{% setvar book_path %}/reference/androidx/_book.yaml{% endsetvar %} {% include "_shared/_reference-head-tags.html" %}

OnPlacedModifierKt

public final class OnPlacedModifierKt


Summary

Public methods

static final @NonNull Modifier
onPlaced(
    @NonNull Modifier receiver,
    @NonNull Function1<@NonNull LayoutCoordinatesUnit> onPlaced
)

Invoke onPlaced after the parent LayoutModifier and parent layout has been placed and before child LayoutModifier is placed.

Public methods

onPlaced

public static final @NonNull Modifier onPlaced(
    @NonNull Modifier receiver,
    @NonNull Function1<@NonNull LayoutCoordinatesUnit> onPlaced
)

Invoke onPlaced after the parent LayoutModifier and parent layout has been placed and before child LayoutModifier is placed. This allows child LayoutModifier to adjust its own placement based on where the parent is.

import androidx.compose.animation.core.Animatable
import androidx.compose.animation.core.AnimationVector2D
import androidx.compose.animation.core.Spring.StiffnessMediumLow
import androidx.compose.animation.core.VectorConverter
import androidx.compose.animation.core.spring
import androidx.compose.foundation.background
import androidx.compose.foundation.border
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.offset
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.layout.size
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.rememberCoroutineScope
import androidx.compose.ui.Alignment
import androidx.compose.ui.ExperimentalComposeUiApi
import androidx.compose.ui.Modifier
import androidx.compose.ui.composed
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.layout.onPlaced
import androidx.compose.ui.layout.positionInParent
import androidx.compose.ui.unit.IntOffset
import androidx.compose.ui.unit.dp
import androidx.compose.ui.unit.round

@OptIn(ExperimentalComposeUiApi::class)
fun Modifier.animatePlacement(): Modifier = composed {
    val scope = rememberCoroutineScope()
    var targetOffset by remember { mutableStateOf(IntOffset.Zero) }
    var animatable by remember {
        mutableStateOf<Animatable<IntOffset, AnimationVector2D>?>(null)
    }
    this.onPlaced {
        // Calculate the position in the parent layout
        targetOffset = it.positionInParent().round()
    }.offset {
        // Animate to the new target offset when alignment changes.
        val anim = animatable ?: Animatable(targetOffset, IntOffset.VectorConverter)
            .also { animatable = it }
        if (anim.targetValue != targetOffset) {
            scope.launch {
                anim.animateTo(targetOffset, spring(stiffness = StiffnessMediumLow))
            }
        }
        // Offset the child in the opposite direction to the targetOffset, and slowly catch
        // up to zero offset via an animation to achieve an overall animated movement.
        animatable?.let { it.value - targetOffset } ?: IntOffset.Zero
    }
}

@OptIn(ExperimentalComposeUiApi::class)
@Composable
fun AnimatedChildAlignment(alignment: Alignment) {
    Box(
        Modifier.fillMaxSize().padding(4.dp).border(1.dp, Color.Red)
    ) {
        Box(
            modifier = Modifier.animatePlacement().align(alignment).size(100.dp)
                .background(Color.Red)
        )
    }
}