Animation: withAnimation and the animation modifier

[colemancda]

Final parity tier: animation. Swift describes intent; Compose animates.

How it works

• withAnimation(_:_:) sets the animation on a transaction while its body runs; a state write inside captures it, and the next (coalesced) evaluation stamps the root node with animationCurve/animationDurationMs.
• .animation(_:value:) marks a single view: its modifier carries the curve, duration, and a token of the observed value.
• Animation — .default, .linear/.easeIn/.easeOut/.easeInOut(duration:), .spring().
• The interpreter folds numeric modifier values (offset, opacity, scale, rotation, frame, background color, corner radius) through animate*AsState unconditionally — spec is a tween/spring while an animated tree (or the node's own .animation) applies, snap otherwise — so each Animatable persists across trees and eases from wherever it currently is.

The interesting bug

The first cut wrapped the tree in a conditional CompositionLocalProvider — and an animated tree arriving switched Compose branches, tearing down every remembered Animatable, so values snapped. The provider is now structurally unconditional (inheriting when a node carries no spec — only the root ever does), keeping the composition stable.

Verification

• swift test — 3 new tests (transaction stamping + one-shot clearing, scoping/restore, implicit emission), 56 passing
• Emulator (new Animation screen), frame-sampled with burst screencaps:
  • implicit color swap: caught a mid-blend frame between the two colors
  • explicit 2s linear offset: five distinct intermediate positions (x = 44 → 190 → 302 → 402 → 498 → 630 → 650) — genuine easing, not a snap
  • default, easeOut, and spring examples all settle at correct end states

Deferred

AnimatedVisibility for inserted/removed children; per-value curve fidelity beyond the five presets; gesture-driven/interruptible animation semantics (a mid-flight non-animated tree snaps to target).