From c032401b6724c99fbb80f158c82bd4fe690bf40e Mon Sep 17 00:00:00 2001
From: Paige Sun <paigesun@fb.com>
Date: Mon, 11 Apr 2022 15:30:14 -0700
Subject: [PATCH] Fix sticky headers for scrollviews by sending onScroll event
 to legacy RCTEventDispatcher

Summary:
Changelog: [Internal][Bridgeless] Fix sticky headers for scrollviews by sending onScroll event to legacy RCTEventDispatcher

## Extra Context

FYI. <ScrollView> is the only Fabric component view that needs to send events via the legacy RCTEventDispatcher. Ideally, all component views should only use `ViewEventEmitter` to send events to JS and not use RCTEventDispatcher. This ScrollView does use ScrollViewEventEmitter, a subclass of ViewEventEmitter:
```
std::static_pointer_cast<ScrollViewEventEmitter const>(_eventEmitter)->onScroll([self _scrollViewMetrics]);
```

However, it also needs RCTEventDispatcher for animations using `Animated.event` for `useNativeDriver: true`. See [ScrollView.js](https://github.com/facebook/react-native/blob/370c65b94379a72e74f5dfd5b6202f282841a950/Libraries/Components/ScrollView/ScrollView.js#L1124-L1129).

Reviewed By: RSNara

Differential Revision: D35540277

fbshipit-source-id: a28535ed10cac8e003523ecda6080574fbb89b85
---
 .../ScrollView/RCTScrollViewComponentView.mm  | 21 ++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm b/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
index 79f3cf315ed..7752626d2d8 100644
--- a/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
+++ b/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
@@ -52,7 +52,11 @@ static UIScrollViewIndicatorStyle RCTUIScrollViewIndicatorStyleFromProps(ScrollV
   }
 }
 
-static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteger tag)
+// Once Fabric implements proper NativeAnimationDriver, this should be removed.
+// This is just a workaround to allow animations based on onScroll event.
+// This is only used to animate sticky headers in ScrollViews, and only the contentOffset and tag is used.
+// TODO: T116850910 [Fabric][iOS] Make Fabric not use legacy RCTEventDispatcher for native-driven AnimatedEvents
+static void RCTSendScrollEventForNativeAnimations_DEPRECATED(UIScrollView *scrollView, NSInteger tag)
 {
   static uint16_t coalescingKey = 0;
   RCTScrollEvent *scrollEvent = [[RCTScrollEvent alloc] initWithEventName:@"onScroll"
@@ -64,7 +68,15 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
                                                       scrollViewZoomScale:scrollView.zoomScale
                                                                  userData:nil
                                                             coalescingKey:coalescingKey];
-  [[RCTBridge currentBridge].eventDispatcher sendEvent:scrollEvent];
+  RCTBridge *bridge = [RCTBridge currentBridge];
+  if (bridge) {
+    [bridge.eventDispatcher sendEvent:scrollEvent];
+  } else {
+    NSDictionary *userInfo = [NSDictionary dictionaryWithObjectsAndKeys:scrollEvent, @"event", nil];
+    [[NSNotificationCenter defaultCenter] postNotificationName:@"RCTSendEventToLegacyEventDispatcher"
+                                                        object:nil
+                                                      userInfo:userInfo];
+  }
 }
 
 @interface RCTScrollViewComponentView () <
@@ -403,9 +415,8 @@ static void RCTSendPaperScrollEvent_DEPRECATED(UIScrollView *scrollView, NSInteg
     if (_eventEmitter) {
       std::static_pointer_cast<ScrollViewEventEmitter const>(_eventEmitter)->onScroll([self _scrollViewMetrics]);
     }
-    // Once Fabric implements proper NativeAnimationDriver, this should be removed.
-    // This is just a workaround to allow animations based on onScroll event.
-    RCTSendPaperScrollEvent_DEPRECATED(scrollView, self.tag);
+
+    RCTSendScrollEventForNativeAnimations_DEPRECATED(scrollView, self.tag);
   }
 
   [self _remountChildrenIfNeeded];