axiom-swiftui-layout-ref

# 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

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

    // Second choice
    HStack {
        icon
        title
        button
    }

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

### With Axis Constraint

```swift
// 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

```swift
struct AdaptiveView: View {
    @Environment(\.horizontalSizeClass) var si