactPackageTurboModuleManagerDelegate.Builder mTMMDelegateBuilder;
   private @Nullable SurfaceDelegateFactory mSurfaceDelegateFactory;
-  private JSInterpreter jsInterpreter = JSInterpreter.OLD_LOGIC;
+  private JSEngineResolutionAlgorithm jsEngineResolutionAlgorithm = null;
 
   /* package protected */ ReactInstanceManagerBuilder() {}
 
@@ -118,28 +121,12 @@ public ReactInstanceManagerBuilder setJSBundleLoader(JSBundleLoader jsBundleLoad
   }
 
   /**
-   * Sets the jsEngine as JSC or HERMES as per the setJsEngineAsHermes call Uses the enum {@link
-   * JSInterpreter}
-   *
-   * @param jsInterpreter
-   */
-  private void setJSEngine(JSInterpreter jsInterpreter) {
-    this.jsInterpreter = jsInterpreter;
-  }
-
-  /**
-   * Utility setter to set the required JSEngine as HERMES or JSC Defaults to OLD_LOGIC if not
-   * called by the host app
-   *
-   * @param hermesEnabled hermesEnabled = true sets the JS Engine as HERMES and JSC otherwise
+   * Sets the JS Engine to load as either Hermes or JSC. If not set, the default is JSC with a
+   * Hermes fallback.
    */
-  public ReactInstanceManagerBuilder setJsEngineAsHermes(boolean hermesEnabled) {
-    if (hermesEnabled) {
-      setJSEngine(JSInterpreter.HERMES);
-    } else {
-      setJSEngine(JSInterpreter.JSC);
-    }
-    return this;
+  private void setJSEngineResolutionAlgorithm(
+      @Nullable JSEngineResolutionAlgorithm jsEngineResolutionAlgorithm) {
+    this.jsEngineResolutionAlgorithm = jsEngineResolutionAlgorithm;
   }
 
   /**
@@ -365,7 +352,11 @@ private JavaScriptExecutorFactory getDefaultJSExecutorFactory(
 
     // if nothing is specified, use old loading method
     // else load the required engine
-    if (jsInterpreter == JSInterpreter.OLD_LOGIC) {
+    if (jsEngineResolutionAlgorithm == null) {
+      FLog.w(
+          TAG,
+          "You're not setting the JS Engine Resolution Algorithm. "
+              + "We'll try to load JSC first, and if it fails we'll fallback to Hermes");
       try {
         // If JSC is included, use it as normal
         initializeSoLoaderIfNecessary(applicationContext);
@@ -378,7 +369,7 @@ private JavaScriptExecutorFactory getDefaultJSExecutorFactory(
         HermesExecutor.loadLibrary();
         return new HermesExecutorFactory();
       }
-    } else if (jsInterpreter == JSInterpreter.HERMES) {
+    } else if (jsEngineResolutionAlgorithm == JSEngineResolutionAlgorithm.HERMES) {
       HermesExecutor.loadLibrary();
       return new HermesExecutorFactory();
     } else {

ReactAndroid/src/main/java/com/facebook/react/ReactNativeHost.java

@@ -190,4 +190,12 @@ protected String getJSMainModuleName() {
    * default ones, you'll want to include more packages here.
    */
   protected abstract List<ReactPackage> getPackages();
+
+  /**
+   * Returns the {@link JSEngineResolutionAlgorithm} to be used when loading the JS engine. If null,
+   * will try to load JSC first and fallback to Hermes if JSC is not available.
+   */
+  protected @Nullable JSEngineResolutionAlgorithm getJSEngineResolutionAlgorithm() {
+    return null;
+  }
 }

ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultReactNativeHost.kt

@@ -8,6 +8,7 @@
 package com.facebook.react.defaults
 
 import android.app.Application
+import com.facebook.react.JSEngineResolutionAlgorithm
 import com.facebook.react.ReactNativeHost
 import com.facebook.react.ReactPackageTurboModuleManagerDelegate
 import com.facebook.react.bridge.JSIModulePackage
@@ -20,8 +21,10 @@ import com.facebook.react.bridge.JSIModulePackage
  * providing the default TurboModuleManagerDelegateBuilder and the default JSIModulePackage,
  * provided the name of the dynamic library to load.
  */
-abstract class DefaultReactNativeHost protected constructor(application: Application) :
-    ReactNativeHost(application) {
+abstract class DefaultReactNativeHost
+protected constructor(
+    application: Application,
+) : ReactNativeHost(application) {
 
   override fun getReactPackageTurboModuleManagerDelegateBuilder():
       ReactPackageTurboModuleManagerDelegate.Builder? =
@@ -38,6 +41,13 @@ abstract class DefaultReactNativeHost protected constructor(application: Applica
         null
       }
 
+  override fun getJSEngineResolutionAlgorithm(): JSEngineResolutionAlgorithm? =
+      when (isHermesEnabled) {
+        true -> JSEngineResolutionAlgorithm.HERMES
+        false -> JSEngineResolutionAlgorithm.JSC
+        null -> null
+      }
+
   /**
    * Returns whether the user wants to use the New Architecture or not.
    *
@@ -48,4 +58,14 @@ abstract class DefaultReactNativeHost protected constructor(application: Applica
    */
   protected open val isNewArchEnabled: Boolean
     get() = false
+
+  /**
+   * Returns whether the user wants to use Hermes.
+   *
+   * If true, the app will load the Hermes engine, and fail if not found. If false, the app will
+   * load the JSC engine, and fail if not found. If null, the app will attempt to load JSC first and
+   * fallback to Hermes if not found.
+   */
+  protected open val isHermesEnabled: Boolean?
+    get() = null
 }

packages/rn-tester/android/app/build.gradle

@@ -139,9 +139,11 @@ android {
     productFlavors {
         hermes {
             dimension "vm"
+            buildConfigField("boolean", "IS_HERMES_ENABLED_IN_FLAVOR", "true")
         }
         jsc {
             dimension "vm"
+            buildConfigField("boolean", "IS_HERMES_ENABLED_IN_FLAVOR", "false")
         }
     }
 

packages/rn-tester/android/app/src/main/java/com/facebook/react/uiapp/RNTesterApplication.java

@@ -115,6 +115,11 @@ public List<ViewManager> createViewManagers(
         protected boolean isNewArchEnabled() {
           return true;
         }
+
+        @Override
+        protected Boolean isHermesEnabled() {
+          return BuildConfig.IS_HERMES_ENABLED_IN_FLAVOR;
+        }
       };
 
   @Override

template/android/app/src/main/java/com/helloworld/MainApplication.java

@@ -38,6 +38,11 @@ protected String getJSMainModuleName() {
         protected boolean isNewArchEnabled() {
           return BuildConfig.IS_NEW_ARCHITECTURE_ENABLED;
         }
+
+        @Override
+        protected Boolean isHermesEnabled() {
+          return BuildConfig.IS_HERMES_ENABLED;
+        }
       };
 
   @Override