swiftui-animation

What this skill does

# SwiftUI Animation (iOS 26+)

Review, write, and fix SwiftUI animations. Apply modern animation APIs with
correct timing, transitions, and accessibility handling using Swift 6.3 patterns.

## Contents

- [Triage Workflow](#triage-workflow)
- [withAnimation (Explicit Animation)](#withanimation-explicit-animation)
- [Implicit Animation](#implicit-animation)
- [Spring Type (iOS 17+)](#spring-type-ios-17)
- [PhaseAnimator (iOS 17+)](#phaseanimator-ios-17)
- [KeyframeAnimator (iOS 17+)](#keyframeanimator-ios-17)
- [`@Animatable Macro`](#animatable-macro)
- [matchedGeometryEffect (iOS 14+)](#matchedgeometryeffect-ios-14)
- [Navigation Zoom Transition (iOS 18+)](#navigation-zoom-transition-ios-18)
- [Transitions (iOS 17+)](#transitions-ios-17)
- [ContentTransition (iOS 16+)](#contenttransition-ios-16)
- [Symbol Effects (iOS 17+)](#symbol-effects-ios-17)
- [Symbol Rendering Modes](#symbol-rendering-modes)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)

## Triage Workflow

### Step 1: Identify the animation category

| Category | API | When to use |
|---|---|---|
| State-driven | `withAnimation`, `.animation(_:body:)`, `.animation(_:value:)` | Explicit state changes, selective modifier animation, or simple value-bound changes |
| Multi-phase | `PhaseAnimator` | Sequenced multi-step animations |
| Keyframe | `KeyframeAnimator` | Complex multi-property choreography |
| Shared element | `matchedGeometryEffect` | Layout-driven hero transitions |
| Navigation | `matchedTransitionSource` + `.navigationTransition(.zoom)` | NavigationStack push/pop zoom |
| View lifecycle | `.transition()` | Insertion and removal |
| Text content | `.contentTransition()` | In-place text/number changes |
| Symbol | `.symbolEffect()` | SF Symbol animations |
| Custom | `CustomAnimation` protocol | Novel timing curves |

### Step 2: Choose the animation curve

```swift
// Timing curves
.linear                              // constant speed
.easeIn(duration: 0.3)              // slow start
.easeOut(duration: 0.3)             // slow end
.easeInOut(duration: 0.3)           // slow start and end

// Spring presets (preferred for natural motion)
.smooth                              // no bounce, fluid
.smooth(duration: 0.5, extraBounce: 0.0)
.snappy                              // small bounce, responsive
.snappy(duration: 0.4, extraBounce: 0.1)
.bouncy                              // visible bounce, playful
.bouncy(duration: 0.5, extraBounce: 0.2)

// Custom spring
.spring(duration: 0.5, bounce: 0.3, blendDuration: 0.0)
.spring(Spring(duration: 0.6, bounce: 0.2), blendDuration: 0.0)
.interactiveSpring(response: 0.15, dampingFraction: 0.86)
```

### Step 3: Apply and verify

- Confirm animation triggers on the correct state change.
- Test with Accessibility > Reduce Motion enabled.
- Verify no expensive work runs inside animation content closures.

## withAnimation (Explicit Animation)

```swift
withAnimation(.spring) { isExpanded.toggle() }

// With completion (iOS 17+)
withAnimation(.smooth(duration: 0.35), completionCriteria: .logicallyComplete) {
    isExpanded = true
} completion: { loadContent() }
```

## Implicit Animation

Prefer `.animation(_:body:)` when only specific modifiers should animate.
Use `.animation(_:value:)` for simple value-bound changes that can animate the
view's animatable modifiers together.

```swift
Badge()
    .foregroundStyle(isActive ? .green : .secondary)
    .animation(.snappy) { content in
        content
            .scaleEffect(isActive ? 1.15 : 1.0)
            .opacity(isActive ? 1.0 : 0.7)
    }
```

```swift
Circle()
    .scaleEffect(isActive ? 1.2 : 1.0)
    .opacity(isActive ? 1.0 : 0.6)
    .animation(.bouncy, value: isActive)
```

## Spring Type (iOS 17+)

Four initializer forms for different mental models.

```swift
// Perceptual (preferred)
Spring(duration: 0.5, bounce: 0.3)

// Physical
Spring(mass: 1.0, stiffness: 100.0, damping: 10.0)

// Response-based
Spring(response: 0.5, dampingRatio: 0.7)

// Settling-based
Spring(settlingDuration: 1.0, dampingRatio: 0.8)
```

Three presets mirror Animation presets: `.smooth`, `.snappy`, `.bouncy`.

