Modifiers to draw scrollbars in Jetpack Compose

Scrollbar.kt

/*
 * MIT License
 *
 * Copyright (c) 2022 Albert Chang
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

import android.view.ViewConfiguration
import androidx.compose.animation.core.Animatable
import androidx.compose.animation.core.tween
import androidx.compose.foundation.ScrollState
import androidx.compose.foundation.gestures.Orientation
import androidx.compose.foundation.horizontalScroll
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.Row
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.LazyListState
import androidx.compose.foundation.lazy.LazyRow
import androidx.compose.foundation.lazy.grid.LazyGridState
import androidx.compose.foundation.lazy.rememberLazyListState
import androidx.compose.foundation.rememberScrollState
import androidx.compose.foundation.verticalScroll
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.LaunchedEffect
import androidx.compose.runtime.remember
import androidx.compose.ui.Modifier
import androidx.compose.ui.composed
import androidx.compose.ui.draw.drawWithContent
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.geometry.Size
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.drawscope.DrawScope
import androidx.compose.ui.input.nestedscroll.NestedScrollConnection
import androidx.compose.ui.input.nestedscroll.NestedScrollSource
import androidx.compose.ui.input.nestedscroll.nestedScroll
import androidx.compose.ui.platform.LocalLayoutDirection
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.LayoutDirection
import androidx.compose.ui.unit.dp
import androidx.compose.ui.util.fastSumBy
import kotlinx.coroutines.channels.BufferOverflow
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.MutableSharedFlow
import kotlinx.coroutines.flow.collectLatest

fun Modifier.drawHorizontalScrollbar(
    state: ScrollState,
    reverseScrolling: Boolean = false
): Modifier = drawScrollbar(state, Orientation.Horizontal, reverseScrolling)

fun Modifier.drawVerticalScrollbar(
    state: ScrollState,
    reverseScrolling: Boolean = false
): Modifier = drawScrollbar(state, Orientation.Vertical, reverseScrolling)

private fun Modifier.drawScrollbar(
    state: ScrollState,
    orientation: Orientation,
    reverseScrolling: Boolean
): Modifier = drawScrollbar(
    orientation, reverseScrolling
) { reverseDirection, atEnd, color, alpha ->
    if (state.maxValue > 0) {
        val canvasSize = if (orientation == Orientation.Horizontal) size.width else size.height
        val totalSize = canvasSize + state.maxValue
        val thumbSize = canvasSize / totalSize * canvasSize
        val startOffset = state.value / totalSize * canvasSize
        drawScrollbar(
            orientation, reverseDirection, atEnd, color, alpha, thumbSize, startOffset
        )
    }
}

fun Modifier.drawHorizontalScrollbar(
    state: LazyListState,
    reverseScrolling: Boolean = false
): Modifier = drawScrollbar(state, Orientation.Horizontal, reverseScrolling)

fun Modifier.drawVerticalScrollbar(
    state: LazyListState,
    reverseScrolling: Boolean = false
): Modifier = drawScrollbar(state, Orientation.Vertical, reverseScrolling)

private fun Modifier.drawScrollbar(
    state: LazyListState,
    orientation: Orientation,
    reverseScrolling: Boolean
): Modifier = drawScrollbar(
    orientation, reverseScrolling
) { reverseDirection, atEnd, color, alpha ->
    val layoutInfo = state.layoutInfo
    val viewportSize = layoutInfo.viewportEndOffset - layoutInfo.viewportStartOffset
    val items = layoutInfo.visibleItemsInfo
    val itemsSize = items.fastSumBy { it.size }
    if (items.size < layoutInfo.totalItemsCount || itemsSize > viewportSize) {
        val estimatedItemSize = if (items.isEmpty()) 0f else itemsSize.toFloat() / items.size
        val totalSize = estimatedItemSize * layoutInfo.totalItemsCount
        val canvasSize = if (orientation == Orientation.Horizontal) size.width else size.height
        val thumbSize = viewportSize / totalSize * canvasSize
        val startOffset = if (items.isEmpty()) 0f else items.first().run {
            (estimatedItemSize * index - offset) / totalSize * canvasSize
        }
        drawScrollbar(
            orientation, reverseDirection, atEnd, color, alpha, thumbSize, startOffset
        )
    }
}

