Get the current scroll position of a SwiftUI ScrollView

Question

With the new ScrolLViewReader, it seems possible to set the scroll offset programmatically.

But I was wondering if it is also possible to get the current scroll position?

It seems like the ScrollViewProxy only comes with the scrollTo method, allowing us to set the offset.

Thanks!

Answer 1

It was possible to read it and before. Here is a solution based on view preferences.

struct DemoScrollViewOffsetView: View {
    @State private var offset = CGFloat.zero
    var body: some View {
        ScrollView {
            VStack {
                ForEach(0..<100) { i in
                    Text("Item \(i)").padding()
                }
            }.background(GeometryReader {
                Color.clear.preference(key: ViewOffsetKey.self,
                    value: -$0.frame(in: .named("scroll")).origin.y)
            })
            .onPreferenceChange(ViewOffsetKey.self) { print("offset >> \($0)") }
        }.coordinateSpace(name: "scroll")
    }
}

struct ViewOffsetKey: PreferenceKey {
    typealias Value = CGFloat
    static var defaultValue = CGFloat.zero
    static func reduce(value: inout Value, nextValue: () -> Value) {
        value += nextValue()
    }
}

backup

Answer 2

I found a version without using PreferenceKey. The idea is simple - by returning Color from GeometryReader, we can set scrollOffset directly inside background modifier.

struct DemoScrollViewOffsetView: View {
    @State private var offset = CGFloat.zero
    var body: some View {
        ScrollView {
            VStack {
                ForEach(0..<100) { i in
                    Text("Item \(i)").padding()
                }
            }.background(GeometryReader { proxy -> Color in
                DispatchQueue.main.async {
                    scrollOffset = -proxy.frame(in: .named("scroll")).origin.y
                }
                return Color.clear
            })
        }.coordinateSpace(name: "scroll")
    }
}

Answer 3

I had a similar need but with List instead of ScrollView, and wanted to know wether items in the lists are visible or not (List preloads views not yet visible, so onAppear()/onDisappear() are not suitable).

After a bit of "beautification" I ended up with this usage:

struct ContentView: View {
    var body: some View {
        GeometryReader { geometry in
            List(0..<100) { i in
                Text("Item \(i)")
                    .onItemFrameChanged(listGeometry: geometry) { (frame: CGRect?) in
                        print("rect of item \(i): \(String(describing: frame)))")
                    }
            }
            .trackListFrame()
        }
    }
}

which is backed by this Swift package: https://github.com/Ceylo/ListItemTracking