swift - SwiftUI ScrollViewReader 在调整大小时保留滚动位置

Question

当使用 ScrollViewReader 滚动到某个位置时，例如到宽度的 40% - 当 ScrollView 的内容调整大小时，如何保留滚动位置？

我写了一个小示例应用程序来说明问题：

最初，ScrollView 的 child 的宽度为 1600，滚动位置为 40%。当您单击“将宽度更改为 800”按钮时，子宽度更改为 800，并且 ScrollView 滚动到末尾。我希望 ScrollView 在调整大小后保留滚动位置，或者在调整大小后始终滚动到 40%。

struct ContentView: View {

    @State private var relativeScrollPosition: Double?
    @State private var childWidth: CGFloat = 1600


    var body: some View {
        VStack {
            Button("Change width to 800") {
                childWidth = 800
            }
            Button("Change width to 1600") {
                childWidth = 1600
            }
            RelativeScrollView(
                relativeScrollPosition: $relativeScrollPosition,
                childWidth: childWidth
            ) {
                HStack{
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("1")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("2")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("3")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("4")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("5")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("6")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("7")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("8")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("9")
                    }
                    ZStack {
                        Spacer().frame(height: 100).background(Color.green)
                        Text("10")
                    }
                }
                .frame(width: childWidth, height: 100, alignment: .center)
            }
            .onAppear {
                scrollTo40percent()
            }
            .onChange(of: childWidth, perform: { _ in
                scrollTo40percent()
            })
        }
    }

    private func scrollTo40percent() {
        relativeScrollPosition = 0.4
    }
}


struct RelativeScrollView<Content: View>: View {

    @Binding var relativeScrollPosition: Double?
    let childWidth: CGFloat
    let isAnimating = true
    var child: () -> Content

    var body: some View {
        ScrollViewReader { reader in
            ScrollView(.horizontal) {
                ZStack {
                    HStack {
                        // swiftlint:disable identifier_name
                        ForEach(0..<101) { i in
                            Spacer().id(i)
                        }
                    }
                    .frame(width: childWidth)
                    self.child()
                }
            }
            .onAppear {
                scroll(reader, to: relativeScrollPosition)
            }
            .onChange(of: relativeScrollPosition) { newPos in
                scroll(reader, to: newPos)
            }
        }
    }

    private func scroll(_ reader: ScrollViewProxy, to position: Double?) {
        guard let unwrappedPosition = position else { return }
        assert(unwrappedPosition >= 0 && unwrappedPosition <= 1)
        let elementToScrollTo = Int(unwrappedPosition * 100)
        if isAnimating {
            withAnimation {
                reader.scrollTo(elementToScrollTo, anchor: .center)
            }
        } else {
            reader.scrollTo(elementToScrollTo, anchor: .center)
        }
    }
}