One card, four announcements — merging is the fix
A composite component produces one semantics node per child by default, so a card becomes four swipes. mergeDescendants, clearAndSetSemantics and invisibleToUser control the shape of the tree.
compose
129 posts
Compose builds a second tree, and it's the one your users hear
Every Compose UI produces a semantics tree alongside the layout tree. It's what screen readers read, what tests query, and what autofill inspects — so getting it right serves three audiences at once.
Take the clock away and animations become testable
Compose tests idle-wait by default, which makes animated UI either flaky or untested. Disabling autoAdvance gives you frame-level control, turning 'wait and hope' into deterministic assertions at chosen moments.
A shared element transition is one element pretending to be two
Shared element transitions match two composables by key and animate between their bounds. Understanding that neither element travels — a third overlay draws the motion — explains the scope requirements and the common failures.
You cannot fade out something that has already left the composition
An element removed by an if is gone immediately, so an exit animation has nothing to animate. AnimatedVisibility keeps it composed until the exit finishes — and AnimatedContent does the same for swaps.
updateTransition is a state machine that happens to animate
updateTransition coordinates several animations driven by one state. It handles multi-state enums, per-property specs via transitionSpec, and child transitions — all with coherent interruption a group of animate*AsState calls cannot give you.
You don't animate from A to B, you declare B and Compose handles the rest
animate*AsState animates toward a target that can change at any moment. Understanding it as declarative rather than imperative explains interruption, redirection, and why spring is the right default.
Your data class is unstable and Compose told you by recomposing everything
A List parameter is unstable because the compiler cannot prove it will not change.
Text is the most expensive composable you use without thinking
Layout measures text to resolve constraints, and a changing style invalidates the cache.
Compose has a dozen animation APIs and one decision tree
The Compose animation surface looks large because it is organised by what you're animating rather than by how. Three questions pick the right API, and picking wrong is why animation code gets complicated.
derivedStateOf exists because you were recomposing on every keystroke
A computed value read in composition invalidates on every change of its source. derivedStateOf narrows it to the result.
State hoisting has a limit, and you have probably passed it
Hoisting everything to the root means every keystroke recomposes the tree.