Version-sensitive. Every API name, SDK gate and browser-support claim below was verified on 2026-09-08 against primary sources. What against, and when, is in _jutsu/VERSIONS.md. If that date is old, re-verify before acting on a version number.

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).

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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 (needs Xcode 26 / SwiftUI 2025 SDK to compile; runtime availability iOS 13.0+ / macOS 10.15+ / watchOS 6.0+) or the Animatable protocol by hand (iOS 13+)
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.

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Springs (the only easing you should care about)

SwiftUI ships named spring presets (iOS 17+): .snappy, .bouncy, .smooth, plus the parameterised .spring(duration:bounce:blendDuration:) and .interactiveSpring(...). Each of the three named ones also takes (duration:extraBounce:) overloads. Use them. Tune response / dampingFraction only when a preset is wrong.