Tracking scroll position in a List SwiftUI - xcode

Question

So in the past I have been tracking the scroll position using a scroll view but I've fallen into a situation where I need to track the position using a List. I am using a List because I want some of the built in real estate to create my views such as the default List styles.
I can get the value using PreferenceKeys, but the issue is when I scroll to far upwards, the PreferenceKey value will default back to its position 0, breaking my show shy header view logic.
This is the TrackableListView code
struct TrackableListView<Content: View>: View {
let offsetChanged: (CGPoint) -> Void
let content: Content
init(offsetChanged: #escaping (CGPoint) -> Void = { _ in }, #ViewBuilder content: () -> Content) {
self.offsetChanged = offsetChanged
self.content = content()
}
var body: some View {
List {
GeometryReader { geometry in
Color.clear.preference(key: ScrollOffsetPreferenceKey.self, value: geometry.frame(in: .named("ListView")).origin)
}
.frame(width: 0, height: 0)
content
.offset(y: -10)
}
.coordinateSpace(name: "ListView")
.onPreferenceChange(ScrollOffsetPreferenceKey.self, perform: offsetChanged)
}
}
private struct ScrollOffsetPreferenceKey: PreferenceKey {
static var defaultValue: CGPoint = .zero
static func reduce(value: inout CGPoint, nextValue: () -> CGPoint) { }
}
And this is my ContentView:
struct ContentView: View {
#State private var contentOffset = CGFloat(0)
#State private var offsetPositionValue: CGFloat = 0
#State private var isShyHeaderVisible = false
var body: some View {
NavigationView {
ZStack {
TrackableListView { offset in
withAnimation {
contentOffset = offset.y
}
} content: {
Text("\(contentOffset)")
}
.overlay(
ZStack {
HStack {
Text("Total points")
.foregroundColor(.white)
.lineLimit(1)
Spacer()
Text("20,000 pts")
.foregroundColor(.white)
.padding(.leading, 50)
}
.padding(.horizontal)
.padding(.vertical, 8)
.frame(width: UIScreen.main.bounds.width)
.background(Color.green)
.offset(y: contentOffset < 50 ? 0 : -5)
.opacity(contentOffset < 50 ? 1 : 0)
.transition(.move(edge: .top))
}
.frame(maxHeight: .infinity, alignment: .top)
)
}
.navigationTitle("Hello")
.navigationViewStyle(StackNavigationViewStyle())
.navigationBarTitleDisplayMode(.inline)
.frame(maxHeight: .infinity, alignment: .top)
.background(AccountBackground())
}
}
}

Answer 1

The issue was that Other Views in my hierarchy (probably introduced by SwiftUI) could be sending the default value, which is why you start getting zero sometimes.
What I needed was a way to determine when to use or forward the new value, versus when to ignore it.
To do this I had to make my value optional GCPoint, with a nil default value, then in your reduce method when using PreferenceKeys you have to do:
if let nextValue = nextValue() {
value = nextValue
}
Then make sure your CGPoint is an optional value.