Simplify new app template for bridgeless (#40929)

Summary:
Pull Request resolved: #40929

This diff reduces the footprint that bridgeless is imposing on the new app template. Specifically:
- I've created a `.toReactHost` method that converts a DefaultReactNativeHost to a DefaultReactHost
- I've updated RN Tester to use the same setup as the New App template which reduces code duplication.

I also had to remove a couple of `UnstableReactNativeAPI` as those were bleeding in the new app template.
I don't think we should ask users to opt-in in `UnstableReactNativeAPI` in the New App template itself as
this means that all the apps will get this opt-in.
Instead we should keep it only for specific APIs that we want the users to opt into.

Changelog:
[Internal] [Changed] - Simplify new app template for bridgeless

Reviewed By: cipolleschi, luluwu2032

Differential Revision: D50227693

fbshipit-source-id: e86c54d5156cc27f1f898b43ca89c57d5cf148b8

Author: cortinico

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactHost.kt

@@ -13,7 +13,6 @@ import android.os.Bundle
 import com.facebook.react.bridge.ReactContext
 import com.facebook.react.bridge.queue.ReactQueueConfiguration
 import com.facebook.react.common.LifecycleState
-import com.facebook.react.common.annotations.UnstableReactNativeAPI
 import com.facebook.react.devsupport.interfaces.DevSupportManager
 import com.facebook.react.interfaces.TaskInterface
 import com.facebook.react.interfaces.fabric.ReactSurface
@@ -26,7 +25,6 @@ import com.facebook.react.modules.core.DefaultHardwareBackBtnHandler
  *
  * The implementation of this interface should be Thread Safe
  */
-@UnstableReactNativeAPI
 interface ReactHost {
 
   /** The current [LifecycleState] for React Host */

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/defaults/DefaultReactHost.kt

@@ -8,7 +8,9 @@
 package com.facebook.react.defaults
 
 import android.content.Context
+import com.facebook.react.JSEngineResolutionAlgorithm
 import com.facebook.react.ReactHost
+import com.facebook.react.ReactNativeHost
 import com.facebook.react.ReactPackage
 import com.facebook.react.bridge.JSBundleLoader
 import com.facebook.react.common.annotations.UnstableReactNativeAPI
@@ -18,10 +20,29 @@ import com.facebook.react.runtime.JSCInstance
 import com.facebook.react.runtime.ReactHostImpl
 import com.facebook.react.runtime.hermes.HermesInstance
 
-@UnstableReactNativeAPI
+/**
+ * A utility class that allows you to simplify the setup of a [ReactHost] for new apps in Open
+ * Source.
+ *
+ * [ReactHost] is an interface responsible of handling the lifecycle of a React Native app when
+ * running in bridgeless mode.
+ */
 object DefaultReactHost {
   private var reactHost: ReactHost? = null
 
+  /**
+   * Util function to create a default [ReactHost] to be used in your application. This method is
+   * used by the New App template.
+   *
+   * @param context the Android [Context] to use for creating the [ReactHost]
+   * @param packageList the list of [ReactPackage]s to use for creating the [ReactHost]
+   * @param jsMainModulePath the path to your app's main module on Metro. Usually `index` or
+   *   `index.<platform>`
+   * @param jsBundleAssetPath the path to the JS bundle relative to the assets directory. Will be
+   *   composed in a `asset://...` URL
+   * @param isHermesEnabled whether to use Hermes as the JS engine, default to true.
+   */
+  @OptIn(UnstableReactNativeAPI::class)
   @JvmStatic
   fun getDefaultReactHost(
       context: Context,
@@ -47,13 +68,42 @@ object DefaultReactHost {
       // TODO: T164788699 find alternative of accessing ReactHostImpl for initialising reactHost
       reactHost =
           ReactHostImpl(
-              context,
-              defaultReactHostDelegate,
-              componentFactory,
-              true,
-              reactJsExceptionHandler,
-              true)
+                  context,
+                  defaultReactHostDelegate,
+                  componentFactory,
+                  true,
+                  reactJsExceptionHandler,
+                  true)
+              .apply {
+                jsEngineResolutionAlgorithm =
+                    if (isHermesEnabled) {
+                      JSEngineResolutionAlgorithm.HERMES
+                    } else {
+                      JSEngineResolutionAlgorithm.JSC
+                    }
+              }
     }
     return reactHost as ReactHost
   }
+
+  /**
+   * Util function to create a default [ReactHost] to be used in your application. This method is
+   * used by the New App template.
+   *
+   * This method takes in input a [ReactNativeHost] (bridge-mode) and uses its configuration to
+   * create an equivalent [ReactHost] (bridgeless-mode).
+   *
+   * @param context the Android [Context] to use for creating the [ReactHost]
+   * @param reactNativeHost the [ReactNativeHost] to use for creating the [ReactHost]
+   */
+  @JvmStatic
+  fun getDefaultReactHost(
+      context: Context,
+      reactNativeHost: ReactNativeHost,
+  ): ReactHost {
+    require(reactNativeHost is DefaultReactNativeHost) {
+      "You can call getDefaultReactHost only with instances of DefaultReactNativeHost"
+    }
+    return reactNativeHost.toReactHost(context)
+  }
 }

