overscroll

fun Modifier.overscroll(overscrollEffect: OverscrollEffect?): Modifier

import androidx.compose.animation.core.Animatable
import androidx.compose.animation.core.spring
import androidx.compose.foundation.OverscrollEffect
import androidx.compose.foundation.background
import androidx.compose.foundation.gestures.Orientation
import androidx.compose.foundation.gestures.rememberScrollableState
import androidx.compose.foundation.gestures.scrollable
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.size
import androidx.compose.foundation.overscroll
import androidx.compose.material.Text
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.rememberCoroutineScope
import androidx.compose.runtime.setValue
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.input.nestedscroll.NestedScrollSource
import androidx.compose.ui.layout.Measurable
import androidx.compose.ui.layout.MeasureResult
import androidx.compose.ui.layout.MeasureScope
import androidx.compose.ui.node.DelegatableNode
import androidx.compose.ui.node.LayoutModifierNode
import androidx.compose.ui.text.TextStyle
import androidx.compose.ui.unit.Constraints
import androidx.compose.ui.unit.IntOffset
import androidx.compose.ui.unit.Velocity
import androidx.compose.ui.unit.dp
import androidx.compose.ui.unit.sp

// our custom offset overscroll that offset the element it is applied to when we hit the bound
// on the scrollable container.
class OffsetOverscrollEffect(val scope: CoroutineScope) : OverscrollEffect {
    private val overscrollOffset = Animatable(0f)

    override fun applyToScroll(
        delta: Offset,
        source: NestedScrollSource,
        performScroll: (Offset) -> Offset,
    ): Offset {
        // in pre scroll we relax the overscroll if needed
        // relaxation: when we are in progress of the overscroll and user scrolls in the
        // different direction = substract the overscroll first
        val sameDirection = sign(delta.y) == sign(overscrollOffset.value)
        val consumedByPreScroll =
            if (abs(overscrollOffset.value) > 0.5 && !sameDirection) {
                val prevOverscrollValue = overscrollOffset.value
                val newOverscrollValue = overscrollOffset.value + delta.y
                if (sign(prevOverscrollValue) != sign(newOverscrollValue)) {
                    // sign changed, coerce to start scrolling and exit
                    scope.launch { overscrollOffset.snapTo(0f) }
                    Offset(x = 0f, y = delta.y + prevOverscrollValue)
                } else {
                    scope.launch { overscrollOffset.snapTo(overscrollOffset.value + delta.y) }
                    delta.copy(x = 0f)
                }
            } else {
                Offset.Zero
            }
        val leftForScroll = delta - consumedByPreScroll
        val consumedByScroll = performScroll(leftForScroll)
        val overscrollDelta = leftForScroll - consumedByScroll
        // if it is a drag, not a fling, add the delta left to our over scroll value
        if (abs(overscrollDelta.y) > 0.5 && source == NestedScrollSource.UserInput) {
            scope.launch {
                // multiply by 0.1 for the sake of parallax effect
                overscrollOffset.snapTo(overscrollOffset.value + overscrollDelta.y * 0.1f)
            }
        }
        return consumedByPreScroll + consumedByScroll
    }

    override suspend fun applyToFling(
        velocity: Velocity,
        performFling: suspend (Velocity) -> Velocity,
    ) {
        val consumed = performFling(velocity)
        // when the fling happens - we just gradually animate our overscroll to 0
        val remaining = velocity - consumed
        overscrollOffset.animateTo(
            targetValue = 0f,
            initialVelocity = remaining.y,
            animationSpec = spring(),
        )
    }

    override val isInProgress: Boolean
        get() = overscrollOffset.value != 0f

    // Create a LayoutModifierNode that offsets by overscrollOffset.value
    override val node: DelegatableNode =
        object : Modifier.Node(), LayoutModifierNode {
            override fun MeasureScope.measure(
                measurable: Measurable,
                constraints: Constraints,
            ): MeasureResult {
                val placeable = measurable.measure(constraints)
                return layout(placeable.width, placeable.height) {
                    val offsetValue = IntOffset(x = 0, y = overscrollOffset.value.roundToInt())
                    placeable.placeRelativeWithLayer(offsetValue.x, offsetValue.y)
                }
            }
        }
}

val offset = remember { mutableStateOf(0f) }
val scope = rememberCoroutineScope()
// Create the overscroll controller
val overscroll = remember(scope) { OffsetOverscrollEffect(scope) }
// let's build a scrollable that scroll until -512 to 512
val scrollStateRange = (-512f).rangeTo(512f)
Box(
    Modifier.size(150.dp)
        .scrollable(
            orientation = Orientation.Vertical,
            state =
                rememberScrollableState { delta ->
                    // use the scroll data and indicate how much this element consumed.
                    val oldValue = offset.value
                    // coerce to our range
                    offset.value = (offset.value + delta).coerceIn(scrollStateRange)

                    offset.value - oldValue // indicate that we consumed what's needed
                },
            // pass the overscroll to the scrollable so the data is updated
            overscrollEffect = overscroll,
        )
        .background(Color.LightGray),
    contentAlignment = Alignment.Center,
) {
    Text(
        offset.value.roundToInt().toString(),
        style = TextStyle(fontSize = 32.sp),
        modifier =
            Modifier
                // show the overscroll only on the text, not the containers (just for fun)
                .overscroll(overscroll),
    )
}