ApproachLayoutModifierNode

ApproachLayoutModifierNode is designed to support gradually approaching the destination layout calculated in the lookahead pass. This can be particularly helpful when the destination layout is anticipated to change drastically and would consequently result in visual disruptions.

In order to create a smooth approach, an interpolation (often through animations) can be used in approachMeasure to interpolate the measurement or placement from a previously recorded size and/or position to the destination/target size and/or position. The destination size is available in ApproachMeasureScope as ApproachMeasureScope.lookaheadSize. And the target position can also be acquired in ApproachMeasureScope during placement by using LookaheadScope.localLookaheadPositionOf with the layout's Placeable.PlacementScope.coordinates. The sample code below illustrates how that can be achieved.

During the lookahead pass, measure will be invoked. By default measure simply passes the incoming constraints to its child, and returns the child measure result to parent without any modification. The default behavior for measure is simply a pass through of constraints and measure results without modification. This can be overridden as needed. approachMeasure will be invoked during the approach pass after lookahead.

isMeasurementApproachInProgress signals whether the measurement is in progress of approaching destination size. It will be queried after the destination has been determined by the lookahead pass, before approachMeasure is invoked. The lookahead size is provided to isMeasurementApproachInProgress for convenience in deciding whether the destination size has been reached.

isPlacementApproachInProgress indicates whether the position is actively approaching destination defined by the lookahead, hence it's a signal to the system for whether additional approach placements are necessary. isPlacementApproachInProgress will be invoked after the destination position has been determined by lookahead pass, and before the placement phase in approachMeasure.

IMPORTANT: When both isMeasurementApproachInProgress and isPlacementApproachInProgress become false, the approach is considered complete. Approach pass will subsequently snap the measurement and placement to lookahead measurement and placement. Once approach is complete, approachMeasure may never be invoked until either isMeasurementApproachInProgress or isPlacementApproachInProgress becomes true again. Therefore it is important to ensure approachMeasure and measure result in the same measurement and placement when the approach is complete. Otherwise, there may be visual discontinuity when we snap the measurement and placement to lookahead.

It is important to be accurate in isPlacementApproachInProgress and isMeasurementApproachInProgress. A prolonged indication of incomplete approach will prevent the system from potentially skipping approach pass when possible.

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
MeasureResult
ApproachMeasureScope.approachMeasure(
    measurable: Measurable,
    constraints: Constraints
)

approachMeasure defines how the measurement and placement of the layout approach the destination size and position.
Cmn
Boolean

isMeasurementApproachInProgress signals whether the measurement is currently approaching destination size.
Cmn
open Boolean

isPlacementApproachInProgress indicates whether the position is approaching destination defined by the lookahead, hence it's a signal to the system for whether additional approach placements are necessary.
Cmn
open Int

The function used to calculate maxIntrinsicHeight for the approach pass changes.
Cmn
open Int

The function used to calculate maxIntrinsicWidth for the approach pass changes.
Cmn
open MeasureResult
MeasureScope.measure(measurable: Measurable, constraints: Constraints)

The function used to measure the modifier.
Cmn
open Int

The function used to calculate minIntrinsicHeight for the approach pass changes.
Cmn
open Int

The function used to calculate minIntrinsicWidth for the approach pass changes.
Cmn

Inherited functions
From androidx.compose.ui.node.DelegatableNode
open Unit

Invoked when the density changes for this node.
Cmn
open Unit

Invoked when the layout direction changes for this node.
Cmn
From androidx.compose.ui.node.LayoutModifierNode
open Int
IntrinsicMeasureScope.maxIntrinsicHeight(
    measurable: IntrinsicMeasurable,
    width: Int
)

The lambda used to calculate IntrinsicMeasurable.maxIntrinsicHeight.
Cmn
open Int
IntrinsicMeasureScope.maxIntrinsicWidth(
    measurable: IntrinsicMeasurable,
    height: Int
)

The function used to calculate IntrinsicMeasurable.maxIntrinsicWidth.
Cmn
open Int
IntrinsicMeasureScope.minIntrinsicHeight(
    measurable: IntrinsicMeasurable,
    width: Int
)

The lambda used to calculate IntrinsicMeasurable.minIntrinsicHeight.
Cmn
open Int
IntrinsicMeasureScope.minIntrinsicWidth(
    measurable: IntrinsicMeasurable,
    height: Int
)