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

@@ -8,7 +8,9 @@
 package com.facebook.react.defaults
 
 import android.app.Application
+import android.content.Context
 import com.facebook.react.JSEngineResolutionAlgorithm
+import com.facebook.react.ReactHost
 import com.facebook.react.ReactNativeHost
 import com.facebook.react.ReactPackageTurboModuleManagerDelegate
 import com.facebook.react.bridge.JSIModulePackage
@@ -68,4 +70,18 @@ protected constructor(
    */
   protected open val isHermesEnabled: Boolean?
     get() = null
+
+  /**
+   * Converts this [ReactNativeHost] (bridge-mode) to a [ReactHost] (bridgeless-mode).
+   *
+   * @param context the Android [Context] to use for creating the [ReactHost]
+   */
+  fun toReactHost(context: Context): ReactHost =
+      DefaultReactHost.getDefaultReactHost(
+          context,
+          packages,
+          jsMainModuleName,
+          bundleAssetName ?: "index",
+          isHermesEnabled ?: true,
+      )
 }

packages/react-native/template/android/app/src/main/java/com/helloworld/MainApplication.kt

@@ -6,14 +6,12 @@ import com.facebook.react.ReactApplication
 import com.facebook.react.ReactHost
 import com.facebook.react.ReactNativeHost
 import com.facebook.react.ReactPackage
-import com.facebook.react.common.annotations.UnstableReactNativeAPI
 import com.facebook.react.defaults.DefaultNewArchitectureEntryPoint.load
-import com.facebook.react.defaults.DefaultReactHost
+import com.facebook.react.defaults.DefaultReactHost.getDefaultReactHost
 import com.facebook.react.defaults.DefaultReactNativeHost
 import com.facebook.react.flipper.ReactNativeFlipper
 import com.facebook.soloader.SoLoader
 
-@UnstableReactNativeAPI
 class MainApplication : Application(), ReactApplication {
 
   override val reactNativeHost: ReactNativeHost =
@@ -33,12 +31,7 @@ class MainApplication : Application(), ReactApplication {
       }
 
   override val reactHost: ReactHost
-    get() =
-        DefaultReactHost.getDefaultReactHost(
-            context = this,
-            packageList = PackageList(this).packages,
-            jsMainModulePath = "index",
-            isHermesEnabled = BuildConfig.IS_HERMES_ENABLED)
+    get() = getDefaultReactHost(this.applicationContext, reactNativeHost)
 
   override fun onCreate() {
     super.onCreate()

