axiom-swiftui-layout-ref

name	axiom-swiftui-layout-ref
description	Reference — Complete SwiftUI adaptive layout API guide covering ViewThatFits, AnyLayout, Layout protocol, onGeometryChange, GeometryReader, size classes, and iOS 26 window APIs
license	MIT
metadata	{"version":"1.0.0"}

SwiftUI Layout API Reference

Comprehensive API reference for SwiftUI adaptive layout tools. For decision guidance and anti-patterns, see the axiom-swiftui-layout skill.

Overview

This reference covers all SwiftUI layout APIs for building adaptive interfaces:

• ViewThatFits — Automatic variant selection (iOS 16+)
• AnyLayout — Type-erased animated layout switching (iOS 16+)
• Layout Protocol — Custom layout algorithms (iOS 16+)
• onGeometryChange — Efficient geometry reading (iOS 16+ backported)
• GeometryReader — Layout-phase geometry access (iOS 13+)
• Safe Area Padding — .safeAreaPadding() vs .padding() (iOS 17+)
• Size Classes — Trait-based adaptation
• iOS 26 Window APIs — Free-form windows, menu bar, resize anchors

---

ViewThatFits

Evaluates child views in order and displays the first one that fits in the available space.

Basic Usage

ViewThatFits {
    // First choice
    HStack {
        icon
        title
        Spacer()
        button
    }

    // Second choice
    HStack {
        icon
        title
        button
    }

    // Fallback
    VStack {
        HStack { icon; title }
        button
    }
}

With Axis Constraint

// Only consider horizontal fit
ViewThatFits(in: .horizontal) {
    wideVersion
    narrowVersion
}

// Only consider vertical fit
ViewThatFits(in: .vertical) {
    tallVersion
    shortVersion
}

How It Works

1. Applies fixedSize() to each child
2. Measures ideal size against available space
3. Returns first child that fits
4. Falls back to last child if none fit

Limitations

• Does not expose which variant was selected
• Cannot animate between variants (use AnyLayout instead)
• Measures all variants (performance consideration for complex views)

---

AnyLayout

Type-erased layout container enabling animated transitions between layouts.

Basic Usage

struct AdaptiveView: View {
    @Environment(\.horizontalSizeClass) var sizeClass

    var layout: AnyLayout {
        sizeClass == .compact
            ? AnyLayout(VStackLayout(spacing: 12))
            : AnyLayout(HStackLayout(spacing: 20))
    }

    var body: some View {
        layout {
            ForEach(items) { item in
                ItemView(item: item)
            }
        }
        .animation(.default, value: sizeClass)
    }
}

Available Layout Types

AnyLayout(HStackLayout(alignment: .top, spacing: 10))
AnyLayout(VStackLayout(alignment: .leading, spacing: 8))
AnyLayout(ZStackLayout(alignment: .center))
AnyLayout(GridLayout(alignment: .leading, horizontalSpacing: 10, verticalSpacing: 10))

Custom Conditions

// Based on Dynamic Type
@Environment(\.dynamicTypeSize) var typeSize

var layout: AnyLayout {
    typeSize.isAccessibilitySize
        ? AnyLayout(VStackLayout())
        : AnyLayout(HStackLayout())
}

// Based on geometry
@State private var isWide = true

var layout: AnyLayout {
    isWide
        ? AnyLayout(HStackLayout())
        : AnyLayout(VStackLayout())
}

Why Use Over Conditional Views

// ❌ Loses view identity, no animation
if isCompact {
    VStack { content }
} else {
    HStack { content }
}

// ✅ Preserves identity, smooth animation
let layout = isCompact ? AnyLayout(VStackLayout()) : AnyLayout(HStackLayout())
layout { content }

---

Layout Protocol

Create custom layout containers with full control over positioning.

