SwiftUI ViewModifier to create an offset fading-in/out navigation bar for a view based on a scroll view offset.

NavBarOffsetModifier.swift

// Assumes usage of: 
//  - https://gist.github.com/LePips/3640ad0cd9b6e2ceb407e9d0e9e32b5c

// Embedding view for content
struct NavBarOffsetView<Content: View>: UIViewControllerRepresentable {
    
    @Binding
    private var scrollViewOffset: CGFloat
    
    private let start: CGFloat
    private let end: CGFloat
    private let content: () -> Content
    
    init(scrollViewOffset: Binding<CGFloat>, start: CGFloat, end: CGFloat, @ViewBuilder content: @escaping () -> Content) {
        self._scrollViewOffset = scrollViewOffset
        self.start = start
        self.end = end
        self.content = content
    }
    
    init(start: CGFloat, end: CGFloat, @ViewBuilder body: @escaping () -> Content) {
        self._scrollViewOffset = Binding(get: { 0 }, set: { _ in })
        self.start = start
        self.end = end
        self.content = body
    }
    
    func makeUIViewController(context: Context) -> NavBarOffsetHostingController<Content> {
        NavBarOffsetHostingController(rootView: content())
    }
    
    func updateUIViewController(_ uiViewController: NavBarOffsetHostingController<Content>, context: Context) {
        uiViewController.scrollViewDidScroll(scrollViewOffset, start: start, end: end)
    }
}

class NavBarOffsetHostingController<Content: View>: UIHostingController<Content> {
    
    private var lastScrollViewOffset: CGFloat = 0
    
    private lazy var navBarBlurView: UIVisualEffectView = {
        let blurView = UIVisualEffectView(effect: UIBlurEffect(style: .systemThinMaterial))
        blurView.translatesAutoresizingMaskIntoConstraints = false
        return blurView
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Customize to fit needs. My cases require a clear background.
        view.backgroundColor = nil
        
        view.addSubview(navBarBlurView)
        navBarBlurView.alpha = 0

        NSLayoutConstraint.activate([
            navBarBlurView.topAnchor.constraint(equalTo: view.topAnchor),
            navBarBlurView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            navBarBlurView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            navBarBlurView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        ])
    }
    
    func scrollViewDidScroll(_ offset: CGFloat, start: CGFloat, end: CGFloat) {
        let diff = end - start
        let currentProgress = (offset - start) / diff
        let offset = min(max(currentProgress, 0), 1)
        
        self.navigationController?.navigationBar
            .titleTextAttributes = [NSAttributedString.Key.foregroundColor: UIColor.label.withAlphaComponent(offset)]
        navBarBlurView.alpha = offset
        lastScrollViewOffset = offset
    }
    
    // Restore custom NavigationBar state
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        self.navigationController?.navigationBar
            .titleTextAttributes = [NSAttributedString.Key.foregroundColor: UIColor.label.withAlphaComponent(lastScrollViewOffset)]
        self.navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)
        self.navigationController?.navigationBar.shadowImage = UIImage()
    }
  
    // Restore default NavigationBar for other views
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        self.navigationController?.navigationBar.titleTextAttributes = [NSAttributedString.Key.foregroundColor: UIColor.label]
        self.navigationController?.navigationBar.setBackgroundImage(nil, for: .default)
        self.navigationController?.navigationBar.shadowImage = nil
    }
}

struct NavBarOffsetModifier: ViewModifier {
    
    @Binding
    var scrollViewOffset: CGFloat
    
    // Begin fading in navbar at offset. If offset < start, navbar hidden
    let start: CGFloat
    // End fading in navbar at offset. If offset > end, navbar shown
    let end: CGFloat
    
    func body(content: Content) -> some View {
        // Required .ignoresSafeArea() due to bug where view will hide under blank NavigationBar
        NavBarOffsetView(scrollViewOffset: $scrollViewOffset, start: start, end: end) {
            content
        }
        .ignoresSafeArea()
    }
}

extension View {
    func navBarOffset(_ scrollViewOffset: Binding<CGFloat>, start: CGFloat, end: CGFloat) -> some View {
        self.modifier(NavBarOffsetModifier(scrollViewOffset: scrollViewOffset, start: start, end: end))
    }
}

// Usage

struct ContentView: View {
    
    @State
    private var scrollViewOffset: CGFloat = 0
  
    var body: some View {
        NavigationView {
            ScrollView {
                VStack {

                    Color.clear
                        .frame(height: 500)

                    VStack {
                        ForEach(0..<100) { _ in
                            Text("Hello There")
                        }

                        // Prove restoration of native NavigationBar
                        // on new views
                        NavigationLink("Navigate") {
                            ScrollView {
                                VStack {
                                    ForEach(0..<50) { _ in
                                        Text("Hello There")
                                    }
                                }
                                .frame(maxWidth: .infinity)
                            }
                            .navigationTitle("Another Test")
                        }
                    }
                    .background(Color.black)
                }
                .frame(maxWidth: .infinity)
            }
            .navigationTitle("Test")
            .navigationBarTitleDisplayMode(.inline)
            .ignoresSafeArea()
            .scrollViewOffset($scrollViewOffset)
            .navBarOffset($scrollViewOffset, start: 300, end: 350)
        }
    } 
}