LookaheadScope

Known direct subclasses
ExtendedPaneScaffoldScope

Extended scope for pane scaffolds.

SharedTransitionScope

SharedTransitionScope provides a coordinator space in which shared elements/ shared bounds (when matched) will transform their bounds from one to another.

Known indirect subclasses
ExtendedPaneScaffoldPaneScope

Extended scope for the panes of pane scaffolds.

ThreePaneScaffoldPaneScope

Scope for the panes of ThreePaneScaffold.

ThreePaneScaffoldScope

Scope for the panes of ThreePaneScaffold.


LookaheadScope provides a receiver scope for all (direct and indirect) child layouts in LookaheadScope. This receiver scope allows access to lookaheadScopeCoordinates from any child's Placeable.PlacementScope. It also allows any child to convert LayoutCoordinates (which can be retrieved in Placeable.PlacementScope) to LayoutCoordinates in lookahead coordinate space using toLookaheadCoordinates.

import androidx.compose.animation.core.AnimationVector2D
import androidx.compose.animation.core.DeferredTargetAnimation
import androidx.compose.animation.core.VectorConverter
import androidx.compose.foundation.background
import androidx.compose.foundation.clickable
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.Row
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.layout.size
import androidx.compose.foundation.layout.width
import androidx.compose.foundation.shape.RoundedCornerShape
import androidx.compose.runtime.movableContentOf
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.ui.Modifier
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.layout.ApproachLayoutModifierNode
import androidx.compose.ui.layout.ApproachMeasureScope
import androidx.compose.ui.layout.LayoutCoordinates
import androidx.compose.ui.layout.LookaheadScope
import androidx.compose.ui.layout.Measurable
import androidx.compose.ui.layout.MeasureResult
import androidx.compose.ui.layout.Placeable
import androidx.compose.ui.layout.layout
import androidx.compose.ui.node.ModifierNodeElement
import androidx.compose.ui.unit.Constraints
import androidx.compose.ui.unit.IntOffset
import androidx.compose.ui.unit.IntSize
import androidx.compose.ui.unit.dp
import androidx.compose.ui.unit.round

/**
 * Creates a custom implementation of ApproachLayoutModifierNode to approach the placement of
 * the layout using an animation.
 */
class AnimatedPlacementModifierNode(var lookaheadScope: LookaheadScope) :
    ApproachLayoutModifierNode, Modifier.Node() {
    // Creates an offset animation, the target of which will be known during placement.
    val offsetAnimation: DeferredTargetAnimation<IntOffset, AnimationVector2D> =
        DeferredTargetAnimation(IntOffset.VectorConverter)

    override fun isMeasurementApproachInProgress(lookaheadSize: IntSize): Boolean {
        // Since we only animate the placement here, we can consider measurement approach
        // complete.
        return false
    }

    // Returns true when the offset animation is in progress, false otherwise.
    override fun Placeable.PlacementScope.isPlacementApproachInProgress(
        lookaheadCoordinates: LayoutCoordinates
    ): Boolean {
        val target =
            with(lookaheadScope) {
                lookaheadScopeCoordinates.localLookaheadPositionOf(lookaheadCoordinates).round()
            }
        offsetAnimation.updateTarget(target, coroutineScope)
        return !offsetAnimation.isIdle
    }

    override fun ApproachMeasureScope.approachMeasure(
        measurable: Measurable,
        constraints: Constraints
    ): MeasureResult {
        val placeable = measurable.measure(constraints)
        return layout(placeable.width, placeable.height) {
            val coordinates = coordinates
            if (coordinates != null) {
                // Calculates the target offset within the lookaheadScope
                val target =
                    with(lookaheadScope) {
                        lookaheadScopeCoordinates.localLookaheadPositionOf(coordinates).round()
                    }

