feat: move notifying observers to event dispatcher (#44474)

Summary:
Based on the discussion starting here: https://discord.com/channels/514829729862516747/1073566663825432587/1237407161991172157, I suggest moving the call to `_notifyEventDispatcherObserversOfEvent_DEPRECATED` straight to `RCTEventDispatcher.mm`. It was previously in `RCTInstance.mm` which is only relevant on bridgeless mode. We want to mimic the behavior of https://github.com/facebook/react-native/blob/06eea61c19cd730cf0c14a436f042d30791c3f4a/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm#L75-L78 but without using `currentBridge` since it is considered bad practice: software-mansion/react-native-reanimated#5497 (comment).

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[IOS] [CHANGED] - Move `notifyObservers` straight to `RCTEventDispatcher.mm`.

Pull Request resolved: #44474

Test Plan:
See that example with `stickyHeaders` still works correctly on both bridgeless and bridge mode.

Videos with it on https://github.com/facebook/react-native/blob/deee037c62a7d62a349d34db427b14d3560ddf83/packages/rn-tester/js/examples/FlatList/FlatList-stickyHeaders.js example with more items for visibility:

- bridgeless:

https://github.com/facebook/react-native/assets/32481228/8b78104a-226b-466a-9f32-60ba4ec14100

- bridge:

https://github.com/facebook/react-native/assets/32481228/f2ca67cb-578f-45d4-954f-3249c6fa9410

- old arch:

https://github.com/facebook/react-native/assets/32481228/7d642923-ddda-4dd3-8f14-c9982a03bc2e

Reviewed By: javache

Differential Revision: D57097880

Pulled By: cipolleschi

fbshipit-source-id: de1504e90529fe4f001f44f02ace329386cf7727

packages/react-native/React/CoreModules/RCTEventDispatcher.mm

@@ -54,6 +54,12 @@ - (void)initialize
   _eventsDispatchScheduled = NO;
   _observers = [NSHashTable weakObjectsHashTable];
   _observersLock = [NSRecursiveLock new];
+
+  NSNotificationCenter *defaultCenter = [NSNotificationCenter defaultCenter];
+  [defaultCenter addObserver:self
+                    selector:@selector(_notifyEventDispatcherObserversOfEvent_DEPRECATED:)
+                        name:@"RCTNotifyEventDispatcherObserversOfEvent_DEPRECATED"
+                      object:nil];
 }
 
 - (void)sendViewEventWithName:(NSString *)name reactTag:(NSNumber *)reactTag
@@ -225,6 +231,15 @@ - (void)flushEventsQueue
   }
 }
 
+- (void)_notifyEventDispatcherObserversOfEvent_DEPRECATED:(NSNotification *)notification
+{
+  NSDictionary *userInfo = notification.userInfo;
+  id<RCTEvent> event = [userInfo objectForKey:@"event"];
+  if (event) {
+    [self notifyObserversOfEvent:event];
+  }
+}
+
 - (std::shared_ptr<facebook::react::TurboModule>)getTurboModule:
     (const facebook::react::ObjCTurboModule::InitParams &)params
 {

packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm

@@ -68,15 +68,10 @@ static void RCTSendScrollEventForNativeAnimations_DEPRECATED(UIScrollView *scrol
                                                       scrollViewZoomScale:scrollView.zoomScale
                                                                  userData:nil
                                                             coalescingKey:coalescingKey];
-  RCTBridge *bridge = [RCTBridge currentBridge];
-  if (bridge) {
-    [bridge.eventDispatcher sendEvent:scrollEvent];
-  } else {
-    NSDictionary *userInfo = [NSDictionary dictionaryWithObjectsAndKeys:scrollEvent, @"event", nil];
-    [[NSNotificationCenter defaultCenter] postNotificationName:@"RCTNotifyEventDispatcherObserversOfEvent_DEPRECATED"
-                                                        object:nil
-                                                      userInfo:userInfo];
-  }
+  NSDictionary *userInfo = [NSDictionary dictionaryWithObjectsAndKeys:scrollEvent, @"event", nil];
+  [[NSNotificationCenter defaultCenter] postNotificationName:@"RCTNotifyEventDispatcherObserversOfEvent_DEPRECATED"
+                                                      object:nil
+                                                    userInfo:userInfo];
 }
 
 @interface RCTScrollViewComponentView () <

packages/react-native/ReactCommon/react/runtime/platform/ios/ReactCommon/RCTInstance.mm

@@ -128,13 +128,6 @@ - (instancetype)initWithDelegate:(id<RCTInstanceDelegate>)delegate
     }
     _launchOptions = launchOptions;
 
-    NSNotificationCenter *defaultCenter = [NSNotificationCenter defaultCenter];
-
-    [defaultCenter addObserver:self
-                      selector:@selector(_notifyEventDispatcherObserversOfEvent_DEPRECATED:)
-                          name:@"RCTNotifyEventDispatcherObserversOfEvent_DEPRECATED"
-                        object:nil];
-
     [self _start];
   }
   return self;
@@ -481,19 +474,6 @@ - (void)_loadScriptFromSource:(RCTSource *)source
   }
 }
 
-- (void)_notifyEventDispatcherObserversOfEvent_DEPRECATED:(NSNotification *)notification
-{
-  NSDictionary *userInfo = notification.userInfo;
-  id<RCTEvent> event = [userInfo objectForKey:@"event"];
-
-  RCTModuleRegistry *moduleRegistry = _bridgeModuleDecorator.moduleRegistry;
-  if (event && moduleRegistry) {
-    id<RCTEventDispatcherProtocol> legacyEventDispatcher = [moduleRegistry moduleForName:"EventDispatcher"
-                                                                   lazilyLoadIfNecessary:YES];
-    [legacyEventDispatcher notifyObserversOfEvent:event];
-  }
-}
-
 - (void)_handleJSError:(const JsErrorHandler::ParsedError &)error
 {
   NSString *message = [NSString stringWithCString:error.message.c_str() encoding:[NSString defaultCStringEncoding]];