Basic Custom Layout

struct FlowLayout: Layout {
    var spacing: CGFloat = 8

    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        let sizes = subviews.map { $0.sizeThatFits(.unspecified) }
        return calculateSize(for: sizes, in: proposal.width ?? .infinity)
    }

    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        var point = bounds.origin
        var lineHeight: CGFloat = 0

        for subview in subviews {
            let size = subview.sizeThatFits(.unspecified)

            if point.x + size.width > bounds.maxX {
                point.x = bounds.origin.x
                point.y += lineHeight + spacing
                lineHeight = 0
            }

            subview.place(at: point, proposal: .unspecified)
            point.x  size.width  spacing
            lineHeight  (lineHeight, size.height)
        }
    }
}


(spacing: ) {
    (tags) { tag 
        (tag: tag)
    }
}

With Cache

struct CachedLayout: Layout {
    struct CacheData {
        var sizes: [CGSize] = []
    }

    func makeCache(subviews: Subviews) -> CacheData {
        CacheData(sizes: subviews.map { $0.sizeThatFits(.unspecified) })
    }

    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout CacheData) -> CGSize {
        // Use cache.sizes instead of measuring again
    }
}

Layout Values

// Define custom layout value
struct Rank: LayoutValueKey {
    static let defaultValue: Int = 0
}

extension View {
    func rank(_ value: Int) -> some View {
        layoutValue(key: Rank.self, value: value)
    }
}

// Read in layout
func placeSubviews(...) {
    let sorted = subviews.sorted { $0[Rank.self] < $1[Rank.self] }
}

---

onGeometryChange

Efficient geometry reading without layout side effects. Backported to iOS 16+.

Basic Usage

@State private var size: CGSize = .zero

var body: some View {
    content
        .onGeometryChange(for: CGSize.self) { proxy in
            proxy.size
        } action: { newSize in
            size = newSize
        }
}

Reading Specific Values

// Width only
.onGeometryChange(for: CGFloat.self) { proxy in
    proxy.size.width
} action: { width in
    columnCount = max(1, Int(width / 150))
}

// Frame in coordinate space
.onGeometryChange(for: CGRect.self) { proxy in
    proxy.frame(in: .global)
} action: { frame in
    globalFrame = frame
}

// Aspect ratio
.onGeometryChange(for: Bool.self) { proxy in
    proxy.size.width > proxy.size.height
} action: { isWide in
    self.isWide = isWide
}

Coordinate Spaces

// Named coordinate space
ScrollView {
    content
        .onGeometryChange(for: CGFloat.self) { proxy in
            proxy.frame(in: .named("scroll")).minY
        } action: { offset in
            scrollOffset = offset
        }
}
.coordinateSpace(name: "scroll")

Comparison with GeometryReader

Aspect	onGeometryChange	GeometryReader
Layout impact	None	Greedy (fills space)
When evaluated	After layout	During layout
Use case	Side effects	Layout calculations
iOS version	16+ (backported)	13+

---

GeometryReader

Provides geometry information during layout phase. Use sparingly due to greedy sizing.

Basic Usage (Constrained)

// ✅ Always constrain GeometryReader
GeometryReader { proxy in
    let width = proxy.size.width
    HStack(spacing: 0) {
        Rectangle().frame(width: width * 0.3)
        Rectangle().frame(width: width * 0.7)
    }
}
.frame(height: 100)  // Required constraint

GeometryProxy Properties

GeometryReader { proxy in
    // Container size
    let size = proxy.size  // CGSize

    // Safe area insets
    let insets = proxy.safeAreaInsets  // EdgeInsets

    // Frame in coordinate space
    let globalFrame = proxy.frame(in: .global)
    let localFrame = proxy.frame(in: .local)
    let namedFrame = proxy.frame(in: .named("container"))
}

Common Patterns

// Proportional sizing
GeometryReader { geo in
    VStack {
        header.frame(height: geo.size.height * 0.2)
        content.frame(height: geo.size.height * 0.8)
    }
}

// Centering with offset
GeometryReader { geo in
    content
        .position(x: geo.size.width / 2, y: geo.size.height / 2)
}

Avoiding Common Mistakes

// ❌ Unconstrained in VStack
VStack {
    GeometryReader { ... }  // Takes ALL space
    Button("Next") { }       // Invisible
}

// ✅ Constrained
VStack {
    GeometryReader { ... }
        .frame(height: 200)
    Button("Next") { }
}

// ❌ Causing layout loops
GeometryReader { geo in
    content
        .frame(width: geo.size.width)  // Can cause infinite loop
}

---

Safe Area Padding

SwiftUI provides two primary approaches for handling spacing around content: .padding() and .safeAreaPadding(). Understanding when to use each is critical for proper layout on devices with safe areas (notch, Dynamic Island, home indicator).