                // Uses the target offset to start an offset animation
                val animatedOffset = offsetAnimation.updateTarget(target, coroutineScope)
                // Calculates the *current* offset within the given LookaheadScope
                val placementOffset =
                    with(lookaheadScope) {
                        lookaheadScopeCoordinates
                            .localPositionOf(coordinates, Offset.Zero)
                            .round()
                    }
                // Calculates the delta between animated position in scope and current
                // position in scope, and places the child at the delta offset. This puts
                // the child layout at the animated position.
                val (x, y) = animatedOffset - placementOffset
                placeable.place(x, y)
            } else {
                placeable.place(0, 0)
            }
        }
    }
}

// Creates a custom node element for the AnimatedPlacementModifierNode above.
data class AnimatePlacementNodeElement(val lookaheadScope: LookaheadScope) :
    ModifierNodeElement<AnimatedPlacementModifierNode>() {

    override fun update(node: AnimatedPlacementModifierNode) {
        node.lookaheadScope = lookaheadScope
    }

    override fun create(): AnimatedPlacementModifierNode {
        return AnimatedPlacementModifierNode(lookaheadScope)
    }
}

val colors = listOf(Color(0xffff6f69), Color(0xffffcc5c), Color(0xff264653), Color(0xff2a9d84))

var isInColumn by remember { mutableStateOf(true) }
LookaheadScope {
    // Creates movable content containing 4 boxes. They will be put either in a [Row] or in a
    // [Column] depending on the state
    val items = remember {
        movableContentOf {
            colors.forEach { color ->
                Box(
                    Modifier.padding(15.dp)
                        .size(100.dp, 80.dp)
                        .then(AnimatePlacementNodeElement(this))
                        .background(color, RoundedCornerShape(20))
                )
            }
        }
    }

    Box(modifier = Modifier.fillMaxSize().clickable { isInColumn = !isInColumn }) {
        // As the items get moved between Column and Row, their positions in LookaheadScope
        // will change. The `animatePlacementInScope` modifier created above will
        // observe that final position change via `localLookaheadPositionOf`, and create
        // a position animation.
        if (isInColumn) {
            Column(Modifier.fillMaxSize()) { items() }
        } else {
            Row { items() }
        }
    }
}

Summary

Public functions

open Offset
LayoutCoordinates.localLookaheadPositionOf(
    sourceCoordinates: LayoutCoordinates,
    relativeToSource: Offset,
    includeMotionFrameOfReference: Boolean
)

Converts relativeToSource in sourceCoordinates's lookahead coordinate space into local lookahead coordinates.

Cmn
LayoutCoordinates

Converts a LayoutCoordinates into a LayoutCoordinates in the Lookahead coordinate space.

Cmn

Public functions

localLookaheadPositionOf

open fun LayoutCoordinates.localLookaheadPositionOf(
    sourceCoordinates: LayoutCoordinates,
    relativeToSource: Offset = Offset.Zero,
    includeMotionFrameOfReference: Boolean = true
): Offset

Converts relativeToSource in sourceCoordinates's lookahead coordinate space into local lookahead coordinates. This is a convenient method for 1) converting both this coordinates and sourceCoordinates into lookahead space coordinates using toLookaheadCoordinates, and 2) invoking LayoutCoordinates.localPositionOf with the converted coordinates.

For layouts where LayoutCoordinates.introducesMotionFrameOfReference returns true (placed under Placeable.PlacementScope.withMotionFrameOfReferencePlacement) you may pass includeMotionFrameOfReference as false to get their position while excluding the additional Offset.

toLookaheadCoordinates

fun LayoutCoordinates.toLookaheadCoordinates(): LayoutCoordinates

Converts a LayoutCoordinates into a LayoutCoordinates in the Lookahead coordinate space. This can be used for layouts within LookaheadScope.

Public properties

lookaheadScopeCoordinates

val Placeable.PlacementScope.lookaheadScopeCoordinatesLayoutCoordinates

Returns the LayoutCoordinates of the LookaheadScope. This is only accessible from Placeable.PlacementScope (i.e. during placement time).

Note: The returned coordinates is not coordinates in the lookahead coordinate space. If the lookahead coordinates of the lookaheadScope is needed, suggest converting the returned coordinates using toLookaheadCoordinates.