gistfile1.txt

struct  SampleView: View {
    @State private var offset: CGFloat = .zero
    @State var shouldShowStickyView: Bool = true

    var body: some View {
        ScrollView {
            offsetReader
            VStack {
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
                rowView("Foo")
            }
            .padding()
        }
        .safeAreaInset(edge: .top, spacing: 0) {
            Text("ヘッダー")
                .frame(maxWidth: .infinity)
                .frame(height: 64)
                .background(Color.white)
                .offset(y: shouldShowStickyView ? 0 : -64)
                .opacity(shouldShowStickyView ? 1 : 0)
                .animation(.easeIn, value: shouldShowStickyView)
        }
        .background(Color.gray)
        .coordinateSpace(name: "scrollViewCoordinateSpace")
        .onPreferenceChange(ScrollViewOffsetPreferenceKey.self) {
            if $0 >= 0 {
                shouldShowStickyView = true
            } else {
                shouldShowStickyView = false
            }
        }
    }
}

extension SampleView {
    func rowView(_ text: String) -> some View {
        Text(text)
            .padding()
            .frame(maxWidth: .infinity)
            .background(Color.white)
            .clipShape(RoundedRectangle(cornerRadius: 10))
    }
    
    var offsetReader: some View {
        GeometryReader { proxy in
            Color.clear
                .preference (
                    key: ScrollViewOffsetPreferenceKey.self,
                    value: proxy.frame(in: .named("scrollViewCoordinateSpace")).minY
                )
        }
        .frame (height: 0)
    }
}

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