Add more customization points in RCTAppDelegate (#34943)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/34943

The RCTAppDelegate class is the new way we have to encapsulate logic we don't want to leak to the average user.

However, some advanced users, like Expo users, may need more options to customize their setup.

This Diff provides more methods to extend the AppDelegate.

## Changelog
[iOS][Added] - Add more extension points for RCTAppDelegate

Reviewed By: cortinico

Differential Revision: D40262513

fbshipit-source-id: 9365a51d938a586b1508233bfa63693cf9aebf7a

Libraries/AppDelegate/RCTAppDelegate.h

@@ -35,7 +35,12 @@
  * If you need to customize the default implementation, you can invoke `[super <method_name>]` and use the returned
  object.
  *
- * Overridable methods (New Architecture):
+ * Overridable methods
+ * Shared:
+ *   - (RCTBridge *)createBridgeWithDelegate:(id<RCTBridgeDelegate>)delegate launchOptions:(NSDictionary *)launchOptions;
+ *   - (UIView *)createRootViewWithBridge:(RCTBridge *)bridge moduleName:(NSString*)moduleName initProps:(NSDictionary *)initProps;
+ *   - (UIViewController *)createRootViewController;
+ * New Architecture:
  *   - (BOOL)concurrentRootEnabled
  *   - (NSDictionary *)prepareInitialProps
  *   - (Class)getModuleClassFromName:(const char *)name
@@ -53,6 +58,41 @@
 @property (nonatomic, strong) RCTBridge *bridge;
 @property (nonatomic, strong) NSString *moduleName;
 
+/**
+ * It creates a `RCTBridge` using a delegate and some launch options.
+ * By default, it is invoked passing `self` as a delegate.
+ * You can override this function to customize the logic that creates the RCTBridge
+ *
+ * @parameter: delegate - an object that implements the `RCTBridgeDelegate` protocol.
+ * @parameter: launchOptions - a dictionary with a set of options.
+ *
+ * @returns: a newly created instance of RCTBridge.
+ */
+- (RCTBridge *)createBridgeWithDelegate:(id<RCTBridgeDelegate>)delegate launchOptions:(NSDictionary *)launchOptions;
+
+/**
+ * It creates a `UIView` starting from a bridge, a module name and a set of initial properties.
+ * By default, it is invoked using the bridge created by `createBridgeWithDelegate:launchOptions` and
+ * the name in the `self.moduleName` variable.
+ * You can override this function to customize the logic that creates the Root View.
+ *
+ * @parameter: bridge - an instance of the `RCTBridge` object.
+ * @parameter: moduleName - the name of the app, used by Metro to resolve the module.
+ * @parameter: initProps - a set of initial properties.
+ *
+ * @returns: a UIView properly configured with a bridge for React Native.
+ */
+- (UIView *)createRootViewWithBridge:(RCTBridge *)bridge moduleName:(NSString*)moduleName initProps:(NSDictionary *)initProps;
+
+/**
+ * It creates the RootViewController.
+ * By default, it creates a new instance of a `UIViewController`.
+ * You can override it to provide your own initial ViewController.
+ *
+ * @return: an instance of `UIViewController`.
+ */
+- (UIViewController *)createRootViewController;
+
 @end
 
 #if RCT_NEW_ARCH_ENABLED

Libraries/AppDelegate/RCTAppDelegate.mm

@@ -32,7 +32,7 @@ static NSString *const kRNConcurrentRoot = @"concurrentRoot";
   RCTAppSetupPrepareApp(application);
 
   if (!self.bridge) {
-    self.bridge = [[RCTBridge alloc] initWithDelegate:self launchOptions:launchOptions];
+    self.bridge = [self createBridgeWithDelegate:self launchOptions:launchOptions];
   }
 #if RCT_NEW_ARCH_ENABLED
   _contextContainer = std::make_shared<facebook::react::ContextContainer const>();
@@ -43,7 +43,7 @@ static NSString *const kRNConcurrentRoot = @"concurrentRoot";
 #endif
 
   NSDictionary *initProps = [self prepareInitialProps];
-  UIView *rootView = RCTAppSetupDefaultRootView(self.bridge, self.moduleName, initProps);
+  UIView *rootView = [self createRootViewWithBridge:self.bridge moduleName:self.moduleName initProps:initProps];
 
   if (@available(iOS 13.0, *)) {
     rootView.backgroundColor = [UIColor systemBackgroundColor];
@@ -52,7 +52,7 @@ static NSString *const kRNConcurrentRoot = @"concurrentRoot";
   }
 
   self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
-  UIViewController *rootViewController = [UIViewController new];
+  UIViewController *rootViewController = [self createRootViewController];
   rootViewController.view = rootView;
   self.window.rootViewController = rootViewController;
   [self.window makeKeyAndVisible];
@@ -88,6 +88,21 @@ static NSString *const kRNConcurrentRoot = @"concurrentRoot";
   return initProps;
 }
 
+- (RCTBridge *)createBridgeWithDelegate:(id<RCTBridgeDelegate>)delegate launchOptions:(NSDictionary *)launchOptions
+{
+  return [[RCTBridge alloc] initWithDelegate:delegate launchOptions:launchOptions];
+}
+
+- (UIView *)createRootViewWithBridge:(RCTBridge *)bridge moduleName:(NSString*)moduleName initProps:(NSDictionary *)initProps
+{
+  return RCTAppSetupDefaultRootView(bridge, moduleName, initProps);
+}
+
+- (UIViewController *)createRootViewController
+{
+  return [UIViewController new];
+}
+
 #if RCT_NEW_ARCH_ENABLED
 #pragma mark - RCTCxxBridgeDelegate