SwiftUI: How do I sync/change progress bar progress based on scrollview’s user current scroll position?

You can do this with a few GeometryReaders.

My method:

1. Get total height of ScrollView container
2. Get inner height of content
3. Find the difference for the total scrollable height
4. Get the distance between the scroll view container top and the content top
5. Divide the distance between tops by the total scrollable height
6. Use PreferenceKeys to set the proportion @State value

Code:

struct ContentView: View {
    @State private var scrollViewHeight: CGFloat = 0
    @State private var proportion: CGFloat = 0

    var body: some View {
        VStack {
            ScrollView {
                VStack {
                    ForEach(0 ..< 100) { index in
                        Text("Item: \(index + 1)")
                    }
                }
                .frame(maxWidth: .infinity)
                .background(
                    GeometryReader { geo in
                        let scrollLength = geo.size.height - scrollViewHeight
                        let rawProportion = -geo.frame(in: .named("scroll")).minY / scrollLength
                        let proportion = min(max(rawProportion, 0), 1)

                        Color.clear
                            .preference(
                                key: ScrollProportion.self,
                                value: proportion
                            )
                            .onPreferenceChange(ScrollProportion.self) { proportion in
                                self.proportion = proportion
                            }
                    }
                )
            }
            .background(
                GeometryReader { geo in
                    Color.clear.onAppear {
                        scrollViewHeight = geo.size.height
                    }
                }
            )
            .coordinateSpace(name: "scroll")

            ProgressView(value: proportion, total: 1)
                .padding(.horizontal)
        }
    }
}

struct ScrollProportion: PreferenceKey {
    static let defaultValue: CGFloat = 0

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

Result: