swiftui-motion

# SwiftUI Motion

> SwiftUI animation core. Loaded for any SwiftUI project (iOS, macOS, multi-target Apple).
> Concise rules here. Deep-dive in `references/`.
> Pair with `../motion-principles/SKILL.md` (foundation) and `../mobile-principles/SKILL.md` (touch UX).

---

## Animation API decision tree

| Need | API |
|---|---|
| Single value over time | `withAnimation { } + @State` or `.animation(_, value:)` |
| Multiple coordinated states | `PhaseAnimator(phases)` (iOS 17+) |
| Time-based keyframes | `KeyframeAnimator(initialValue:repeating:content:)` (iOS 17+) |
| Custom property animations | `@Animatable` macro (iOS 17+) or `Animatable` protocol |
| Shared element transitions | `matchedGeometryEffect(id:in:)` |
| Gesture-driven | `DragGesture` / `MagnifyGesture` + `.offset` / `.scaleEffect` |
| Loop forever | `.animation(.linear.repeatForever(autoreverses: true), value: ...)` or `.phaseAnimator` |

**Rule:** start with `withAnimation`. Reach for `PhaseAnimator` only when you have 3+ ordered states. Reach for `KeyframeAnimator` only when you need parallel time-based tracks.

---

## Springs (the only easing you should care about)

SwiftUI ships 4 named springs (iOS 17+). Use them. Tune `response` / `dampingFraction` only when a preset is wrong.

| Preset (iOS 17+) | Equivalent | Mood |
|---|---|---|
| `.snappy` | `.spring(response: 0.3, dampingFraction: 0.85)` | UI snappy |
| `.bouncy` | `.spring(response: 0.5, dampingFraction: 0.7)` | playful |
| `.smooth` | `.spring(response: 0.5, dampingFraction: 1.0)` | calm, no bounce |
| `.interactiveSpring()` | `.spring(response: 0.15, dampingFraction: 0.86)` | gesture follow |

`response` is the time the spring takes to settle (lower = snappier, higher = softer). `dampingFraction` is the overshoot intensity in `0...1` (1 = no overshoot, 0 = perpetual oscillation - never use 0). For UI work, stay in `response: 0.2...0.5` and `dampingFraction: 0.7...1.0`. Deep-dive: `references/springs-cheatsheet.md`.

iOS 17+ also exposes `.spring(duration:bounce:)` where `bounce` is `0...1` (0 = critically damped, 1 = full bounce). It's the same spring, exposed in a more designer-friendly way:

```swift
.animation(.spring(duration: 0.4, bounce: 0.3), value: state)
```

---

## Implicit vs explicit animations

```swift
// Implicit - via .animation modifier (binds to a value)
Circle()
    .scaleEffect(scale)
    .animation(.spring(.snappy), value: scale)
```

```swift
// Explicit - via withAnimation block
Button("Grow") {
    withAnimation(.smooth) { scale = 1.5 }
}
```

**Rule:** prefer explicit (`withAnimation`) for state changes triggered by user actions; use implicit when *any* change to a value should always animate (e.g., a progress bar that updates from anywhere). Never both on the same property - the outer `withAnimation` wins, but the implicit `.animation` modifier still runs and stacks confusingly.

---

## Transitions

Transitions drive insertion / removal of views inside an `if`, `switch`, or `ForEach`. They run when the parent's animation context fires (so wrap state mutations in `withAnimation`).

```swift
if visible {
    Card().transition(.asymmetric(
        insertion: .move(edge: .bottom).combined(with: .opacity),
        removal: .opacity.animation(.easeIn(duration: 0.15))
    ))
}
```

**BAD - vanish into a black hole:**
```swift
Card().transition(.scale)  // scales to 0, the universal "broken" feel
```

**GOOD - never scale to 0:**
```swift
Card().transition(
    .scale(scale: 0.95).combined(with: .opacity)
)
```

iOS 17+ also has the `.transition(_:)` modifier with custom transitions via the `Transition` protocol - useful for shared timing across many views. For 90% of work, the built-in combinators (`.move`, `.opacity`, `.scale`, `.slide`, `.push`, `.asymmetric`, `.combined(with:)`) are enough.

---

## matchedGeometryEffect (hero animations)

Tag two views with the same `id` in the same `Namespace`. SwiftUI interpolates frame and position when the source view is replaced.

```swift
struct Gallery: View {
    @Namespace private var ns
    @State private var expanded = false

    var body: some View {
        ZStack {
            if expanded {
                LargeCard()
                    .matchedGeometryEffect(id: "card", in: ns)
                    .onTapGesture { withAnimation(.spring(.smooth)) { expanded = false } }
            } else {
                SmallCard()
                    .matchedGeometryEffect(id: "card", in: ns)
                    .onTapGesture { withAnimation(.spring(.smooth)) { expanded = true } }
            }
        }
    }
}
```

`isSource: true` (default on the source-of-truth view) tells SwiftUI which frame to interpolate from. Common gotchas: id collisions across unrelated namespaces, view identity instability (use stable ids, not array indices), and animating out of an `if` branch where the destination view doesn't exist yet (wrap both branches inside the same parent, use opacity to hide instead of removing).

---

## PhaseAnimator (iOS 17+)

For ordered state choreography. Define a `CaseIterable + Hashable` enum, SwiftUI walks through phases sequentially, settling on the last one.

```swift
enum SuccessPhase: CaseIterable { case start, scaleUp, rotate, settle }

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

    var body: some View {
        Image(systemName: "checkmark.circle.fill")
            .font(.system(size: 64))
            .foregroundStyle(.green)
            .phaseAnimator(SuccessPhase.allCases, trigger: trigger) { view, phase in
                view
                    .scaleEffect(phase == .start ? 0 : phase == .settle ? 1 : 1.2)
                    .rotationEffect(.degrees(phase == .rotate ? 360 : 0))
                    .opacity(phase == .start ? 0 : 1)
            } animation: { phase in
                switch phase {
                case .start: .smooth(duration: 0.05)
                case .scaleUp: .spring(.bouncy, blendDuration: 0.25)
                case .rotate: .spring(