The Critical Difference

// ❌ WRONG - Ignores safe areas, content hits notch/home indicator
ScrollView {
    content
}
.padding(.horizontal, 20)

// ✅ CORRECT - Respects safe areas, adds padding beyond them
ScrollView {
    content
}
.safeAreaPadding(.horizontal, 20)

Key insight: .padding() adds fixed spacing from the view's edges. .safeAreaPadding() adds spacing beyond the safe area insets.

When to Use Each

Use .padding() when

• Adding spacing between sibling views within a container
• Creating internal spacing that should be consistent everywhere
• Working with views that already respect safe areas (like List, Form)
• Adding decorative spacing on macOS (no safe area concerns)

VStack(spacing: 0) {
    header
        .padding(.horizontal, 16)  // ✅ Internal spacing

    Divider()

    content
        .padding(.horizontal, 16)  // ✅ Internal spacing
}

Use .safeAreaPadding() when (iOS 17+)

• Adding margin to full-width content that extends to screen edges
• Implementing edge-to-edge scrolling with proper insets
• Creating custom containers that need safe area awareness
• Working with Liquid Glass or full-screen materials

// ✅ Edge-to-edge list with custom padding
List(items) { item in
    ItemRow(item)
}
.listStyle(.plain)
.safeAreaPadding(.horizontal, 20)  // Adds 20pt beyond safe areas

// ✅ Full-screen content with proper margins
ZStack {
    Color.blue.ignoresSafeArea()

    VStack {
        content
    }
    .safeAreaPadding(.all, 16)  // Respects notch, home indicator
}

Platform Availability

iOS 17+, iPadOS 17+, macOS 14+, axiom-visionOS 1.0+

For earlier iOS versions, use manual safe area handling:

// iOS 13-16 fallback
GeometryReader { geo in
    content
        .padding(.horizontal, 20 + geo.safeAreaInsets.leading)
}

Or conditional compilation:

if #available(iOS 17, *) {
    content.safeAreaPadding(.horizontal, 20)
} else {
    content.padding(.horizontal, 20)
        .padding(.leading, safeAreaInsets.leading)
}

Edge-Specific Usage

// Top only (below status bar/notch)
.safeAreaPadding(.top, 8)

// Bottom only (above home indicator)
.safeAreaPadding(.bottom, 16)

// Horizontal (left/right of safe areas)
.safeAreaPadding(.horizontal, 20)

// All edges
.safeAreaPadding(.all, 16)

// Individual edges
.safeAreaPadding(EdgeInsets(top: 8, leading: 20, bottom: 16, trailing: 20))

Common Patterns

Edge-to-Edge ScrollView

ScrollView {
    LazyVStack(spacing: 12) {
        ForEach(items) { item in
            ItemCard(item)
        }
    }
}
.safeAreaPadding(.horizontal, 16)  // Content inset from edges + safe areas
.safeAreaPadding(.vertical, 8)

Full-Screen Background with Safe Content

ZStack {
    // Background extends edge-to-edge
    LinearGradient(...)
        .ignoresSafeArea()

    // Content respects safe areas + custom padding
    VStack {
        header
        Spacer()
        content
        Spacer()
        footer
    }
    .safeAreaPadding(.all, 20)
}

Nested Padding (Combined Approach)

// Outer: Safe area padding for device insets
VStack(spacing: 0) {
    content
}
.safeAreaPadding(.horizontal, 16)  // Beyond safe areas

// Inner: Regular padding for internal spacing
VStack {
    Text("Title")
        .padding(.bottom, 8)  // Internal spacing
    Text("Subtitle")
}

