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

    // Second choice
    HStack {
        icon
        title
        button
    }

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

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

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

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)
    }
}

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

// 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())
}

// ❌ 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 }

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 = max(lineHeight, size.height)
        }
    }
}

// Usage
FlowLayout(spacing: 12) {
    ForEach(tags) { tag in
        TagView(tag: tag)
    }
}

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
    }
}

// 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] }
}

@State private var size: CGSize = .zero

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

// 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
}

// 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")

// ✅ 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

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"))
}

// 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)
}

// ❌ 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
}

// ❌ 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)

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

    Divider()

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

// ✅ 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
}

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

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

// 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))

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

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

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

// 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")
}

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

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

// ❌ 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)

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

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

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

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

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

content
    .environment(\.horizontalSizeClass, .compact)

@Environment(\.dynamicTypeSize) var dynamicTypeSize

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

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
}

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

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

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

@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("-")
            }
        }
    }
}

// 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()
}

@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
            }
        }
}