Show NavigationBar when scrolling down

Question

I have this simple code with a hidden NavigationBar but what I want is to show it when scrolling down. How can I do that?

struct ContentView: View {
    
    var body: some View {
        NavigationView {
            ScrollView(showsIndicators: false) {
                VStack {
                    ForEach(0 ..< 3) { _ in
                        Image(systemName: "rectangle.fill")
                            .resizable()
                            .aspectRatio(contentMode: .fill)
                            .padding()
                    }
                }
            }
            .navigationBarTitle("Title Here", displayMode: .inline)
            .navigationBarHidden(true)
        }.edgesIgnoringSafeArea(.all)
    }
}

Answer 1

Update: Xcode 14 / iOS 16

Now .navigationBarHidden(barHidden) is animatable, and with old NavigationView and with new NavigationStack.

struct ContentView: View {
    @State private var barHidden = true
    
    var body: some View {
        NavigationStack {  // << Tested !!
            ScrollView(showsIndicators: false) {

Test module is here

Original

It is possible but this modifier is not animatable, so bar appears instantly (the same is observed when toggle bar with button). Anyway I think it worth posting.

Tested with Xcode 12 / iOS 14

struct ContentView: View {
    @State private var barHidden = true

    var body: some View {
        NavigationView {
            ScrollView(showsIndicators: false) {
                VStack {
                    ForEach(0 ..< 3) { _ in
                        Image(systemName: "rectangle.fill")
                            .resizable()
                            .aspectRatio(contentMode: .fill)
                            .padding()
                    }
                }.background(GeometryReader {
                    Color.clear.preference(key: ViewOffsetKey.self,
                        value: -$0.frame(in: .named("scroll")).origin.y)
                })
                .onPreferenceChange(ViewOffsetKey.self) {
                    if !barHidden && $0 < 50 {
                        barHidden = true
                        print("<< hiding")
                    } else if barHidden && $0 > 50{
                        barHidden = false
                        print(">> showing")
                    }
                }
            }.coordinateSpace(name: "scroll")
            .navigationBarTitle("Title Here", displayMode: .inline)
            .navigationBarHidden(barHidden)
        }
        .animation(.default, value: barHidden)
        .edgesIgnoringSafeArea(.all)
    }
}

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