Decision Tree

Does your content extend to screen edges?
├─ YES → Use .safeAreaPadding()
│   ├─ Is it scrollable? → .safeAreaPadding(.horizontal/.vertical)
│   └─ Is it full-screen? → .safeAreaPadding(.all)
│
└─ NO (contained within a safe container like List/Form)
    └─ Use .padding() for internal spacing

Visual Debugging

// Visualize safe area padding (iOS 17+)
content
    .safeAreaPadding(.horizontal, 20)
    .background(.red.opacity(0.2))  // Shows padding area
    .border(.blue)  // Shows content bounds

Migration from Manual Safe Area Handling

// ❌ OLD: Manual calculation (iOS 13-16)
GeometryReader { geo in
    content
        .padding(.top, geo.safeAreaInsets.top + 16)
        .padding(.bottom, geo.safeAreaInsets.bottom + 16)
        .padding(.horizontal, 20)
}

// ✅ NEW: .safeAreaPadding() (iOS 17+)
content
    .safeAreaPadding(.vertical, 16)
    .safeAreaPadding(.horizontal, 20)

Related APIs

.safeAreaInset(edge:) - Adds persistent content that shrinks the safe area:

ScrollView {
    content
}
.safeAreaInset(edge: .bottom) {
    // This REDUCES the safe area, content scrolls under it
    toolbarButtons
        .padding()
        .background(.ultraThinMaterial)
}

.ignoresSafeArea() - Opts out of safe area completely:

Color.blue
    .ignoresSafeArea()  // Extends to absolute screen edges

Why It Matters

Before iOS 17: Developers had to manually calculate safe area insets with GeometryReader, leading to:

• Verbose code
• Performance overhead (GeometryReader forces extra layout pass)
• Easy mistakes (forgetting to check all edges)

iOS 17+: .safeAreaPadding() provides:

• Declarative API (matches SwiftUI philosophy)
• Automatic safe area awareness
• Better performance (no extra layout passes)
• Type-safe edge specification

Real-world impact: Using .padding() instead of .safeAreaPadding() on iPhone 15 Pro causes content to:

• Hit the Dynamic Island (top)
• Overlap the home indicator (bottom)
• Get cut off by screen corners (rounded edges)

---

Size Classes

Environment values indicating horizontal and vertical size characteristics.

Reading Size Classes

struct AdaptiveView: View {
    @Environment(\.horizontalSizeClass) var horizontalSizeClass
    @Environment(\.verticalSizeClass) var verticalSizeClass

    var body: some View {
        if horizontalSizeClass == .compact {
            compactLayout
        } else {
            regularLayout
        }
    }
}

Size Class Values

enum UserInterfaceSizeClass {
    case compact    // Constrained space
    case regular    // Ample space
}

Platform Behavior

iPhone:

Orientation	Horizontal	Vertical
Portrait	.compact	.regular
Landscape (small)	.compact	.compact
Landscape (Plus/Max)	.regular	.compact

iPad:

Configuration	Horizontal	Vertical
Any full screen	.regular	.regular
70% Split View	.regular	.regular
50% Split View	.regular	.regular
33% Split View	.compact	.regular
Slide Over	.compact	.regular

Overriding Size Classes

content
    .environment(\.horizontalSizeClass, .compact)

---

Dynamic Type Size

Environment value for user's preferred text size.

Reading Dynamic Type

@Environment(\.dynamicTypeSize) var dynamicTypeSize

var body: some View {
    if dynamicTypeSize.isAccessibilitySize {
        accessibleLayout
    } else {
        standardLayout
    }
}

Size Categories

enum DynamicTypeSize: Comparable {
    case xSmall
    case small
    case medium
    case large           // Default
    case xLarge
    case xxLarge
    case xxxLarge
    case accessibility1  // isAccessibilitySize = true
    case accessibility2
    case accessibility3
    case accessibility4
    case accessibility5
}

Scaled Metric

@ScaledMetric var iconSize: CGFloat = 24
@ScaledMetric(relativeTo: .largeTitle) var headerSize: CGFloat = 44

Image(systemName: "star")
    .frame(width: iconSize, height: iconSize)