The function used to calculate IntrinsicMeasurable.minIntrinsicWidth.
Cmn

Inherited properties
From androidx.compose.ui.node.DelegatableNode
Modifier.Node

A reference of the Modifier.Node that holds this node's position in the node hierarchy.
Cmn

Public functions

approachMeasure

Cmn 
fun ApproachMeasureScope.approachMeasure(
    measurable: Measurable,
    constraints: Constraints
): MeasureResult

approachMeasure defines how the measurement and placement of the layout approach the destination size and position. In order to achieve a smooth approach from the current size and position to the destination, an interpolation (often through animations) can be used in approachMeasure to interpolate the measurement or placement from a previously recorded size and position to the destination/target size and position. The destination size is available in ApproachMeasureScope as ApproachMeasureScope.lookaheadSize. And the target position can also be acquired in ApproachMeasureScope during placement by using LookaheadScope.localLookaheadPositionOf with the layout's Placeable.PlacementScope.coordinates. Please see sample code below for how that can be achieved.

Note: approachMeasure is only guaranteed to be invoked when either isMeasurementApproachInProgress or isMeasurementApproachInProgress is true. Otherwise, the system will consider the approach complete (i.e. destination reached) and may skip the approach pass when possible.

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() }
        }
    }
}

isMeasurementApproachInProgress

Cmn 
fun isMeasurementApproachInProgress(lookaheadSize: IntSize): Boolean

isMeasurementApproachInProgress signals whether the measurement is currently approaching destination size. It will be queried after the destination has been determined by the lookahead pass, before approachMeasure is invoked. The lookahead size is provided to isMeasurementApproachInProgress for convenience in deciding whether the destination size has been reached.

Note: It is important to be accurate in isPlacementApproachInProgress and isMeasurementApproachInProgress. A prolonged indication of incomplete approach will prevent the system from potentially skipping approach pass when possible.

isPlacementApproachInProgress

Cmn 
open fun Placeable.PlacementScope.isPlacementApproachInProgress(
    lookaheadCoordinates: LayoutCoordinates
): Boolean

isPlacementApproachInProgress indicates whether the position is approaching destination defined by the lookahead, hence it's a signal to the system for whether additional approach placements are necessary. isPlacementApproachInProgress will be invoked after the destination position has been determined by lookahead pass, and before the placement phase in approachMeasure.

Note: It is important to be accurate in isPlacementApproachInProgress and isMeasurementApproachInProgress. A prolonged indication of incomplete approach will prevent the system from potentially skipping approach pass when possible.

By default, isPlacementApproachInProgress returns false.

maxApproachIntrinsicHeight

Cmn 
open fun ApproachIntrinsicMeasureScope.maxApproachIntrinsicHeight(
    measurable: IntrinsicMeasurable,
    width: Int
): Int

The function used to calculate maxIntrinsicHeight for the approach pass changes.

maxApproachIntrinsicWidth

Cmn 
open fun ApproachIntrinsicMeasureScope.maxApproachIntrinsicWidth(
    measurable: IntrinsicMeasurable,
    height: Int
): Int

The function used to calculate maxIntrinsicWidth for the approach pass changes.

measure

Cmn 
open fun MeasureScope.measure(measurable: Measurable, constraints: Constraints): MeasureResult

The function used to measure the modifier. The measurable corresponds to the wrapped content, and it can be measured with the desired constraints according to the logic of the LayoutModifierNode. The modifier needs to choose its own size, which can depend on the size chosen by the wrapped content (the obtained Placeable), if the wrapped content was measured. The size needs to be returned as part of a MeasureResult, alongside the placement logic of the Placeable, which defines how the wrapped content should be positioned inside the LayoutModifierNode. A convenient way to create the MeasureResult is to use the MeasureScope.layout factory function.

A LayoutModifierNode uses the same measurement and layout concepts and principles as a androidx.compose.ui.layout.Layout, the only difference is that they apply to exactly one child. For a more detailed explanation of measurement and layout, see androidx.compose.ui.layout.MeasurePolicy.

minApproachIntrinsicHeight

Cmn 
open fun ApproachIntrinsicMeasureScope.minApproachIntrinsicHeight(
    measurable: IntrinsicMeasurable,
    width: Int
): Int

The function used to calculate minIntrinsicHeight for the approach pass changes.

minApproachIntrinsicWidth

Cmn 
open fun ApproachIntrinsicMeasureScope.minApproachIntrinsicWidth(
    measurable: IntrinsicMeasurable,
    height: Int
): Int

The function used to calculate minIntrinsicWidth for the approach pass changes.