onReceive(self.timer) doesn't work inside a NavigationView

Question

I have this page of a Scrollview with a custom header only shown when it scrolls past a certain height. I use GeometryReader with onReceive to constantly check the current scrolling height:

@State var userInfoUpateInterval = Timer.publish(every: 0.1, on: .current, in: .tracking).autoconnect()
@State var showHeader: Bool = false

var body: some View {
    NavigationView {
        ZStack(alignment: .top) {
            ScrollView(.vertical) {
                GeometryReader { geometry in
                    Text("User info component").onReceive(self.userInfoUpateInterval) { (_) in
                        self.onUserInfoLayoutChange(geometry)
                    }
                }
                
                VStack {
                    Text("content")
                }.frame(width: UIScreen.screenWidth, height: 1500)
                
            }
        
            ProfileHeader(title: "user.userName", showHeader: $showHeader)
        }
    }
}

The scrolling and header hiding/showing works perfectly until I wrapped the ZStack in a NavigationView. onReceive is simply not triggered anymore. If I swap NavigationView with a ZStack everything works as expected again.

I have seen this Timer onReceive not working inside NavigationView question but I don't have conditional component. Is this a SwiftUI bug or I'm doing something wrong?

Answer 1

Here is a demo of possible solution for your case. Tested with Xcode 11.4 / iOS 13.4 (and it is forward compatible)

The idea is to react not by timer but by view position change that has been read/tracked by view preferences.

struct ViewOffsetKey: PreferenceKey {
    typealias Value = CGFloat
    static var defaultValue: CGFloat { 0 }
    static func reduce(value: inout Value, nextValue: () -> Value) {
        value = value + nextValue()
    }
}


struct DemoView: View {
    @State var showHeader: Bool = false

    var body: some View {
        NavigationView {
            ZStack(alignment: .top) {
                ScrollView(.vertical) {
                    Text("User info component")
                        .background(GeometryReader {
                            Color.clear.preference(key: ViewOffsetKey.self,
                                                   value: -$0.frame(in: .named("scroll_area")).origin.y) })
                    VStack {
                        Text("content")
                    }.frame(maxWidth: .infinity, minHeight: 1500)
                }.coordinateSpace(name: "scroll_area")

                if showHeader {
                    Text("ProfileHeader")
                }
            }
        }
        .onPreferenceChange(ViewOffsetKey.self) {
            self.showHeader = $0 > 200    // << your condition
        }
    }
}