---

iOS 26 Window APIs

Window Resize Anchor

WindowGroup {
    ContentView()
}
.windowResizeAnchor(.topLeading)  // Resize originates from top-left
.windowResizeAnchor(.center)      // Resize from center

Menu Bar Commands (iPad)

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .commands {
            CommandMenu("View") {
                Button("Show Sidebar") {
                    showSidebar.toggle()
                }
                .keyboardShortcut("s", modifiers: [.command, .option])

                Divider()

                Button("Zoom In") { zoom += 0.1 }
                    .keyboardShortcut("+")
                Button("Zoom Out") { zoom -= 0.1 }
                    .keyboardShortcut("-")
            }
        }
    }
}

NavigationSplitView Column Control

// iOS 26: Automatic column visibility
NavigationSplitView {
    Sidebar()
} content: {
    ContentList()
} detail: {
    DetailView()
}
// Columns auto-hide/show based on available width

// Manual control (when needed)
@State private var columnVisibility: NavigationSplitViewVisibility = .all

NavigationSplitView(columnVisibility: $columnVisibility) {
    Sidebar()
} detail: {
    DetailView()
}

Scene Phase

@Environment(\.scenePhase) var scenePhase

var body: some View {
    content
        .onChange(of: scenePhase) { oldPhase, newPhase in
            switch newPhase {
            case .active:
                // Window is visible and interactive
            case .inactive:
                // Window is visible but not interactive
            case .background:
                // Window is not visible
            }
        }
}

---

Coordinate Spaces

Built-in Coordinate Spaces

// Global (screen coordinates)
proxy.frame(in: .global)

// Local (view's own bounds)
proxy.frame(in: .local)

// Named (custom)
proxy.frame(in: .named("mySpace"))

Creating Named Spaces

ScrollView {
    content
        .onGeometryChange(for: CGFloat.self) { proxy in
            proxy.frame(in: .named("scroll")).minY
        } action: { offset in
            scrollOffset = offset
        }
}
.coordinateSpace(name: "scroll")

// iOS 17+ typed coordinate space
extension CoordinateSpaceProtocol where Self == NamedCoordinateSpace {
    static var scroll: Self { .named("scroll") }
}

---

ScrollView Geometry (iOS 18+)

onScrollGeometryChange

ScrollView {
    content
}
.onScrollGeometryChange(for: CGFloat.self) { geometry in
    geometry.contentOffset.y
} action: { offset in
    scrollOffset = offset
}

ScrollGeometry Properties

.onScrollGeometryChange(for: ScrollGeometry.self) { $0 } action: { geo in
    let offset = geo.contentOffset      // Current scroll position
    let size = geo.contentSize          // Total content size
    let visible = geo.visibleRect       // Currently visible rect
    let insets = geo.contentInsets      // Content insets
}

---

Lazy Container Gotchas

Recycling Behavior

LazyVStack and LazyHStack create views on demand and recycle them when off-screen. This means:

• View identity matters: If cells flash/disappear during fast scrolling, the view identity is unstable. Use explicit .id() on items.
• onAppear/onDisappear fire repeatedly: Views are created and destroyed as you scroll. Don't use these for one-time setup.
• State resets on recycle: @State in lazy items resets when recycled. Lift state to the model layer.

// ❌ Items flash during fast scroll — unstable identity
LazyVStack {
    ForEach(Array(items.enumerated()), id: \.offset) { index, item in
        ItemRow(item: item)  // Identity changes when array mutates
    }
}

// ✅ Stable identity prevents flash/disappear
LazyVStack {
    ForEach(items) { item in  // Uses item.id (Identifiable)
        ItemRow(item: item)
    }
}

When NOT to Use Lazy Containers

Scenario	Use Instead	Why
< 50 items	VStack / HStack	No recycling overhead, simpler
Nested in another lazy container	VStack (inner)	Nested lazy causes layout issues
Need all items measured upfront	VStack	Lazy containers don't know total size

---

Resources

WWDC: 2025-208, 2024-10074, 2022-10056

Docs: /swiftui/layout, /swiftui/viewthatfits

Skills: axiom-swiftui-layout, axiom-swiftui-debugging