21.5 Traditional vs. Lazy Stacks

So far in this chapter we have only covered the HStack, VStack and ZStack views. Although the stack examples shown so far contain relatively few child views, it is possible for a stack to contain large quantities of views. This is particularly common when a stack is embedded in a ScrollView. ScrollView is a view which allows the user to scroll through content that extends beyond the visible area of either the containing view or device the screen.

When using the traditional HStack and VStack views, the system will create all the views child views at initialization, regardless of whether those views are currently visible to the user. While this may not be an issue for most requirements, this can lead to performance degradation in situations where a stack has thousands of child views.

To address this issue, SwiftUI also provides “lazy” vertical and horizontal stack views. These views (named LazyVStack and LazyHStack) use exactly the...