fun Modifier.drawVerticalScrollbar(
    state: LazyGridState,
    spanCount: Int,
    reverseScrolling: Boolean = false
): Modifier = drawScrollbar(
    Orientation.Vertical, reverseScrolling
) { reverseDirection, atEnd, color, alpha ->
    val layoutInfo = state.layoutInfo
    val viewportSize = layoutInfo.viewportEndOffset - layoutInfo.viewportStartOffset
    val items = layoutInfo.visibleItemsInfo
    val rowCount = (items.size + spanCount - 1) / spanCount
    var itemsSize = 0
    for (i in 0 until rowCount) {
        itemsSize += items[i * spanCount].size.height
    }
    if (items.size < layoutInfo.totalItemsCount || itemsSize > viewportSize) {
        val estimatedItemSize = if (rowCount == 0) 0f else itemsSize.toFloat() / rowCount
        val totalRow = (layoutInfo.totalItemsCount + spanCount - 1) / spanCount
        val totalSize = estimatedItemSize * totalRow
        val canvasSize = size.height
        val thumbSize = viewportSize / totalSize * canvasSize
        val startOffset = if (rowCount == 0) 0f else items.first().run {
            val rowIndex = index / spanCount
            (estimatedItemSize * rowIndex - offset.y) / totalSize * canvasSize
        }
        drawScrollbar(
            Orientation.Vertical, reverseDirection, atEnd, color, alpha, thumbSize, startOffset
        )
    }
}

private fun DrawScope.drawScrollbar(
    orientation: Orientation,
    reverseDirection: Boolean,
    atEnd: Boolean,
    color: Color,
    alpha: () -> Float,
    thumbSize: Float,
    startOffset: Float
) {
    val thicknessPx = Thickness.toPx()
    val topLeft = if (orientation == Orientation.Horizontal) {
        Offset(
            if (reverseDirection) size.width - startOffset - thumbSize else startOffset,
            if (atEnd) size.height - thicknessPx else 0f
        )
    } else {
        Offset(
            if (atEnd) size.width - thicknessPx else 0f,
            if (reverseDirection) size.height - startOffset - thumbSize else startOffset
        )
    }
    val size = if (orientation == Orientation.Horizontal) {
        Size(thumbSize, thicknessPx)
    } else {
        Size(thicknessPx, thumbSize)
    }

    drawRect(
        color = color,
        topLeft = topLeft,
        size = size,
        alpha = alpha()
    )
}

private fun Modifier.drawScrollbar(
    orientation: Orientation,
    reverseScrolling: Boolean,
    onDraw: DrawScope.(
        reverseDirection: Boolean,
        atEnd: Boolean,
        color: Color,
        alpha: () -> Float
    ) -> Unit
): Modifier = composed {
    val scrolled = remember {
        MutableSharedFlow<Unit>(
            extraBufferCapacity = 1,
            onBufferOverflow = BufferOverflow.DROP_OLDEST
        )
    }
    val nestedScrollConnection = remember(orientation, scrolled) {
        object : NestedScrollConnection {
            override fun onPostScroll(
                consumed: Offset,
                available: Offset,
                source: NestedScrollSource
            ): Offset {
                val delta = if (orientation == Orientation.Horizontal) consumed.x else consumed.y
                if (delta != 0f) scrolled.tryEmit(Unit)
                return Offset.Zero
            }
        }
    }

    val alpha = remember { Animatable(0f) }
    LaunchedEffect(scrolled, alpha) {
        scrolled.collectLatest {
            alpha.snapTo(1f)
            delay(ViewConfiguration.getScrollDefaultDelay().toLong())
            alpha.animateTo(0f, animationSpec = FadeOutAnimationSpec)
        }
    }

    val isLtr = LocalLayoutDirection.current == LayoutDirection.Ltr
    val reverseDirection = if (orientation == Orientation.Horizontal) {
        if (isLtr) reverseScrolling else !reverseScrolling
    } else reverseScrolling
    val atEnd = if (orientation == Orientation.Vertical) isLtr else true

    val color = BarColor

    Modifier
        .nestedScroll(nestedScrollConnection)
        .drawWithContent {
            drawContent()
            onDraw(reverseDirection, atEnd, color, alpha::value)
        }
}

private val BarColor: Color
    @Composable get() = MaterialTheme.colorScheme.onSurfaceVariant.copy(alpha = 0.5f)

private val Thickness = 4.dp
private val FadeOutAnimationSpec =
    tween<Float>(durationMillis = ViewConfiguration.getScrollBarFadeDuration())

@Preview(widthDp = 400, heightDp = 400, showBackground = true)
@Composable
internal fun ScrollbarPreview() {
    val state = rememberScrollState()
    Column(
        modifier = Modifier
            .drawVerticalScrollbar(state)
            .verticalScroll(state),
    ) {
        repeat(50) {
            Text(
                text = "Item ${it + 1}",
                modifier = Modifier
                    .fillMaxWidth()
                    .padding(16.dp)
            )
        }
    }
}

