This ScrollView has a modifier called `onScroll`, which is updated when scrolls occur.

A - Usage.swift

struct ContentView: View {

    @State var scrollOffset: CGPoint = .zero

    var body: some View {
        ObservableScrollView {
            Text("Hello, world!")
                .foregroundColor(self.scrollOffset.y == 0 ? .blue : .red)
        }
        .onScroll { self.scrollOffset = $0 }
    }
}

ObservableScrollView.swift

struct ScrollOffsetPreferenceKey: PreferenceKey {
    typealias Value = CGPoint
    static var defaultValue = CGPoint.zero

    static func reduce(value: inout CGPoint, nextValue: () -> CGPoint) {
        value = nextValue()
    }
}

struct ObservableScrollView<Content> : View where Content : View {

    var content: Content

    var axes: Axis.Set

    var showsIndicators: Bool

    init(_ axes: Axis.Set = .vertical, showsIndicators: Bool = true, @ViewBuilder content: () -> Content) {
        self.content = content()
        self.axes = axes
        self.showsIndicators = showsIndicators
    }

    var body: some View {
        GeometryReader { outerGeometry in
            ScrollView(self.axes, showsIndicators: self.showsIndicators) {
                ZStack(alignment: self.axes == .vertical ? .top : .leading) {
                    GeometryReader { innerGeometry in
                        Color.clear
                            .preference(key: ScrollOffsetPreferenceKey.self, value: CGPoint(x: (outerGeometry.frame(in: .global).minX - innerGeometry.frame(in: .global).minX), y: (outerGeometry.frame(in: .global).minY - innerGeometry.frame(in: .global).minY)))
                    }
                    VStack {
                        self.content
                    }
                }
            }
        }
    }
}

extension ObservableScrollView {
    func onScroll(_ onScroll: @escaping (CGPoint) -> ()) -> some View {
        self.onPreferenceChange(ScrollOffsetPreferenceKey.self, perform: onScroll)
    }
}

Wrong implementation of reduce func. Closure will not called.