SwiftUI: Maintain scroll position when a View within a ScrollView changes size

Question

Goal

I have a list of items, and each can be expanded to show some additional content. However, when expanding/collapsing the item, the scroll position sometimes jumps about. Is there a way to maintain scroll position when one item changes size?

Ideally, the top edge of the view that's expanding/collapsing should remain in a fixed position on the screen.

Alternative Design?

Alternatively, is this expanding style inappropriate on iOS? If so, is there a recommended best practice for showing more content (like seeing "more comments", expanding a quote, see a longer caption, etc.)

Attempted Solutions

Using ScrollViewReader and scrollTo() - Unfortunately the timing of the expanded Bool toggle and scrollTo are inconsistent. This means that the view sometimes scrolls before the size has been adjusted (thus landing in the wrong spot). I tried a few permutations of grouping the two actions in the same withAnimation block, Tasks, etc. - but it always looked janky & incorrect
.scrollPosition() modifier - Only available in iOS 17, so realistically won't apply to most users for quite a while
withAnimation completion block - Also only in iOS 17

Example Code

struct MyList: View {
    var items: [String] = ["Item A", "Item B", "Item C", "Item D", "Item E", "Item F", "Item G", "Item H", "Item I", "Item J" ]
    
    var body: some View {
        ScrollView {
            LazyVStack(alignment: .center) {
                ForEach(items, id: \.self) { item in
                    ItemView(item: item)
                }
            }
            .background(.ultraThinMaterial)
        }
    }
}

struct ItemView: View {
    var item: String
    
    @State var expanded: Bool = false
    
    var body: some View {
        
        VStack {
            Text(item)
            Spacer(minLength: 50)
            Button("Expand/Collapse") {
                expanded.toggle()
            }
            if expanded {
                Text("Shown When Expanded")
                Text("\n -\n -\n -\n -\n -\n -\n -\n -\n -\n -\n -\n -")
            }
        }
        .frame(maxWidth: .infinity)
        .background()
        .padding(.bottom)
    }
}

David B. · Accepted Answer · 2023-12-14 13:40:17Z

1

Use a VStack instead of a LazyVStack and you'll get the behavior you're looking for:

struct MyList: View {
    var items: [String] = ["Item A", "Item B", "Item C", "Item D", "Item E", "Item F", "Item G", "Item H", "Item I", "Item J" ]
    
    var body: some View {
        ScrollView {
            VStack(alignment: .center) {
                ForEach(items, id: \.self) { item in
                    ItemView(item: item)
                }
            }
            .background(.ultraThinMaterial)
        }
    }
}

answered Dec 14, 2023 at 13:40

David B.

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1 Comment

Kia Abdi Over a year ago

I was facing a jumpy scroll when I used ScrollView with scrollPosition and a LazyVStack. replacing it with VStack solved the problem but I can't leave it as a VStack. Is there a way to solve this problem while still using LazyVStack?

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