fix(ios): emit onPageScroll and onPageScrollStateChanged on iOS 15

ios/PagerScrollDelegate.swift

@@ -4,7 +4,7 @@ import UIKit
  Scroll delegate used to control underlying TabView's collection view.
  */
 class PagerScrollDelegate: NSObject, UIScrollViewDelegate, UICollectionViewDelegate {
-  weak var originalDelegate: UICollectionViewDelegate?
+  weak var originalDelegate: UIScrollViewDelegate?
   weak var delegate: PagerViewProviderDelegate?
   var orientation: UICollectionView.ScrollDirection = .horizontal
 
@@ -78,11 +78,13 @@ class PagerScrollDelegate: NSObject, UIScrollViewDelegate, UICollectionViewDeleg
   }
 
   func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
-    originalDelegate?.collectionView?(collectionView, didEndDisplaying: cell, forItemAt: indexPath)
+    (originalDelegate as? UICollectionViewDelegate)?.collectionView?(collectionView, didEndDisplaying: cell, forItemAt: indexPath)
   }
 
   func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
-    originalDelegate?.collectionView?(collectionView, willDisplay: cell, forItemAt: indexPath)
+    // iOS 15 only, see PagerViewIOS15Compat.swift (delete together with the iOS 15 support).
+    PagerViewIOS15Compat.attachScrollDelegate(to: collectionView, delegate: delegate)
+    (originalDelegate as? UICollectionViewDelegate)?.collectionView?(collectionView, willDisplay: cell, forItemAt: indexPath)
   }
 
   override func responds(to aSelector: Selector!) -> Bool {

ios/PagerViewIOS15Compat.swift

@@ -7,14 +7,18 @@ import UIKit
 /// collection view (private class `_UICollectionViewOrthogonalScrollerEmbeddedScrollView`), while
 /// the collection view itself only scrolls vertically. Because `PagerView` can only toggle the
 /// collection view, on iOS 15 `scrollEnabled={false}` / `overdrag={false}` never reached the view
-/// that actually handles the swipe.
+/// that actually handles the swipe. For the same reason `PagerScrollDelegate`, installed on the
+/// collection view, never saw the paging scroll, so `onPageScroll` / `onPageScrollStateChanged`
+/// were never emitted.
 ///
 /// On iOS 16 and later the collection view itself is the pager, so none of this is needed — the
 /// `#available` checks below make every function in here a no-op on those versions.
 ///
 /// - Note: Delete this file together with the two `PagerViewIOS15Compat` calls in `PagerView.swift`
-///   once the minimum supported iOS version is 16.
+///   and the one in `PagerScrollDelegate.swift` once the minimum supported iOS version is 16.
 enum PagerViewIOS15Compat {
+  private static var scrollDelegateKey: UInt8 = 0
+
   /// Mirrors `scrollEnabled` onto the scroll views SwiftUI embedded in the paging collection view.
   static func applyScrollEnabled(_ scrollEnabled: Bool, to collectionView: UICollectionView?) {
     if #available(iOS 16, *) {
@@ -33,6 +37,23 @@ enum PagerViewIOS15Compat {
     pagingScrollViews(in: collectionView).forEach { $0.bounces = overdrag }
   }
 
+  /// Puts a `PagerScrollDelegate` in front of the delegate of the scroll views SwiftUI embedded in
+  /// the paging collection view. They only exist once the collection view has laid out its pages.
+  static func attachScrollDelegate(to collectionView: UICollectionView, delegate: PagerViewProviderDelegate?) {
+    if #available(iOS 16, *) {
+      return
+    }
+
+    for scrollView in pagingScrollViews(in: collectionView) where !(scrollView.delegate is PagerScrollDelegate) {
+      let scrollDelegate = PagerScrollDelegate()
+      scrollDelegate.originalDelegate = scrollView.delegate
+      scrollDelegate.delegate = delegate
+      // `UIScrollView.delegate` is weak; the scroll view owns its delegate instead.
+      objc_setAssociatedObject(scrollView, &scrollDelegateKey, scrollDelegate, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
+      scrollView.delegate = scrollDelegate
+    }
+  }
+
   /// Scroll views that SwiftUI embedded in the paging collection view.
   ///
   /// Scroll views that belong to the pages' React Native content live inside the paging cells, so