Compose Animations

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 for state-holder details.