SwiftUI ScrollView performance

Had performance issues with ScrollView in SwiftUI. The default behavior doesn't virtualize, which can cause problems with long lists.

For long lists, you should use List instead of ScrollView:

// Bad for long lists
ScrollView {
    VStack {
        ForEach(items) { item in
            ItemRow(item: item)
        }
    }
}

// Better for long lists
List(items) { item in
    ItemRow(item: item)
}

But here's what I learned - if you need custom layout in a scrollable view, you can use LazyVStack or LazyHStack:

ScrollView {
    LazyVStack {
        ForEach(items) { item in
            ItemRow(item: item)
        }
    }
}

The Lazy variants only render visible items, which is much more efficient for long lists.

Also, if you're using List, make sure your row views are efficient. Heavy computation in row views will cause jank:

// Bad - creates new formatter every time
struct ItemRow: View {
    let item: Item
    
    var body: some View {
        Text(item.date, formatter: DateFormatter()) // Creates new formatter
    }
}

// Better - use static formatter
struct ItemRow: View {
    let item: Item
    private static let formatter = DateFormatter()
    
    var body: some View {
        Text(item.date, formatter: Self.formatter)
    }
}

One gotcha - List has different styling options than ScrollView. If you need custom styling, you might need to use LazyVStack in a ScrollView instead.

And if you're loading data as you scroll, make sure to implement proper pagination. Loading all data at once will hurt performance.

Thanks for reading! Check out more posts on the blog if you'd like.