PartialScroll

PartialScroll.swift

import SwiftUI

struct ContentView: View {
    
    @State var offset: CGFloat = 0.0
    @State var previousOffset: CGFloat = 0.0
    
    init() {
        let appearance = UINavigationBarAppearance()
        appearance.configureWithTransparentBackground()
        appearance.backgroundColor = .clear
        appearance.shadowColor = .clear
        UINavigationBar.appearance().standardAppearance = appearance
    }
    
    var body: some View {
        NavigationView {
            ZStack {
                ScrollView(
                    showsIndicators: false,
                    offsetChanged: {
                        previousOffset = offset
                        offset = $0.y
                    }
                ) {
                    VStack(spacing: 0) {
                            ForEach(1..<10) { _ in
                                Rectangle()
                                    .fill(Color.blue)
                                    .frame(height: 100)
                                Rectangle()
                                    .fill(Color.green)
                                    .frame(height: 100)
                            }
                    }.padding(.top, 300 + 104)
                }
                
                VStack(spacing: 0) {
                    Rectangle()
                        .fill(Color.red)
                        .frame(height: 300)
                    Rectangle()
                        .fill(Color.yellow)
                        .frame(height: 104)
                    Spacer()
                }
                .offset(y: max(-300, offset))
            }
            .ignoresSafeArea()
            .navigationTitle(Text("\(offset)"))
            .navigationBarTitleDisplayMode(.inline)
        }
    }
}

struct ScrollView<Content: View>: View {
    let axes: Axis.Set
    let showsIndicators: Bool
    let offsetChanged: (CGPoint) -> Void
    let content: Content
    
    init(
        axes: Axis.Set = .vertical,
        showsIndicators: Bool = true,
        offsetChanged: @escaping (CGPoint) -> Void = { _ in },
        @ViewBuilder content: () -> Content
    ) {
        self.axes = axes
        self.showsIndicators = showsIndicators
        self.offsetChanged = offsetChanged
        self.content = content()
    }
    
    var body: some View {
        SwiftUI.ScrollView(axes, showsIndicators: showsIndicators) {
            VStack(spacing: 0) {
                GeometryReader { geometry in
                    Color.clear.preference(
                        key: ScrollOffsetPreferenceKey.self,
                        value: geometry.frame(in: .named("scrollView")).origin
                    )
                }.frame(width: 0, height: 0)
                content
            }
        }
        .coordinateSpace(name: "scrollView")
        .onPreferenceChange(ScrollOffsetPreferenceKey.self, perform: offsetChanged)
    }
}

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

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Simulator.Screen.Recording.-.iPhone.14.Pro.-.2022-10-06.at.19.15.21.mp4