Expose unstable_loadFusebox API on Android (#44858)

Summary:
Pull Request resolved: #44858

- Enables an opt-in to the Fusebox stack on Android for both architectures in open source.
- Templates use of this opt-in in RNTester.

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D58359907

packages/react-native/ReactAndroid/api/ReactAndroid.api

@@ -1995,6 +1995,7 @@ public final class com/facebook/react/defaults/DefaultNewArchitectureEntryPoint
 	public static final fun load (ZZ)V
 	public static final fun load (ZZZ)V
 	public static synthetic fun load$default (ZZZILjava/lang/Object;)V
+	public static final fun unstable_loadFusebox (Z)V
 }
 
 public class com/facebook/react/defaults/DefaultReactActivityDelegate : com/facebook/react/ReactActivityDelegate {

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

@@ -55,6 +55,10 @@ public object DefaultNewArchitectureEntryPoint {
             override fun batchRenderingUpdatesInEventLoop(): Boolean = true
 
             override fun useNativeViewConfigsInBridgelessMode(): Boolean = true
+
+            // We need to assign this now as we can't call ReactNativeFeatureFlags.override()
+            // more than once.
+            override fun fuseboxEnabledDebug(): Boolean = isFuseboxEnabled
           })
     }
 
@@ -67,21 +71,25 @@ public object DefaultNewArchitectureEntryPoint {
   }
 
   private var privateFabricEnabled: Boolean = false
+
   @JvmStatic
   public val fabricEnabled: Boolean
     get() = privateFabricEnabled
 
   private var privateTurboModulesEnabled: Boolean = false
+
   @JvmStatic
   public val turboModulesEnabled: Boolean
     get() = privateTurboModulesEnabled
 
   private var privateConcurrentReactEnabled: Boolean = false
+
   @JvmStatic
   public val concurrentReactEnabled: Boolean
     get() = privateConcurrentReactEnabled
 
   private var privateBridgelessEnabled: Boolean = false
+
   @JvmStatic
   public val bridgelessEnabled: Boolean
     get() = privateBridgelessEnabled
@@ -101,4 +109,29 @@ public object DefaultNewArchitectureEntryPoint {
                 "bridgelessEnabled=true requires (turboModulesEnabled=true AND fabricEnabled=true) - Please update your DefaultNewArchitectureEntryPoint.load() parameters."
         else -> true to ""
       }
+
+  /**
+   * If called, enables the new debugger stack (codename Fusebox). Must be called before
+   * {@link #load()}.
+   *
+   * @param isNewArchEnabled Please pass {@code BuildConfig.IS_NEW_ARCH_ENABLED} here.
+   */
+  @JvmStatic
+  public fun unstable_loadFusebox(
+      isNewArchEnabled: Boolean,
+  ) {
+    isFuseboxEnabled = true
+
+    if (!isNewArchEnabled) {
+      ReactNativeFeatureFlags.override(
+          object : ReactNativeFeatureFlagsDefaults() {
+            override fun fuseboxEnabledDebug(): Boolean = true
+          })
+    } else {
+      // no-op as this will be taken care of inside load() instead
+    }
+  }
+
+  /** Set to {@code true} if {@link #unstable_loadFusebox()} was called. */
+  private var isFuseboxEnabled: Boolean = false
 }

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

@@ -136,6 +136,10 @@ class RNTesterApplication : Application(), ReactApplication {
     ReactFontManager.getInstance().addCustomFont(this, "Rubik", R.font.rubik)
     super.onCreate()
     SoLoader.init(this, /* native exopackage */ false)
+
+    // [Experiment] Enable the new debugger stack (codename Fusebox)
+    // unstable_loadFusebox(BuildConfig.IS_NEW_ARCHITECTURE_ENABLED)
+
     if (BuildConfig.IS_NEW_ARCHITECTURE_ENABLED) {
       load()
     }