/layout

name: layout
description: SwiftUI layout beyond stacks — the Layout protocol (when custom layout beats GeometryReader), Grid vs lazy grids, custom containers with sections and container values, and lazy-stack/ScrollView performance rules (what breaks laziness, prefetch discipline, scroll APIs). Use when building custom layouts or containers, fixing lazy-stack jank or memory growth, or wiring programmatic/snapping scrolling.
allowed-tools: [Read, Write, Edit, Glob, Grep]
last_verified: 2026-07-16
review_by: 2027-06-22

SwiftUI Layout & Containers

The layer between "stacks and spacers" and "it scrolls like butter with 100k rows" — Apple's Layout protocol, container composition, and the lazy-stack rules from the WWDC26 deep dive. View identity/data-flow questions route to `swiftui/data-flow`.

When This Skill Activates

• "Make these buttons equal width" / measurement-dependent layout
• Building a reusable container (custom List/board/carousel) that should accept ForEach + sections
• Lazy stack jank, memory growth, scroll-position bugs, broken scroll targeting
• Programmatic scrolling, paging/snapping, scroll-linked effects
• GeometryReader causing layout loops or mangled sizing

Custom Layout protocol (not GeometryReader)

Reach for a custom `Layout` whenever you must **measure subviews and feed the measurement back into layout** — GeometryReader only measures its container and can't influence the engine. Canonical case: equal-width buttons.

• `sizeThatFits`: propose `.unspecified` to read each subview's ideal size

(`subviews.map { $0.sizeThatFits(.unspecified) }`); guard empty subviews; `replacingUnspecifiedDimensions()` for nil proposal dimensions.

• `placeSubviews`: never assume origin (0,0) — use `bounds.minX/midX` (non-zero origins are

what make layouts composable); `place(at:anchor:proposal:)` with a proposal that may differ from the ideal size (that's how equal widths happen).

• **Respect spacing preferences**: `subviews[i].spacing.distance(to:along:)`, taking the larger

of conflicting preferences — matching built-in containers. No hardcoded 8s.

• Per-subview data via `LayoutValueKey` (+ a `layoutValue` convenience modifier), read as

`subview[Key.self]`.

• Cache only after Instruments shows layout cost — it's an optimization, not a requirement.
• **Switch layouts without killing identity**: `AnyLayout(HStackLayout())` ↔ custom layout with

`.animation(_:value:)` — SwiftUI sees one changing view, so state survives and it animates.

• Don't build fallbacks into the layout — wrap alternatives in `ViewThatFits`.

Grid decisions

| Need | Use | |---|---| | Static 2D with cross-row alignment | `Grid`/`GridRow` (+ `gridCellColumns` to span, `gridColumnAlignment` per column) | | Scrollable, large content | `LazyVGrid`/`LazyHGrid` (only visible views load; one axis fixed up front) | | "First arrangement that fits" | `ViewThatFits` |

Custom containers (Demystify Containers)

Make containers that compose like `List` does:

• API shape: a trailing `@ViewBuilder var content: Content` — callers can then mix static

views, `ForEach`, and conditionals.

• Iterate **resolved** children with `ForEach(subviews: content)`; need the whole collection

(count/chunking)? `Group(subviews: content) { subviews in … }`.

• Internalize **declared vs resolved**: one declared ForEach resolves to N subviews; Group to

its children; EmptyView to zero; `if` conditionally. Counting declared views is a bug.

• Sections are opt-in: `ForEach(sections: content)`, reading `section.header` /

`section.content`; check `header.isEmpty` before rendering the slot.

• Per-child customization via container values: `extension ContainerValues { @Entry var … }`,

set with a convenience modifier, read via `subview.containerValues`. Scoping model: **Environment flows down · Preferences flow up · container values reach only the direct container.** Setting one on a `Section` styles the whole section.

Lazy stacks & scrolling performance (WWDC26 rules)

LazyVStack builds views only until the viewport fills; totals and offsets are **estimated** from average placed-view size and corrected as you scroll. Everything below follows from that:

• **One subview per ForEach element, always.** An `if` inside a row (0-or-1 views) forces the

stack to keep off-screen views + their `@State` alive to preserve indices — and environment changes then re-evaluate off-screen bodies. Filter at the data layer (`@Query` predicate); gate auth-type conditions *outside* the stack.

• **Never key logic off absolute scroll offset** in a lazy stack (`onScrollGeometryChange` sees

estimates) — use `onScrollTargetVisibilityChange(threshold: 0.8)` for visibility triggers.

• **Set up in `init`, not `onAppear`** (`_model = State(initialValue:)`): body runs during

prefetch; `onAppear` fires only on-screen, throwing prefetch work away and causing post-appearance size jumps. Start async loads in `init`/`task`.

• **Don't persist meaningful state in row `@State`** — off-screen views are eventually

released. Hoist (`@State var highlighted: Set<ID>` outside, `@Binding` down).

• `scrollTransition` transforms must stay inside the original frame (scale ✅; rotations

escaping the frame make views vanish early).

• Don't drive layout from `onGeometryChange` height feedback (content shoves, targeting

breaks) — that's the custom `Layout` case above.

• Nest `LazyHStack` inside `LazyVStack` freely (unscrolled rows stay unloaded) — but fix child

heights (`lineLimit`, explicit frames) in the horizontal stacks.

• `pinnedViews: [.sectionHeaders]` pins headers; infinite scroll = trailing

`ProgressView().onAppear { fetchNextPage() }` after the ForEach.

The scroll API map

• Snapping/paging: `scrollTargetLayout()` + `scrollTargetBehavior(.viewAligned/.paging)`.
• Track/control position: `scrollPosition` binding; programmatic `ScrollPosition` +

`scrollTo(id:)` — works for unloaded targets *if* IDs map to stable one-subview el