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# Compose: animations
## Core principle
Pick the smallest API that expresses the motion and its lifecycle.
## Procedure
1. Identify the job: show or hide a subtree, animate one value, coordinate values from one state, resize content, swap content, or handle user-driven motion.
2. Choose the matching API from the table. Prefer target-state APIs; use `Animatable` only when gestures, interruption, or imperative control require it.
3. Check lifecycle: an alpha animation keeps content composed; `AnimatedVisibility` removes it after exit. Do not use a fade when unmounting is required.
4. For `AnimatedContent`, render from the content lambda target and choose a
`contentKey` only when visual identity differs from payload equality. In a
swap between distinct items, keep the target value as the identity (the
default) or key by a stable item ID; do not collapse different items to a
shared presence or branch key. In a review, name any rendered expression that
captures outer target state and
recommend using the lambda target there. If a test must prove transition
identity, check outgoing and incoming content during the overlap; checking
only the settled destination misses a shared-identity bug. Read
[AnimatedContent identity](references/animated-content.md) for state-holder
details.
5. Keep animated `State` in layout or draw block modifiers when it changes at frame rate; route deeper diagnosis to [Compose performance](../compose-performance/SKILL.md).
6. Use Navigation Compose transitions for destination swaps it owns, and dedicated libraries for art-based motion.
7. Finish when the API, lifecycle, and content identity match the UI, no simpler API fits, and the relevant behavior is verified.
## API choice
| Need | Prefer |
|---|---|
| Show/hide a subtree with enter/exit semantics | [`AnimatedVisibility`](https://developer.android.com/develop/ui/compose/animation/composables-modifiers#animatedvisibility) |
| One value follows state | `animate*AsState` |
| Several values follow one boolean, enum, or sealed state | `rememberTransition` plus child animations |
| Child size changes | `Modifier.animateContentSize()` |
| Different composable trees fill one region | `AnimatedContent`, or `Crossfade` for the simple case |
| Drag, fling, interruption, or imperative control | [`Animatable`](https://developer.android.com/reference/kotlin/androidx/compose/animation/core/Animatable) |
Use an `AnimationSpec` when the default motion is wrong and a distinct `label` when multiple animations need tooling visibility.
```kotlin
val width by animateDpAsState(
targetValue = if (expanded) 200.dp else 56.dp,
animationSpec = spring(dampingRatio = 0.7f),
label = "fabWidth",
)
```
For values that must remain synchronized, define them on one transition rather than several independent `animate*AsState` calls:
```kotlin
val transition = rememberTransition(targetState = phase, label = "phase")
val alpha by transition.animateFloat(label = "alpha") { target ->
if (target == Phase.Visible) 1f else 0f
}
val offset by transition.animateDp(label = "offset") { target ->
if (target == Phase.Visible) 0.dp else 24.dp
}
```
For animated fills, prefer `drawBehind { drawRect(color.value) }` over a value-form background when the color updates every frame. For an API ambiguity, start with the official [Choose an animation API](https://developer.android.com/develop/ui/compose/animation/choose-api) guide; use [`rememberInfiniteTransition`](https://developer.android.com/reference/kotlin/androidx/compose/animation/core/rememberInfiniteTransition) for repeating cycles and [`SeekableTransitionState`](https://developer.android.com/reference/kotlin/androidx/compose/animation/core/SeekableTransitionState) for seekable or test-controlled progress.
## When not to use this skill
- For side-effect timing or click-launched work, use [Compose state and effects](../compose-state-and-effects/SKILL.md).
- For deep state-read or recomposition diagnosis, use [Compose performance](../compose-performance/SKILL.md).
---
## Source and license
Source: [chrisbanes/skills](https://github.com/chrisbanes/skills/tree/72628aea92c9081df0ba96600f24af3f07f4a87a/skills/compose-animations). Author: Chris Banes.
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