import SwiftUI
#if canImport(UIKit)
import UIKit
#endif

#if os(iOS)
@MainActor
struct ScrollOffsetObserver: UIViewRepresentable {
    let onOffsetChange: (CGFloat) -> Void

    func makeUIView(context: Context) -> ScrollOffsetProbeView {
        let view = ScrollOffsetProbeView()
        view.onOffsetChange = onOffsetChange
        return view
    }

    func updateUIView(_ uiView: ScrollOffsetProbeView, context: Context) {
        uiView.onOffsetChange = onOffsetChange
        uiView.attachIfNeeded()
    }
}

@MainActor
final class ScrollOffsetProbeView: UIView {
    var onOffsetChange: (CGFloat) -> Void = { _ in }

    private weak var observedScrollView: UIScrollView?
    private var observation: NSKeyValueObservation?

    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = .clear
        isUserInteractionEnabled = false
    }

    required init?(coder: NSCoder) {
        super.init(coder: coder)
    }

    deinit {
        observation?.invalidate()
    }

    override func didMoveToSuperview() {
        super.didMoveToSuperview()
        attachIfNeeded()
    }

    override func didMoveToWindow() {
        super.didMoveToWindow()
        attachIfNeeded()
    }

    func attachIfNeeded() {
        guard let scrollView = findEnclosingScrollView() else { return }
        guard scrollView !== observedScrollView else { return }

        observation?.invalidate()
        observedScrollView = scrollView
        observation = scrollView.observe(\.contentOffset, options: [.initial, .new]) { [weak self] sv, _ in
            self?.onOffsetChange(sv.contentOffset.y)
        }
    }

    private func findEnclosingScrollView() -> UIScrollView? {
        var view: UIView? = self
        while let current = view {
            if let scrollView = current as? UIScrollView {
                return scrollView
            }
            view = current.superview
        }
        return nil
    }
}
#endif