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

@@ -10,30 +10,26 @@ package com.facebook.react.uiapp
 import android.app.Application
 import com.facebook.fbreact.specs.SampleLegacyModule
 import com.facebook.fbreact.specs.SampleTurboModule
-import com.facebook.react.JSEngineResolutionAlgorithm
 import com.facebook.react.ReactApplication
 import com.facebook.react.ReactHost
 import com.facebook.react.ReactNativeHost
 import com.facebook.react.ReactPackage
 import com.facebook.react.TurboReactPackage
+import com.facebook.react.ViewManagerOnDemandReactPackage
 import com.facebook.react.bridge.NativeModule
 import com.facebook.react.bridge.ReactApplicationContext
-import com.facebook.react.common.annotations.UnstableReactNativeAPI
 import com.facebook.react.common.assets.ReactFontManager
-import com.facebook.react.common.mapbuffer.ReadableMapBuffer
 import com.facebook.react.config.ReactFeatureFlags
-import com.facebook.react.defaults.DefaultComponentsRegistry.Companion.register
 import com.facebook.react.defaults.DefaultNewArchitectureEntryPoint.load
+import com.facebook.react.defaults.DefaultReactHost
 import com.facebook.react.defaults.DefaultReactNativeHost
-import com.facebook.react.fabric.ComponentFactory
 import com.facebook.react.flipper.ReactNativeFlipper.initializeFlipper
-import com.facebook.react.interfaces.exceptionmanager.ReactJsExceptionHandler
 import com.facebook.react.module.model.ReactModuleInfo
 import com.facebook.react.module.model.ReactModuleInfoProvider
-import com.facebook.react.runtime.ReactHostImpl
 import com.facebook.react.shell.MainReactPackage
 import com.facebook.react.uiapp.component.MyLegacyViewManager
 import com.facebook.react.uiapp.component.MyNativeViewManager
+import com.facebook.react.uimanager.ReactShadowNode
 import com.facebook.react.uimanager.ViewManager
 import com.facebook.soloader.SoLoader
 
@@ -96,17 +92,28 @@ class RNTesterApplication : Application(), ReactApplication {
                     }
                   }
             },
-            object : ReactPackage {
+            object : ReactPackage, ViewManagerOnDemandReactPackage {
               override fun createNativeModules(
                   reactContext: ReactApplicationContext
-              ): List<NativeModule> {
-                return emptyList()
-              }
+              ): List<NativeModule> = emptyList()
+
+              override fun getViewManagerNames(reactContext: ReactApplicationContext) =
+                  listOf("RNTMyNativeView", "RNTMyLegacyNativeView")
 
               override fun createViewManagers(
                   reactContext: ReactApplicationContext
               ): List<ViewManager<*, *>> =
                   listOf(MyNativeViewManager(), MyLegacyViewManager(reactContext))
+
+              override fun createViewManager(
+                  reactContext: ReactApplicationContext,
+                  viewManagerName: String
+              ): ViewManager<*, out ReactShadowNode<*>> =
+                  if (viewManagerName == "RNTMyNativeView") {
+                    MyNativeViewManager()
+                  } else {
+                    MyLegacyViewManager(reactContext)
+                  }
             })
       }
 
@@ -115,6 +122,9 @@ class RNTesterApplication : Application(), ReactApplication {
     }
   }
 
+  override val reactHost: ReactHost
+    get() = DefaultReactHost.getDefaultReactHost(this.applicationContext, reactNativeHost)
+
   override fun onCreate() {
     ReactFontManager.getInstance().addCustomFont(this, "Rubik", R.font.rubik)
     super.onCreate()
@@ -124,35 +134,4 @@ class RNTesterApplication : Application(), ReactApplication {
     }
     initializeFlipper(this, reactNativeHost.reactInstanceManager)
   }
-
-  @UnstableReactNativeAPI
-  override val reactHost: ReactHost by lazy {
-    // Create an instance of ReactHost to manager the instance of ReactInstance,
-    // which is similar to how we use ReactNativeHost to manager instance of ReactInstanceManager
-    val reactHostDelegate = RNTesterReactHostDelegate(applicationContext)
-    val reactJsExceptionHandler = RNTesterReactJsExceptionHandler()
-    val componentFactory = ComponentFactory()
-    register(componentFactory)
-    ReactHostImpl(
-            this.applicationContext,
-            reactHostDelegate,
-            componentFactory,
-            true,
-            reactJsExceptionHandler,
-            true)
-        .apply {
-          jsEngineResolutionAlgorithm =
-              if (BuildConfig.IS_HERMES_ENABLED_IN_FLAVOR) {
-                JSEngineResolutionAlgorithm.HERMES
-              } else {
-                JSEngineResolutionAlgorithm.JSC
-              }
-          reactHostDelegate.reactHost = this
-        }
-  }
-
-  @UnstableReactNativeAPI
-  class RNTesterReactJsExceptionHandler : ReactJsExceptionHandler {
-    override fun reportJsException(errorMap: ReadableMapBuffer?) {}
-  }
 }