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

ios/Package.swift

@@ -51,6 +51,7 @@ let package = Package(
                 "Extensions.swift",
                 "PagerGestureObserver.swift",
                 "PagerScrollDelegate.swift",
+                "PagerScrollDelegateIOS15.swift",
                 "PagerView.swift",
                 "PagerViewIOS15Compat.swift",
                 "PagerViewProps.swift",

ios/PagerScrollDelegateIOS15.swift

@@ -0,0 +1,16 @@
+import UIKit
+
+/// iOS 15 only, see `PagerViewIOS15Compat`. Delete once the minimum supported iOS version is 16.
+///
+/// The scroll views that page on iOS 15 only exist once the collection view displays its pages, so
+/// a `PagerScrollDelegate` is attached to them from here.
+final class PagerScrollDelegateIOS15: PagerScrollDelegate {
+  override func collectionView(
+    _ collectionView: UICollectionView,
+    willDisplay cell: UICollectionViewCell,
+    forItemAt indexPath: IndexPath
+  ) {
+    PagerViewIOS15Compat.attachScrollDelegate(to: collectionView, delegate: delegate)
+    super.collectionView(collectionView, willDisplay: cell, forItemAt: indexPath)
+  }
+}

ios/PagerView.swift

@@ -9,7 +9,13 @@ import react_native_pager_view
 
 struct PagerView: View {
   @ObservedObject var props: PagerViewProps
-  @State private var scrollDelegate = PagerScrollDelegate()
+  @State private var scrollDelegate: PagerScrollDelegate = {
+    if #available(iOS 16, *) {
+      return PagerScrollDelegate()
+    }
+    // iOS 15 only, see PagerViewIOS15Compat.swift (delete together with the iOS 15 support).
+    return PagerScrollDelegateIOS15()
+  }()
   @State private var gestureObserver = PagerGestureObserver()
   @State private var didEmitInitialPageSelection = false
   weak var delegate: PagerViewProviderDelegate?

ios/PagerViewIOS15Compat.swift

@@ -1,4 +1,7 @@
 import UIKit
+#if SWIFT_PACKAGE
+import react_native_pager_view
+#endif
 
 /// Workarounds for the SwiftUI paged `TabView` on iOS 15.
 ///
@@ -8,14 +11,20 @@ import UIKit
 /// 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, and the back-gesture arbitration of `PagerGestureObserver`
-/// never reached the pan that blocks it.
+/// never reached the pan that blocks it. 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 `PagerViewIOS15Compat` calls in `PagerView.swift`
+/// - Note: Delete this file and `PagerScrollDelegateIOS15.swift` together with the
+///   `PagerViewIOS15Compat` calls and the `PagerScrollDelegateIOS15` choice in `PagerView.swift`
 ///   once the minimum supported iOS version is 16.
 enum PagerViewIOS15Compat {
+  private static var scrollDelegateKey: UInt8 = 0
+  private static var originalDelegateKey: 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, *) {
@@ -47,6 +56,33 @@ enum PagerViewIOS15Compat {
     pagingScrollViews(in: collectionView).forEach { gestureObserver.attach(to: $0) }
   }
 
+  /// 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) {
+      if scrollView.delegate is PagerScrollDelegate {
+        continue
+      }
+
+      let originalDelegate = ScrollViewDelegateForwarder(target: scrollView.delegate)
+      let scrollDelegate = PagerScrollDelegate()
+      scrollDelegate.originalDelegate = originalDelegate
+      scrollDelegate.delegate = delegate
+      // Both delegate references are weak; the scroll view owns them instead.
+      let policy = objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC
+      objc_setAssociatedObject(scrollView, &scrollDelegateKey, scrollDelegate, policy)
+      objc_setAssociatedObject(scrollView, &originalDelegateKey, originalDelegate, policy)
+      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
@@ -75,3 +111,22 @@ enum PagerViewIOS15Compat {
     return scrollViews
   }
 }
+
+/// `PagerScrollDelegate` forwards to a `UICollectionViewDelegate`, but the embedded scroll views'
+/// own delegate (UIKit's `_UICollectionViewOrthogonalScrollerSectionController`) is only a
+/// `UIScrollViewDelegate`. This passes every call through to it.
+private final class ScrollViewDelegateForwarder: NSObject, UICollectionViewDelegate {
+  private weak var target: UIScrollViewDelegate?
+
+  init(target: UIScrollViewDelegate?) {
+    self.target = target
+  }
+
+  override func responds(to aSelector: Selector!) -> Bool {
+    target?.responds(to: aSelector) ?? false
+  }
+
+  override func forwardingTarget(for aSelector: Selector!) -> Any? {
+    target
+  }
+}