How do you write a custom container with the Layout protocol?
The gist
The Layout protocol (iOS 16) exposes the same mechanism HStack and VStack are built on. A container implements two methods: sizeThatFits(proposal:subviews:cache:) — what size I take for a given proposal, and placeSubviews(in:proposal:subviews:cache:) — where to put each child. Children are available as LayoutSubview proxies: you can ask them for a size for any proposal and call place(at:anchor:proposal:). The container cannot see their body or type — only sizes and layout values.
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