compose-jb/tutorials/Scrollbars/README.md

# Scrollbar usage

## What is covered

In this tutorial, we will show you how to add scrollbars to scrollable lists using Compose for Desktop.

## Scrollbars applying

You can apply scrollbars to scrollable components. The scrollbar and scrollable components share a common state to synchronize with each other. For example, VerticalScrollbar can be attached to Modifier.verticalScroll, and LazyColumnFor and HorizontalScrollbar can be attached to Modifier.horizontalScroll and LazyRowFor.

```kotlin
import androidx.compose.desktop.Window
import androidx.compose.foundation.HorizontalScrollbar
import androidx.compose.foundation.VerticalScrollbar
import androidx.compose.foundation.background
import androidx.compose.foundation.horizontalScroll
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.Spacer
import androidx.compose.foundation.layout.fillMaxHeight
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.layout.width
import androidx.compose.foundation.rememberScrollState
import androidx.compose.foundation.rememberScrollbarAdapter
import androidx.compose.foundation.verticalScroll
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.unit.IntSize
import androidx.compose.ui.unit.dp

fun main() {
    Window(title = "Scrollbars", size = IntSize(250, 400)) {
        Box(
            modifier = Modifier.fillMaxSize()
                .background(color = Color(180, 180, 180))
                .padding(10.dp)
        ) {
            val stateVertical = rememberScrollState(0)
            val stateHorizontal = rememberScrollState(0)

            Box(
                modifier = Modifier
                    .fillMaxSize()
                    .verticalScroll(stateVertical)
                    .padding(end = 12.dp, bottom = 12.dp)
                    .horizontalScroll(stateHorizontal)
            ) {
                Column {
                    for (item in 0..30) {
                        TextBox("Item #$item")
                        if (item < 30) {
                            Spacer(modifier = Modifier.height(5.dp))
                        }
                    }
                }
            }
            VerticalScrollbar(
                modifier = Modifier.align(Alignment.CenterEnd)
                    .fillMaxHeight(),
                adapter = rememberScrollbarAdapter(stateVertical)
            )
            HorizontalScrollbar(
                modifier = Modifier.align(Alignment.BottomStart)
                    .fillMaxWidth()
                    .padding(end = 12.dp),
                adapter = rememberScrollbarAdapter(stateHorizontal)
            )
        }
    }
}

@Composable
fun TextBox(text: String = "Item") {
    Box(
        modifier = Modifier.height(32.dp)
            .width(400.dp)
            .background(color = Color(200, 0, 0, 20))
            .padding(start = 10.dp),
        contentAlignment = Alignment.CenterStart
    ) {
        Text(text = text)
    }
}
```

![Scrollbars](scrollbars.gif)

## Lazy scrollable components with Scrollbar

You can use scrollbars with lazy scrollable components, for example, LazyColumn.

```kotlin
import androidx.compose.desktop.Window
import androidx.compose.foundation.ExperimentalFoundationApi
import androidx.compose.foundation.VerticalScrollbar
import androidx.compose.foundation.background
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.Spacer
import androidx.compose.foundation.layout.fillMaxHeight
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.rememberLazyListState
import androidx.compose.foundation.rememberScrollbarAdapter
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.unit.IntSize
import androidx.compose.ui.unit.dp

fun main() {
    Window(title = "Scrollbars", size = IntSize(250, 400)) {
        LazyScrollable()
    }
}

@OptIn(ExperimentalFoundationApi::class)
@Composable
fun LazyScrollable() {
    Box(
        modifier = Modifier.fillMaxSize()
            .background(color = Color(180, 180, 180))
            .padding(10.dp)
    ) {

        val state = rememberLazyListState()
        val itemCount = 1000

        LazyColumn(Modifier.fillMaxSize().padding(end = 12.dp), state) {
            items(itemCount) { x ->
                TextBox("Item #$x")
                Spacer(modifier = Modifier.height(5.dp))
            }
        }
        VerticalScrollbar(
            modifier = Modifier.align(Alignment.CenterEnd).fillMaxHeight(),
            adapter = rememberScrollbarAdapter(
                scrollState = state,
                itemCount = itemCount,
                averageItemSize = 37.dp // TextBox height + Spacer height
            )
        )
    }
}

@Composable
fun TextBox(text: String = "Item") {
    Box(
        modifier = Modifier.height(32.dp)
            .fillMaxWidth()
            .background(color = Color(0, 0, 0, 20))
            .padding(start = 10.dp),
        contentAlignment = Alignment.CenterStart
    ) {
        Text(text = text)
    }
}
```

![Lazy component](lazy_scrollbar.gif)

## Theme applying

Scrollbars support themes to change their appearance. The example below shows how to use the DesktopTheme appearance for the scrollbar.

```kotlin
import androidx.compose.desktop.DesktopTheme
import androidx.compose.desktop.Window
import androidx.compose.foundation.background
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxHeight
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.layout.Spacer
import androidx.compose.foundation.rememberScrollbarAdapter
import androidx.compose.foundation.rememberScrollState
import androidx.compose.material.Text
import androidx.compose.foundation.VerticalScrollbar
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.verticalScroll
import androidx.compose.material.MaterialTheme
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp
import androidx.compose.ui.unit.IntSize

fun main() {
    Window(title = "Scrollbars", size = IntSize(280, 400)) {
        MaterialTheme {
            DesktopTheme {
                Box(
                    modifier = Modifier.fillMaxSize()
                        .background(color = Color(180, 180, 180))
                        .padding(10.dp)
                ) {
                    val state = rememberScrollState(0)

                    Column(
                        modifier = Modifier
                            .verticalScroll(state)
                            .fillMaxSize()
                            .padding(end = 12.dp)
                    ) {
                        for (item in 0..30) {
                            TextBox("Item #$item")
                            if (item < 30) {
                                Spacer(modifier = Modifier.height(5.dp))
                            }
                        }
                    }
                    VerticalScrollbar(
                        modifier = Modifier.align(Alignment.CenterEnd)
                            .fillMaxHeight(),
                        adapter = rememberScrollbarAdapter(state)
                    )
                }
            }
        }
    }
}

@Composable
fun TextBox(text: String = "Item") {
    Box(
        modifier = Modifier.height(32.dp)
            .fillMaxWidth()
            .background(color = Color(0, 0, 0, 20))
            .padding(start = 10.dp),
        contentAlignment = Alignment.CenterStart
    ) {
        Text(text = text)
    }
}
```

![Themed scrollbar](themed_scrollbar.gif)