## PhaseAnimator (iOS 17+)

Cycle through discrete phases with per-phase animation curves.

```swift
enum PulsePhase: CaseIterable {
    case idle, grow, shrink
}

struct PulsingDot: View {
    var body: some View {
        PhaseAnimator(PulsePhase.allCases) { phase in
            Circle()
                .frame(width: 40, height: 40)
                .scaleEffect(phase == .grow ? 1.4 : 1.0)
                .opacity(phase == .shrink ? 0.5 : 1.0)
        } animation: { phase in
            switch phase {
            case .idle: .easeIn(duration: 0.2)
            case .grow: .spring(duration: 0.4, bounce: 0.3)
            case .shrink: .easeOut(duration: 0.3)
            }
        }
    }
}
```

Trigger-based variant runs one cycle per trigger change:

```swift
PhaseAnimator(PulsePhase.allCases, trigger: tapCount) { phase in
    // ...
} animation: { _ in .spring(duration: 0.4) }
```

## KeyframeAnimator (iOS 17+)

Animate multiple properties along independent timelines.

```swift
struct AnimValues {
    var scale: Double = 1.0
    var yOffset: Double = 0.0
    var opacity: Double = 1.0
}

struct BounceView: View {
    @State private var trigger = false

    var body: some View {
        Button { trigger.toggle() } label: {
            Image(systemName: "star.fill")
                .font(.largeTitle)
                .keyframeAnimator(
                    initialValue: AnimValues(),
                    trigger: trigger
                ) { content, value in
                    content
                        .scaleEffect(value.scale)
                        .offset(y: value.yOffset)
                        .opacity(value.opacity)
                } keyframes: { _ in
                    KeyframeTrack(\.scale) {
                        SpringKeyframe(1.5, duration: 0.3)
                        CubicKeyframe(1.0, duration: 0.4)
                    }
                    KeyframeTrack(\.yOffset) {
                        CubicKeyframe(-30, duration: 0.2)
                        CubicKeyframe(0, duration: 0.4)
                    }
                    KeyframeTrack(\.opacity) {
                        LinearKeyframe(0.6, duration: 0.15)
                        LinearKeyframe(1.0, duration: 0.25)
                    }
                }
        }
        .buttonStyle(.plain)
    }
}
```

Keyframe types: `LinearKeyframe` (linear), `CubicKeyframe` (smooth curve),
`SpringKeyframe` (spring physics), `MoveKeyframe` (instant jump).

Use `repeating: true` for looping keyframe animations.

## `@Animatable` Macro

Replaces manual `AnimatableData` boilerplate. Attach to any type with
animatable stored properties.

```swift
// Replaces manual AnimatableData boilerplate
@Animatable
struct WaveShape: Shape {
    var frequency: Double
    var amplitude: Double
    var phase: Double
    @AnimatableIgnored var lineWidth: CGFloat

    func path(in rect: CGRect) -> Path {
        // draw wave using frequency, amplitude, phase
    }
}
```

Rules:
- Stored properties must conform to `VectorArithmetic`.
- Use `@AnimatableIgnored` to exclude non-animatable properties.
- Computed properties are never included.

## matchedGeometryEffect (iOS 14+)

Synchronize geometry between views for shared-element animations.

```swift
struct HeroView: View {
    @Namespace private var heroSpace
    @State private var isExpanded = false

    var body: some View {
        Group {
            if isExpanded {
                Button {
                    withAnimation(.spring(duration: 0.4, bounce: 0.2)) {
                        isExpanded = false
                    }
                } label: {
                    DetailCard()
                        .matchedGeometryEffect(id: "card", in: heroSpace)
                }
            } else {
                Button {
                    withAnimation(.sp