@Preview(widthDp = 400, heightDp = 400, showBackground = true)
@Composable
internal fun LazyListScrollbarPreview() {
    val state = rememberLazyListState()
    LazyColumn(
        modifier = Modifier.drawVerticalScrollbar(state),
        state = state
    ) {
        items(50) {
            Text(
                text = "Item ${it + 1}",
                modifier = Modifier
                    .fillMaxWidth()
                    .padding(16.dp)
            )
        }
    }
}

@Preview(widthDp = 400, showBackground = true)
@Composable
internal fun HorizontalScrollbarPreview() {
    val state = rememberScrollState()
    Row(
        modifier = Modifier
            .drawHorizontalScrollbar(state)
            .horizontalScroll(state)
    ) {
        repeat(50) {
            Text(
                text = (it + 1).toString(),
                modifier = Modifier
                    .padding(horizontal = 8.dp, vertical = 16.dp)
            )
        }
    }
}

@Preview(widthDp = 400, showBackground = true)
@Composable
internal fun LazyListHorizontalScrollbarPreview() {
    val state = rememberLazyListState()
    LazyRow(
        modifier = Modifier.drawHorizontalScrollbar(state),
        state = state
    ) {
        items(50) {
            Text(
                text = (it + 1).toString(),
                modifier = Modifier
                    .padding(horizontal = 8.dp, vertical = 16.dp)
            )
        }
    }
}

I'd like to use this in a project, but there is no license associated with it. Would it be possible to add an open source license to this gist @mxalbert1996?

(like described in https://zebracatzebra.com/oss/getting-the-gist-of-github-gist-licensing/)

@mattttvaughn Added!

Hello @mxalbert1996,
I tried to use this in my project with LazyColumn. But if I add Arrangement.spacedBy to it, then the height of the scrollbar is bigger than it should be. Could you modify your code to overcome this problem? (I tried to modify itemsSize, but no luck so far.)
Thank you!

For some reason scrollbar stopped moving when scrolling, it shows up when scroll stops

I want to add the Scrollbar on LazyVerticalGrid. For this, I have modified the function but It's not displaying properly.

private fun Modifier.drawScrollbar(
    state: LazyGridState,
    orientation: Orientation,
    reverseScrolling: Boolean
): Modifier = drawScrollbar(
    orientation, reverseScrolling
) { reverseDirection, atEnd, thickness, color, alpha ->
    val layoutInfo = state.layoutInfo
    val viewportSize = layoutInfo.viewportEndOffset - layoutInfo.viewportStartOffset
    val items = layoutInfo.visibleItemsInfo
    val itemsSize = items.fastMap { it.size }.size
    val showScrollbar = items.size < layoutInfo.totalItemsCount || itemsSize > viewportSize
    val estimatedItemSize = if (items.isEmpty()) 0f else itemsSize.toFloat() / items.size
    val totalSize = estimatedItemSize * layoutInfo.totalItemsCount
    val canvasSize = if (orientation == Orientation.Horizontal) size.width else size.height
    val thumbSize = viewportSize / totalSize * canvasSize
    val startOffset = if (items.isEmpty()) 0f else items
        .first()
        .run {
            (estimatedItemSize * index - offset.y) / totalSize * canvasSize
        }
    val drawScrollbar = onDrawScrollbar(
        orientation, reverseDirection, atEnd, showScrollbar,
        thickness, color, alpha, thumbSize, startOffset
    )
    onDrawWithContent {
        drawContent()
        drawScrollbar()
    }
}

I'm experiencing the same issue that @alashow mentioned. With Compose 1.3.0 and 1.3.1, the scrollbar is no longer updated during the scroll, only after the scroll is finished (i.e. when the fade-out animation starts).

It looks like state reads inside drawWithCache are no longer getting picked up in Compose 1.3.0+. I'm not sure whether this is a bug in Compose or in the scrollbar implementation, but as a quick and dirty workaround, accessing the same states from a drawBehind or drawWithContent modifier seems to resolve the issue.

For example: https://gist.github.com/DonTomika/7d161a188722f00d4f580cf3f355958d/revisions

Yeah it seems that the original version is broken when using Compose 1.3 so I've updated the gist.

Pls, create a lib project and add the file. And upload to https://mvnrepository.com/artifact/

@Kolyall There is no plan to make this a library as official support of scrollbars is in focus on the roadmap.