Prepare AppDelegate to enable Concurrent Root on iOS (#33671)

Summary:
Pull Request resolved: #33671

With React 18, we now need to allow users on Fabric to opt-in for Concurrent Root.

This commit adds a new method that can be customized in the AppDelegate to turn the feature on and off.
The flag is passed as an initialProps to the rootView.

## Changelog:
[iOS][Added] - Prepare a method in the AppDelegate to control the concurrentRoot.

Reviewed By: cortinico, dmitryrykun

Differential Revision: D35757833

fbshipit-source-id: 192cf74c796554cba39366aa90c53c191f960c20

Author: Riccardo Cipolleschi

packages/rn-tester/RNTester/AppDelegate.mm

@@ -77,6 +77,8 @@ @interface AppDelegate () <RCTCxxBridgeDelegate, RCTTurboModuleManagerDelegate>
 }
 @end
 
+static NSString *const kRNConcurrentRoot = @"concurrentRoot";
+
 @implementation AppDelegate
 
 - (BOOL)application:(__unused UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
@@ -86,12 +88,7 @@ - (BOOL)application:(__unused UIApplication *)application didFinishLaunchingWith
   _bridge = [[RCTBridge alloc] initWithDelegate:self launchOptions:launchOptions];
 
   // Appetizer.io params check
-  NSDictionary *initProps = @{};
-  NSString *_routeUri = [[NSUserDefaults standardUserDefaults] stringForKey:@"route"];
-  if (_routeUri) {
-    initProps =
-        @{@"exampleFromAppetizeParams" : [NSString stringWithFormat:@"rntester://example/%@Example", _routeUri]};
-  }
+  NSDictionary *initProps = [self prepareInitialProps];
 
 #ifdef RN_FABRIC_ENABLED
   _contextContainer = std::make_shared<facebook::react::ContextContainer const>();
@@ -119,6 +116,28 @@ - (BOOL)application:(__unused UIApplication *)application didFinishLaunchingWith
   return YES;
 }
 
+- (BOOL)concurrentRootEnabled
+{
+  // Switch this bool to turn on and off the concurrent root
+  return true;
+}
+
+- (NSDictionary *)prepareInitialProps
+{
+  NSMutableDictionary *initProps = [NSMutableDictionary new];
+
+  NSString *_routeUri = [[NSUserDefaults standardUserDefaults] stringForKey:@"route"];
+  if (_routeUri) {
+    initProps[@"exampleFromAppetizeParams"] = [NSString stringWithFormat:@"rntester://example/%@Example", _routeUri];
+  }
+
+#ifdef RCT_NEW_ARCH_ENABLED
+  initProps[kRNConcurrentRoot] = @([self concurrentRootEnabled]);
+#endif
+
+  return initProps;
+}
+
 - (NSURL *)sourceURLForBridge:(__unused RCTBridge *)bridge
 {
   return [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"packages/rn-tester/js/RNTesterApp.ios"];

template/ios/HelloWorld/AppDelegate.mm

@@ -16,6 +16,8 @@
 
 #import <react/config/ReactNativeConfig.h>
 
+static NSString *const kRNConcurrentRoot = @"concurrentRoot";
+
 @interface AppDelegate () <RCTCxxBridgeDelegate, RCTTurboModuleManagerDelegate> {
   RCTTurboModuleManager *_turboModuleManager;
   RCTSurfacePresenterBridgeAdapter *_bridgeAdapter;
@@ -41,7 +43,8 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
   bridge.surfacePresenter = _bridgeAdapter.surfacePresenter;
 #endif
 
-  UIView *rootView = RCTAppSetupDefaultRootView(bridge, @"HelloWorld", nil);
+  NSDictionary *initProps = [self prepareInitialProps];
+  UIView *rootView = RCTAppSetupDefaultRootView(bridge, @"HelloWorld", initProps);
 
   if (@available(iOS 13.0, *)) {
     rootView.backgroundColor = [UIColor systemBackgroundColor];
@@ -57,6 +60,28 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
   return YES;
 }
 
+/// This method controls whether the `concurrentRoot`feature of React18 is turned on or off.
+///
+/// @see: https://reactjs.org/blog/2022/03/29/react-v18.html
+/// @note: This requires to be rendering on Fabric (i.e. on the New Architecture).
+/// @return: `true` if the `concurrentRoot` feture is enabled. Otherwise, it returns `false`.
+- (BOOL)concurrentRootEnabled
+{
+  // Switch this bool to turn on and off the concurrent root
+  return true;
+}
+
+- (NSDictionary *)prepareInitialProps
+{
+  NSMutableDictionary *initProps = [NSMutableDictionary new];
+
+#ifdef RCT_NEW_ARCH_ENABLED
+  initProps[kRNConcurrentRoot] = @([self concurrentRootEnabled]);
+#endif
+
+  return initProps;
+}
+
 - (NSURL *)sourceURLForBridge:(RCTBridge *)bridge
 {
 #if DEBUG