compose-state-deferred-reads

Compose state deferred reads

Core principle

State reads invalidate the phase that reads them. If a State<T> is read in a composable body, changes invalidate composition. If it is read in layout or draw, changes can invalidate only layout or draw. Frame-rate state such as scroll offsets, animations, and drag positions usually belongs in layout/draw, not composition.

The fix is structural: keep the State<T> or a provider lambda, and read the value inside a layout/draw callback.

When to use this skill

• val x by animate*AsState(...) is passed to Modifier.offset(x = ...), Modifier.size(...), Modifier.graphicsLayer(...), or another value-form modifier.
• LazyListState.firstVisibleItemScrollOffset, ScrollState.value, Animatable.value, or gesture state is read in a composable body.
• A composable takes scrollOffset: Int, progress: Float, dragOffset: Offset, or similar frame-rate values.
• Recomposition counters climb during scroll, animation, or gestures even when data is stable.

1. Prefer block-form modifiers

Several modifiers have value forms and block forms. The value form receives values already read in composition; the block form can read during layout or draw.

// Before: animated value read in composition by the `by` delegate
@Composable
fun SelectionPill(selectedIndex: Int) {
    val offsetX by animateDpAsState(120.dp * selectedIndex)
    Box(Modifier.offset(x = offsetX))
}

// After: State is kept, value is read in the layout-phase offset block
@Composable
fun SelectionPill(selectedIndex: Int) {
    val offsetX = animateDpAsState(120.dp * selectedIndex)
    Box(
        Modifier.offset {
            IntOffset(offsetX.value.roundToPx(), 0)
        },
    )
}

Common replacements:

The drawBehind block is already draw-phase; the important part is that the State.value read also happens inside that block.

2. Pass providers across composable boundaries

If the fast-changing value would cross a composable boundary, pass a provider lambda instead of a snapshot value:

// Before: HomeScreen reads scroll offset in composition and passes the value down
@Composable
fun HomeScreen() {
    val listState = rememberLazyListState()
    LazyColumn(state = listState) {
        item { HeroImage(scrollOffset = listState.firstVisibleItemScrollOffset) }
    }
}

@Composable
fun HeroImage(scrollOffset: Int, modifier: Modifier = Modifier) {
    AsyncImage(
        model = "...",
        modifier = modifier.graphicsLayer(translationY = -scrollOffset / 2f),
    )
}

// After: the only read happens inside graphicsLayer
@Composable
fun HomeScreen() {
    val listState = rememberLazyListState()
    LazyColumn(state = listState) {
        item {
            HeroImage(
                scrollOffsetProvider = {
                    if (listState.firstVisibleItemIndex == 0) {
                        listState.firstVisibleItemScrollOffset
                    } else {
                        0
                    }
                },
            )
        }
    }
}

@Composable
fun HeroImage(scrollOffsetProvider: () -> Int, modifier: Modifier = Modifier) {
    AsyncImage(
        model = "...",
        modifier = modifier.graphicsLayer {
            translationY = -scrollOffsetProvider() / 2f
        },
    )
}

Suffix provider parameters with Provider when that clarifies the deferred-read contract.

3. Other layout/draw read sites

State reads can also be deferred inside:

• Modifier.layout { measurable, constraints -> ... }
• Custom Alignment.align(...)
• drawWithContent, drawBehind, and other draw modifiers
• Block-form layer/layout modifiers such as graphicsLayer { ... } and offset { ... }

Use these when the state changes where something is placed or painted. If the state decides which composables exist, it belongs in composition.

Quick reference

When NOT to apply

• The state controls which composables are emitted.
• The animation is one-shot, cheap, and clarity wins.
• You are writing tests where direct value assertions are simpler.
• Runtime evidence shows recomposition is not the bottleneck.

Related

• compose-state-holder-ui-split - where state-holder vs plain UI split applies when passing providers/lambdas across boundaries.
• compose-stability-diagnostics - parameter stability and compiler reports.
• compose-modifier-and-layout-style - child composables need a normal modifier parameter before callers can move visual reads into modifiers.