Gesture representable horizontal swipe in `ScrollView` with fallback

ContentView.swift

struct ContentView: View {
    @Environment(\.layoutDirection) private var layoutDirection
    
    private static let height = 64.0
    private static let feedbackGenerator = UIImpactFeedbackGenerator(style: .medium)
    
    // > iOS 18
    @State private var newSecondaryActionDragAmount: CGFloat = 0
    // < iOS 18
    @GestureState private var oldSecondaryActionDragAmount = CGSize.zero
    
    private var translation: CGFloat {
        if #available(iOS 18, *) {
            newSecondaryActionDragAmount
        } else {
            oldSecondaryActionDragAmount.width
        }
    }
    
    @available(iOS 18, *)
    class HorizontalSwipeGestureRecognizer: UIPanGestureRecognizer {
        var direction: LayoutDirection = .leftToRight
        
        private var startedAt: ContinuousClock.Instant? = .none
        
        override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
            super.touchesMoved(touches, with: event)
            
            let now = ContinuousClock.now
            if case .none = startedAt {
                startedAt = now
            }
            
            let velocity = self.velocity(in: self.view)
            
            if now - startedAt! < .milliseconds(30) {
                let distance = abs(velocity.x) - abs(velocity.y)
                let cap = abs(velocity.x) / (5 / 2)
                
                if distance < cap {
                    print("too slow")
                    state = .failed
                    return
                }
                
                /// ensure swipe is horizontal
                if abs(velocity.x) < abs(velocity.y) {
                    print("not horizontal")
                    state = .failed
                    return
                }
                
                /// ensure right swipe direction
                switch direction {
                case .leftToRight where velocity.x < 0:
                    print("wrong direction")
                    state = .failed
                    return
                case .rightToLeft where velocity.x > 0:
                    print("wrong direction")
                    state = .failed
                    return
                default:
                    break
                }
                
                return
            } else if state == .possible {
                state = .began
            } else {
                state = .changed
            }
        }
        
        override func reset() {
            startedAt = .none
        }
    }
    
    @available(iOS 18, *)
    struct HorizontalSwipeGesture: UIGestureRecognizerRepresentable {
        typealias UIGestureRecognizerType = HorizontalSwipeGestureRecognizer
        
        @Binding var dragAmount: CGFloat
        
        @State private var surpassedThreshold = false
        
        let action: () -> Void
        let direction: LayoutDirection
        let threshold: CGFloat
        
        func makeUIGestureRecognizer(context: Context) -> UIGestureRecognizerType {
            let recognizer = HorizontalSwipeGestureRecognizer()
            recognizer.direction = direction
            recognizer.maximumNumberOfTouches = 1
            recognizer.minimumNumberOfTouches = 1
            
            recognizer.delegate = context.coordinator
            return recognizer
        }
        
        func updateUIGestureRecognizer(_ recognizer: UIGestureRecognizerType, context: Context) {
            recognizer.direction = direction
        }
        
        func makeCoordinator(converter _: CoordinateSpaceConverter) -> Coordinator {
            Coordinator()
        }
        
        func handleUIGestureRecognizerAction(_ recognizer: UIGestureRecognizerType, context: Context) {
            let sign: CGFloat = direction == .rightToLeft ? -1 : 1
            
            switch recognizer.state {
            case .changed:
                let translation = recognizer.translation(in: recognizer.view)
                
                dragAmount = translation.x
                
                if translation.x * sign > threshold {
                    if !surpassedThreshold {
                        ContentView.feedbackGenerator.impactOccurred()
                    }
                    surpassedThreshold = true
                } else {
                    surpassedThreshold = false
                }
            case .cancelled, .failed:
                dragAmount = 0
            case .ended:
                dragAmount = 0
                
                let translation = recognizer.translation(in: recognizer.view)
                if translation.x * sign > threshold {
                    action()
                }
            default:
                break
            }
        }
        
        final class Coordinator: NSObject, UIGestureRecognizerDelegate {
            @objc func gestureRecognizer(
                _ gestureRecognizer: UIGestureRecognizer,
                shouldRecognizeSimultaneouslyWith otherGestureRecognizer: UIGestureRecognizer
            ) -> Bool {
                if case .some = otherGestureRecognizer as? UIPanGestureRecognizer {
                    return false
                }
                
                if case .some = otherGestureRecognizer as? UITapGestureRecognizer {
                    return false
                }
                
                return true
            }
        }
    }
    
    @available(iOS 18, *)
    private var dragAmount: Binding<CGFloat> {
        var transaction = Transaction()
        transaction.isContinuous = true
        return $newSecondaryActionDragAmount.transaction(transaction)
    }
    
    @ViewBuilder var inner: some View {
        Rectangle()
            .frame(width: Self.height, height: Self.height)
            .offset(x: translation, y: 0)
    }
    
    func action() {
        print("acted")
    }
    
    var body: some View {
        let threshold = Self.height
        let sign: CGFloat = layoutDirection == .rightToLeft ? -1 : 1
        
        ScrollView {
            VStack {
                if #available(iOS 18, *) {
                    inner
                        .gesture(
                            HorizontalSwipeGesture(
                                dragAmount: dragAmount,
                                action: action,
                                direction: layoutDirection,
                                threshold: threshold
                            )
                        )
                } else {
                    inner
                        .gesture(DragGesture(minimumDistance: 16)
                            .updating($oldSecondaryActionDragAmount) { value, state, _ in
                                if value.translation.width * sign > threshold,
                                   state.width * sign < threshold
                                {
                                    Self.feedbackGenerator.impactOccurred()
                                }
                                
                                state = value.translation
                            }
                            .onEnded { state in
                                if state.translation.width * sign > threshold {
                                    action()
                                }
                            }
                        )
                }
            }
            .frame(maxWidth: .infinity, alignment: .leading)
        }
        .contentMargins(20)
    }
}