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SwiftUI proposal-response layout model

Document status: reviewed. Canonical source.

Summary

SwiftUI custom layout uses proposal–response measurement followed by subview placement. A Layout implementation receives proxy values rather than direct subviews and can query dimensions, spacing, priority and custom layout values.

Evidence

  • The Layout protocol requires sizeThatFits(proposal:subviews:cache:) and placeSubviews(in:proposal:subviews:cache:). Optional methods provide alignment, spacing, axis properties and caches. https://developer.apple.com/documentation/swiftui/layout (retrieved 2026-08-29).
  • LayoutSubview.sizeThatFits(_:) accepts a proposed size. SwiftUI views choose a size while considering the parent proposal; the result is not a CSS-style fixed-width declaration. https://developer.apple.com/documentation/swiftui/layoutsubview/sizethatfits(_:) (retrieved 2026-08-29).
  • ViewThatFits can choose the first child that fits the proposal. Custom layout examples share measurements between sizing and placement through a cache. https://developer.apple.com/documentation/swiftui/composing-custom-layouts-with-swiftui (retrieved 2026-08-29).

NUIF relevance

Borrow proposal–response sizing, intrinsic probes, stack alignment, spacing and explicit separation between sizing and placement.

Adapt profile-zero stacks and fixed/intrinsic/fill sizing into a generated, profile-owned Swift subset. Compiler, SDK, operating-system version, dynamic type, locale and font registry are evaluation-context provenance.

Reject arbitrary SwiftUI import as a lossless document operation. Result builders, modifiers, state, environment values, custom Layout types and platform views are executable semantics.